WATER’S STRUCTURE
Team Leader:
Bob Holzer
Writer:
John Watson
Editor:
CHAOS Communications
Producer:
Michele Boniface
Content Reviewers:
Donna Matovinovic
Stella Shrum
Produced by ACCESS The Education Station
© 1997 Alberta Education
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WATER’S STRUCTURE
Teacher’s Guide
Table of Contents
Introduction ............................................................1
Program Summary ................................................1
Links to Curriculum Standards ...........................1
Pre-Test ....................................................................2
Teacher Preparation/Instructional Notes ..........2
Student Objectives .................................................3
Student Preparation...............................................3
Blackline Masters ...................................................4
Answer Key ............................................................5
Script of Video Narration ................................... 11
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INTRODUCTION
This Teacher’s Guide accompanies Program Five, “Water’s
Structure,” from the Simply Science series.
Simply Science is a series of twenty-five science programs
for high school students. These instructional programs use
practical applications as context to the interdisciplinary concept development emphasizing the connections among science, technology, and society. This comprehensive
Teacher’s Guide and accompanying blackline master activity sheets provide extended practice and additional learning opportunities.
PROGRAM SUMMARY
“Water’s Structure” investigates how atoms are arranged
in a water molecule. Evidence for the chemical composition and polar nature of water is provided. An overview of
atomic models, from Thomson to Bohr, is presented. The
Bohr model is used to explain covalent bonding in water.
LINKS TO CURRICULUM STANDARDS
“Water’s Structure” correlates with the following National
Science Education Standards for grades 9-12:
Physical Science: Structure of atoms
• Matter is made of minute particles called atoms, and
atoms are composed of even smaller components. These
components have measurable properties, such as mass and
electrical charge. Each atom has a positively charged
nucleus surrounded by negatively charged electrons. The
electric force between the nucleus and electrons holds the
atom together.
Physical Science: Structure and properties of matter
• Atoms interact with one another by transferring or sharing electrons that are furthest from the nucleus. These outer
electrons govern the chemical properties of the element.
5
• Bonds between atoms are created when electrons are
paired up by being transferred or shared. A substance composed of a single kind of atom is called an element. The
atoms may be bonded together into molecules or crystalline solids. A compound is formed when two or more kinds
of atoms bind together chemically.
• The physical properties of compounds reflect the nature
of the interaction among its molecules. These interactions
are determined by the structure of the molecule, including
the constituent atoms and the distances and angles between
them.
• Solids, liquids and gases differ in the distance and angles
between the molecules or atoms and therefore the energy
that binds them together. In solids the structure is nearly
rigid; in liquids molecules or atoms move around each other
but do not move apart; and in gases molecules or atoms
move almost independently of each other and are mostly
far apart.
PRE-TEST
A Pre-Test is included with the Blackline Masters for this
program. It is meant to be administered before the video
and its ensuing activities are used. This assessment tool
allows you to gauge student comprehension of the Objectives before completing the lesson; its results may be contrasted with those of the Post-Test, also included herein, to
assess comprehension of the Objectives after completing
the lesson.
TEACHER PREPARATION/INSTRUCTIONAL NOTES
Before presenting this lesson to your students we suggest
that you preview the video and review this guide, and the
accompanying blackline master activities in order to familiarize yourself with their content.
As you review the materials presented in this guide, you
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may find it necessary to make some changes, additions, or
deletions to meet the specific needs of your class. We encourage you to do so, for only by tailoring this program to
your class will they obtain the maximum instructional benefits afforded by the materials.
It is also suggested that the video presentation take place
before the entire group under your supervision. The lesson activities grow out of the context of the video, therefore, the presentation should be a common experience for
all students.
STUDENT OBJECTIVES
After viewing the video and participating in the follow-up
activities, students will be able to:
• Explain why water is considered a polar molecule.
• Describe how water’s polar nature is the source of
many of its properties, such as surface tension and the
density anomaly.
• Define the terms atom and molecule.
• Recognize and describe the tests for oxygen and
hydrogen.
• Appreciate that careful observation helps us to organize our understanding of matter.
STUDENT PREPARATION
This video is one of a series. Before students view this program and complete the follow-up activities, they should
be able to:
1. Recognize the property of liquid water which allows
some insects to “walk” across its surface.
Surface tension allows some insects to walk across liquid water.
2. Identify the property of water which causes ice (solid
water) to float on liquid water, making activities like
ice sailing possible.
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The density anomaly is responsible for ice floating on liquid water.
3. Compare water’s freezing and boiling points to those
of substances with similar-sized molecules (e.g., ammonia).
Water has much higher freezing and boiling points than substances with molecules of a similar size.
4. Compare the heat of fusion for water with that of other
substances.
Water has a very high heat of fusion compared to most other substances.
If students have difficulty with any of the items, you should
review the concepts in reference materials before viewing
the video.
BLACKLINE MASTERS
The following blackline master activity sheets are included
with this guide. Duplicate and distribute those you wish to
use. An Answer Key appears on pages 5-9.
(1.) Blackline Master #1: Pre-Test is to be given to your
students prior to viewing the video to assess their prior
knowledge of the topic. It may be contrasted to Blackline
Masters #7a-7d: Post-Test to gauge student comprehension
of the Objectives after the lesson has been completed.
(2.) Blackline Master #2: Glossary is a list of terms from
the video. Students may find this handout helpful when
completing the activities which accompany this lesson, as
well as for preparation for the Post-Test.
(3.) Blackline Master #3: Water: What’s It Made Of? details the electrolysis of water, resulting in the separation of
water molecules into hydrogen and oxygen.
(4.) Blackline Master #4: The Atomic Model contrasts the
atomic model proposed by four scientists.
(5.) Blackline Masters #5a-5b: Atoms Sharing Electrons
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examines covalent bonds.
(6.) Blackline Master #6: The Shape of Water explores the
shape of the water molecule, and correlates it to the unique
behavior of water.
(7.) Blackline Masters #7a-7d: Post-Test is an assessment
tool to be used after the video and follow-up activities have
been completed. The test is based directly on the Student
Objectives for this program and the National Science Education Standards for grades 9-12.
ANSWER KEY
Blackline Master #1: Pre-Test
1.T
2.F
3.F
4.T
5.T
6.F
7.T
8.F
9.T
10.T
Blackline Masters #3-6
Note that some of these questions have more than one
possible answer.
1. A burning splint is held near the mouth of a small test
tube of hydrogen. A positive result is the “pop” heard
when the hydrogen explodes.
Caution: Hydrogen is explosive; very small quantities should be used with this procedure.
2. A wood splint which is glowing, but not burning, is
inserted into a container of oxygen. A positive result
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3.
4.
5.
6.
7.
8.
9.
is if the splint bursts back into flames.
45 mL
You should observe the stream of water is attracted to
the balloon.
No. Water is attracted to both positive and negative
charges; this test does not show which charge the
balloon has.
He found evidence for the existence of the electron.
The few alpha particles that bounced back caused
Rutherford to propose they had hit a massive, positive part of the atom – the nucleus.
Bohr’s atom identifies specific distances from the
nucleus (energy levels) where the electrons may be;
Rutherford’s atom did not do that.
The non-symmetric shape of the hydrogen sulfide
molecule suggests it is a polar molecule.
10. Hydrogen bonding does not occur in methane. It is a
symmetric molecule; there are no positive and negative sides.
11. Covalent bonds occur when atoms share electrons
and form a molecule. Hydrogen bonds occur between some polar molecules. They are forces between
the slightly positive and negative charged parts of
adjacent molecules.
12. Unequal sharing of electrons leaves oxygen slightly
negative and hydrogen slightly positive. The v-shape
of the water molecule means both of the hydrogen
atoms are on the same side of the molecule.
13. It is a regular, orderly arrangement, but it includes an
amount of empty space – holes between molecules.
These holes are larger in ice than in liquid water.
14. No, methane does not form a crystal lattice; the forces
acting between molecules are too weak to produce the
orderly arrangement we call a lattice.
Note: Other hypotheses are acceptable, but they
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should include an answer and a reason.
15. The glucose molecules get in between water molecules and do not allow the lattice to form.
Answers to Blackline Master #4: The Atomic Model
Scientist
Dalton
Model
Description
Billiard Ball
•All matter is made of atoms
•The atom is solid and indivisible
Thomson
Plum Pudding
•Positive matter makes up most
of the atom
•Electrons are stuck in this matter
Rutherford
Nuclear
•Massive, positive nucleus contains
most matter
•Electrons are in orbit around
the nucleus
Bohr
Energy Level or
Bohr Model
•Electrons orbit the nucleus only at
specific distances from the nucleus
called energy levels
Blackline Masters #7a-7d: Post-Test
Multiple Choice
1.
b.
polar
2.
d.
all of the above
3.
d.
Democritus
4.
a.
electron
5.
c.
hydrogen bonds
Long Answer
1.
a chemical
2.
The volumes of gas collected. One was twice that
of the other.
3.
The glowing splint test for oxygen. Oxygen
supports combustion. The splint igniting indicates
oxygen.
The burning splint test for hydrogen. Hydrogen
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4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
burns vigorously (explodes). The flame ignited
the hydrogen giving the characteristic “pop.”
break
Bring the balloon close to an object of known
charge, positive or negative. Attraction means the
balloon has the opposite charge and repulsion
means it has the same charge.
J.J. Thomson
a very dense (mass), positively charged nucleus,
negatively charged electrons orbiting in a
planetary manner
Using the quantum theory, Bohr proposed that the
electrons exist only in specific, stable energy
levels, or orbits. They would not be found in
between levels, and would not go below a certain
minimum level. The most energetic electrons
would be in an orbit furthest from the nucleus,
and the least energetic electrons would be closest
to the nucleus.
two or eight
share electrons
Water contracts until it reaches 4°C, to this point
the hydrogen bonds dominate. Below 4°C
molecular motion slows and the repulsion
between like poles of the polar water molecules
become more evident. This forces the molecules
apart into the honeycomb like structure, a
hexagonal crystal lattice with empty spaces, of ice.
are polar, they have a positive side and a negative
side
The two hydrogen atoms are bonded to one
oxygen atom in a V-shape. Unequal sharing of the
electrons between the oxygen and hydrogen
atoms make the molecule polar, which in turn
leads to hydrogen bonds between molecules. The
hydrogen bonds are responsible for high specific
heat capacity, surface tension and freezing and
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14.
15.
16.
boiling points.
non-metal atoms share electrons forming, a
covalent bond
The covalent bonds, between the atoms of
hydrogen and oxygen, are due to the sharing of
electrons.
The hydrogen bonds are due to the attraction
between the positive and negative ends of the
polar water molecule.
Glucose molecules get between the water
molecules in the cell cytoplasm preventing the
water from freezing. The glucose acts like anti
freeze in automobile cooling systems.
17.
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SCRIPT OF NARRATION
DANA: WATER CAN BE SQUIRTED, FUNNELED, SPRAYED
AROUND, FROZEN IN BLOCKS, STOCKPILED BEHIND DAMS,
SHOVELED WHEN IT’S SNOW, AND MANIPULATED INTO ALMOST ANY SHAPE OR SIZE. BUT IF YOU HAD TO DESCRIBE THE
“SHAPE” OF WATER, COULD YOU? THREE-QUARTERS OF OUR
PLANET IS MADE UP OF WATER, A COMMON BUT VERY UNUSUAL SUBSTANCE. BEFORE BIRTH, WE FLOAT IN A WARM,
WATERY WORLD. OUT HERE, OUR BODIES ARE STILL ABOUT 70%
WATER. WE CAN LIVE WITHOUT FOOD FOR WEEKS. BUT WITHOUT WATER, WE’D LAST ONLY A FEW DAYS. OUR BODY CELLS
ARE PACKED WITH A LIVING FLUID CALLED CYTOPLASM,
MADE UP OF MOSTLY...WATER. NUTRIENTS DISSOLVED IN WATER ARE ABSORBED BY THE ROOTS OF TREES AND CARRIED UP
TO THE TOP BRANCHES. WE KNOW THAT WATER IS UNUSUAL.
IT HAS: AS IT FREEZES, WATER BECOMES LESS DENSE AND EXPANDS, WHICH MEANS THAT ICE FLOATS ON LIQUID WATER.
SARAH: EVER HEARD OF WATER PIPES BREAKING BECAUSE OF
SEVERE COLD? WHEN WATER EXPANDS DURING FREEZING, IT
EXERTS TREMENDOUS PRESSURE ON WHATEVER HAPPENS TO
BE AROUND IT, ON TOP OF IT, OR JUST CLOSE BY. HERE’S PROOF.
FILL A NARROW-NECKED GLASS JAR WITH WATER RIGHT TO
THE TOP, AND PUT IT INTO THE FREEZER. NOW, WE DON’T
WANT TO CREATE AN AIRTIGHT SEAL, SO LET THAT SIT FOR A
FEW HOURS UNTIL THE ICE IS EXPANDED OUT FROM THE NECK
OF THE JAR. NOW, BALANCE A SAUCER ON TOP, AND PLACE A
WEIGHT, WITH A MASS OF 2 KILOGRAMS, ON TOP OF THAT. LET
THAT SIT OVER NIGHT. AND THE ICE HAS EXPANDED TO LIFT
BOTH THE SAUCER AND THE WEIGHT. THAT’S WHY WE
SHOULDN’T FREEZE THINGS IN SEALED GLASS CONTAINERS.
EXPANDING WATER CAN EASILY BREAK GLASS. IN FACT, FREEZING WATER EXERTS ONE OF THE GREATEST FORCES IN NATURE.
DANA: BUT WHY DOES WATER BEHAVE THIS WAY? I’LL GIVE
YOU A HINT: IT HAS TO DO WITH ITS SHAPE.
STEPHANIE: WATER HAS A SHAPE?
DANA: LIQUID WATER CONFORMS TO THE SHAPE OF THE CONTAINER. THAT’S BECAUSE THE WATER MOLECULES INSIDE ARE
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FREE TO MOVE, WHILE STILL MAINTAINING CONTACT WITH
ONE ANOTHER. THIS WATER APPEARS TO BE STILL. BUT IF YOU
COULD LOOK AT IT IN EXTREME CLOSE-UP, YOU’D SEE BILLIONS
OF MOLECULES ZOOMING AROUND. THE STRUCTURE OF
THESE MOLECULES MAKES WATER A UNIQUE SUBSTANCE ON
EARTH. TO TAKE A REALLY CLOSE LOOK AT WATER, WE NEED
TO GET RIGHT INTO THE STRUCTURE OF THE WATER MOLECULE AND THE ATOMS THAT IT’S MADE OF. YOU CAN’T GET
MUCH CLOSER THAN THAT! WE HAVE ENGINE START.
DARREN: THIS HOFFMAN APPARATUS WILL HELP US SEE WHAT
WATER’S MADE OF. DO YOU KNOW HOW IT WORKS?
STEPHANIE: NOT EXACTLY.
DARREN: IT USES AN ELECTRIC CURRENT TO BREAK DOWN
SUBSTANCES. WE’RE DECOMPOSING WATER, SO WHAT’S THE
FIRST THING WE DO?
STEPHANIE: ADD WATER TO THE APPARATUS.
DARREN: TO MAKE IT WORK, WE NEED TO USE TAP WATER,
NOT DISTILLED WATER.
STEPHANIE: ALL RIGHT, IT’S FULL.
DARREN: GOOD. NOW WE JUST CLOSE THESE. AND THERE’S
ONE MORE THING.
STEPHANIE: TURN ON THE POWER.
DARREN: IF YOU LOOK CLOSELY AT THE ELECTRODES, CAN
YOU SEE ANYTHING?
STEPHANIE: BUBBLES. SO THE CURRENT IS STARTING TO DECOMPOSE THE WATER?
DARREN: THAT’S RIGHT.
STEPHANIE: HOW LONG WILL IT TAKE?
DARREN: WE’LL LEAVE IT ABOUT 20 MINUTES.
STEPHANIE: THE WATER’S BROKEN DOWN, AND THE GASES
HAVE COLLECTED HERE AND HERE.
DARREN: HOW MUCH GAS DO WE HAVE ON EACH SIDE?
STEPHANIE: THERE’S 9.3 MILLILITERS ON THIS SIDE. AND OVER
ON THIS SIDE, THERE’S 18.6 MILLILITERS. SO THERE’S TWICE
AS MUCH HERE.
DARREN: WHAT DO YOU THINK THE GASES ARE?
STEPHANIE: WATER IS H2O. SO THERE SHOULD BE TWO PARTS
HYDROGEN TO ONE PART OXYGEN. THIS SIDE SHOULD BE HY15
DROGEN.
DARREN: OKAY. WE CAN IDENTIFY THE GASES USING A SPLINT
TEST. FIRST WE’LL COLLECT THE GAS FROM EACH SIDE.
STEPHANIE: THANKS. OKAY, I’M TAKING OXYGEN FROM THIS
TEST TUBE.
DARREN: AND I’VE GOT THE HYDROGEN.
STEPHANIE: THERE YOU GO.
DARREN: THANK YOU. HERE’S AN EMPTY TEST TUBE.
STEPHANIE: IT HAS AIR IN IT.
DARREN: YOU’RE ABSOLUTELY RIGHT. NOW I’M GOING TO
LIGHT THIS SPLINT. AND WHAT HAPPENS WHEN YOU PUT THE
GLOWING SPLINT INTO IT?
STEPHANIE: NOTHING MUCH.
DARREN: GOOD. THIS TEST TUBE FULL OF AIR IS OUR “CONTROL.” IT GIVES US SOMETHING TO COMPARE WHEN WE TEST
THE GASES WE COLLECTED. CAN YOU PREDICT WHAT’S GOING TO HAPPEN WITH OUR SPLINT TESTS? YOU KNOW WHAT
HAPPENS WHEN YOU FAN GLOWING COALS?
STEPHANIE: WHEN YOU FAN A FIRE, IT GLOWS BRIGHTER, OR
IGNITES, BECAUSE THINGS CAN’T BURN WITHOUT OXYGEN.
DARREN: EXACTLY. OXYGEN SUPPORTS BURNING. SO WHEN
WE TEST FOR OXYGEN, WHAT DO WE PREDICT THE SPLINT WILL
DO?
STEPHANIE: GLOW BRIGHTER, OR IGNITE.
DARREN: AND?
STEPHANIE: AND IT DOES. SO THIS TEST TUBE HAS OXYGEN.
DARREN: HYDROGEN IS THE MAIN SOURCE OF FUEL THAT
POWERS SPACE SHUTTLES. IT BURNS EXTREMELY VIGOROUSLY.
IF THE BURNING WEREN’T CONTROLLED, THE HYDROGEN
WOULD EXPLODE. ENGINES AT 100% AND THROTTLING DOWN.
DARREN: WHAT WILL HAPPEN WHEN WE PUT THIS NEAR THE
MOUTH OF THE TEST TUBE FULL OF HYDROGEN?
STEPHANIE: AN EXPLOSION?
DARREN: RIGHT. WHEN WE TEST FOR HYDROGEN, WE EXPECT
TO HEAR A “POP,” LIKE A TINY EXPLOSION. LET’S GIVE IT A TRY.
STEPHANIE: SOUNDS LIKE HYDROGEN!
DARREN: SO WE’VE IDENTIFIED THE GASES.
STEPHANIE: TWO PARTS HYDROGEN TO ONE PART OXYGEN.
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DARREN: THIS EVIDENCE SHOWS WHY H2O IS THE FORMULA
WE USE FOR THE WATER MOLECULE. THE HOFFMAN APPARATUS IS HANDY FOR LEARNING ABOUT THE STRUCTURE OF
THIS...THE WATER MOLECULE. ATOMS IN MOLECULES ARE
JOINED BY CHEMICAL BONDS. USING THE HOFFMAN APPARATUS, WE ADD ENERGY TO THE SYSTEM WITH AN ELECTRICAL
CURRENT.
STEPHANIE: AND THAT ENERGY BREAKS THE BONDS HOLDING THE MOLECULES TOGETHER.
DARREN: RIGHT. WHEN THOSE BONDS ARE BROKEN, WE’RE
LEFT WITH THE ELEMENTS THAT MAKE UP WATER.
STEPHANIE: TWO PARTS HYDROGEN TO ONE PART OXYGEN.
THAT’S H2O.
DANA: HYDROGEN BURNS VIGOROUSLY. OXYGEN SUPPORTS
BURNING. WHEN A SPACE SHUTTLE TAKES OFF, THE ROCKETS
THAT FIRE IT ARE POWERED BY THE FORCE OF HYDROGEN AND
OXYGEN UNDER COMBUSTION. YOU KNOW THAT H2O IS MADE
UP OF TWO PARTS HYDROGEN AND ONE PART OXYGEN. BUT
THIS DOESN’T EXPLAIN THE UNUSUAL CHARACTERISTICS
THAT WATER HAS. THERE ARE STRONG “ATTRACTIVE” FORCES
BETWEEN WATER MOLECULES. TO UNDERSTAND THIS, LET’S
THINK ABOUT STATIC ELECTRICITY. IF YOU RUB A BALLOON
ON YOUR HAIR, AND THEN PUT THE BALLOON NEXT TO A
WALL, TWO THINGS HAPPEN: YOU GIVE YOURSELF AN INSTANT BAD HAIR DAY, AND THE BALLOON STICKS TO THE
WALL. DOES WATER REACT TO STATIC ELECTRICITY? IF IT DOES,
THEN IT MUST BE ELECTRICALLY CHARGED. COULD ELECTRICAL CHARGES EXPLAIN THE STRONG BONDS BETWEEN WATER MOLECULES? FIRST, WE’LL SEE WHAT HAPPENS WHEN
THESE RODS ARE GIVEN AN ELECTRICAL CHARGE.
JULIEN: OKAY.
DANA: WHEN WE BRING A NEGATIVELY CHARGED ROD NEAR
A POSITIVELY CHARGED ROD, WHAT HAPPENS?
JULIEN: THEY’RE ATTRACTED.
DANA: GOOD. BUT WHAT HAPPENS WHEN WE BRING A POSITIVELY CHARGED ROD NEAR ANOTHER POSITIVELY CHARGED
ROD?
JULIEN: THEY REPEL.
17
DANA: RIGHT. AND THE SAME THING HAPPENS WHEN WE
BRING TWO NEGATIVELY CHARGED RODS TOGETHER. WHEN
YOU RUBBED THE BALLOON ON YOUR HEAD, ELECTRONS
MOVED FROM YOUR HAIR TO THE
BALLOON. SO WHY DID YOUR HAIR STICK UP?
JULIEN: BECAUSE IT WAS LEFT WITH A POSITIVE CHARGE.
CHARGES THAT ARE THE SAME REPEL, SO MY HAIR WAS REPELLED BY OTHER HAIRS.
DANA: AND WHY DID THE BALLOON STICK TO THE WALL?
JULIEN: THE BALLOON HAD A NEGATIVE CHARGE. OPPOSITE
CHARGES ATTRACT, SO THE NEGATIVE CHARGES IN THE BALLOON WERE ATTRACTED TO POSITIVE CHARGES IN THE WALL.
DANA: GOOD! LET’S SEE IF WATER REACTS TO ELECTRICITY.
HERE’S A GLASS ROD AND AN EBONITE ROD. I’LL GIVE THE
GLASS ROD A POSITIVE CHARGE BY RUBBING IT WITH THIS SILK.
YOU GIVE THE EBONITE ROD A NEGATIVE CHARGE WITH THIS
FUR.
JULIEN: OKAY.
DANA: THEN WE’LL RUN A THIN STREAM OF WATER FROM
THE TAP, HOLD THE POSITIVELY CHARGED ROD IN CLOSE AND
WATCH WHAT HAPPENS.
JULIEN: THE WATER’S ATTRACTED TO IT.
DANA: RIGHT. NOW YOU TRY IT WITH THE NEGATIVELY
CHARGED ROD.
JULIEN: IT MOVES TO THE NEGATIVE ROD, TOO. SO WATER IS
ATTRACTED TO BOTH NEGATIVE AND POSITIVE CHARGES.
WHY IS THAT?
DANA: WATER IS ATTRACTED TO BOTH CHARGES BECAUSE OF
THE STRUCTURE OF THE WATER MOLECULE. WE KNOW THAT
A MOLECULE IS MADE UP OF ATOMS — IN THIS CASE, TWO
ATOMS OF HYDROGEN AND ONE
ATOM OF OXYGEN. BUT HOW DO ATOMS GET ELECTRICAL
CHARGES? WELL, WHAT ARE ATOMS MADE OF?
DARREN: FOR THOUSANDS OF YEARS, SCIENTISTS HAVE STUDIED THE NATURE OF MATTER. A PHILOSOPHER OF ANCIENT
GREECE, DEMOCRITUS FIRST PROPOSED THAT ALL MATTER
WAS MADE UP OF TINY PIECES HE CALLED “ATOMS”. TODAY,
MORE THAN TWENTY CENTURIES LATER, WE KNOW THAT
18
DEMOCRITUS WAS ON TO SOMETHING.
STEPHANIE: BEFORE THE SEVENTEENTH CENTURY, ALCHEMISTS STUDIED MATTER, EXPLORING HOW IT CAN CHANGE
STATES, FROM SOLID TO LIQUID TO GAS. ALCHEMISTS TRIED
TO CHANGE METALS, LIKE IRON, INTO GOLD. THEY NEVER
QUITE MANAGED IT, BUT THEY STILL MADE SOME INTERESTING DISCOVERIES ABOUT MATTER.
DARREN: OTHER SCIENTISTS TOOK THIS FURTHER. THEY TRIED
TO UNDERSTAND WHAT MATTER LOOKED LIKE AT THE
ATOMIC LEVEL. THEY DEVISED MODELS OF THE ATOM THAT
CHANGED AS NEW EVIDENCE WAS GATHERED AND NEW
THEORIES EVOLVED.
JULIEN: JOHN DALTON, AN ENGLISH CHEMIST AND PHYSICIST,
SAID AN ATOM CANNOT BE DIVIDED, CREATED, OR DESTROYED. HE CAME UP WITH THE MODEL OF THE ATOM THAT
WAS A PERFECT SPHERE. AND THAT’S WHY IT’S CALLED THE
“BILLIARD BALL” MODEL.
DARREN: ANOTHER ENGLISH PHYSICIST, J. J. THOMSON,
ELABORATED ON DALTON’S THEORY. EXPERIMENTING WITH
CATHODE-RAY TUBES, THOMSON DISCOVERED THAT CATHODE RAYS WERE NEGATIVELY CHARGED PARTICLES THAT WE
NOW CALL ELECTRONS. THOMSON BELIEVED THAT ALL ATOMS
CONTAINED ELECTRONS THAT FLOWED THROUGH A POSITIVE
“ATMOSPHERE”, LIKE PLUMS IN A PLUM PUDDING. FOR THIS
REASON, HIS VISION OF THE ATOM IS KNOWN AS THE “PLUM
PUDDING” MODEL.
STEPHANIE: ERNEST RUTHERFORD WAS ONE OF THOMSON’S
STUDENTS. RUTHERFORD EXPERIMENTED WITH RADIOACTIVE
ELEMENTS. HE KNEW THAT UNSTABLE ATOMS COME APART
OR DECAY, RELEASING RADIANT ENERGY AS THEY BREAK
DOWN.
DARREN: RUTHERFORD SET OUT TO VERIFY THOMSON’S
MODEL OF THE ATOM. HE BOMBARDED THE ATOM WITH POSITIVELY CHARGED PARTICLES. TO HIS SURPRISE, THESE PARTICLES DIDN’T PENETRATE IN SOME AREAS. RUTHERFORD DISCOVERED THAT MOST OF THE MASS OF THE ATOM IS CONCENTRATED IN A POSITIVELY CHARGED NUCLEUS. HE PROPOSED
THAT ELECTRONS ORBIT THE NUCLEUS LIKE PLANETS
19
AROUND THE SUN. THE QUESTION WAS, WHAT KEPT THOSE
ELECTRONS IN ORBIT?
JULIEN: A DANISH PHYSICIST, NIELS BOHR, CAME UP WITH AN
EXPLANATION AND REVISED THE MODEL. IN 1913, WHEN HE
WAS JUST 28, BOHR SUGGESTED A MODEL OF THE ATOM, WHICH
WE STILL USE PARTS OF TODAY.
DARREN: BOHR’S MODEL OF THE ATOM ALSO HAD A DENSE,
POSITIVELY CHARGED NUCLEUS IN THE CENTER, LIKE THE
SUN, WITH ELECTRONS CIRCLING IT. BOHR PROPOSED THAT
ELECTRONS MOVE IN CIRCULAR PATHWAYS AT FIXED DISTANCES FROM THE NUCLEUS. THE DIFFERENT PATHWAYS REPRESENT DIFFERENT ENERGY LEVELS. HE SAID ELECTRONS
HAVE FIXED AMOUNTS OF ENERGY. ELECTRONS WITH LOW
ENERGY ORBIT CLOSE TO THE NUCLEUS. AND ELECTRONS
WITH HIGH ENERGY ORBIT FARTHER AWAY. WHEN WE LOOK
AT ELECTRONS AND THEIR ENERGY LEVELS, IT HELPS US UNDERSTAND HOW ATOMS CAN “SHARE” ELECTRONS — JOINING
TOGETHER TO FORM MOLECULES, LIKE THIS WATER
MOLECULE!
JULIEN: IN 1922, NIELS BOHR WON THE NOBEL PRIZE FOR HIS
ATOMIC MODEL.
DANA: USING BOHR’S MODEL OF THE ATOM, WE’RE GOING TO
LOOK AT HOW ATOMS JOIN TOGETHER TO FORM A MOLECULE.
BUT FIRST, THERE’S A RULE WE NEED TO KEEP IN MIND. ATOMS TEND TOWARD STABILITY — IN OTHER WORDS, THEY
WANT TO BE STABLE. SO THEY TRY TO CREATE EITHER FULL OR
EMPTY ENERGY LEVELS. IF THE OUTER LEVEL OF ONE ATOM IS
ALMOST FULL, IT TRIES TO GAIN AN ELECTRON FROM ANOTHER ATOM.
JULIEN: AND IF THE OUTER LEVEL IS ALMOST EMPTY, IT WILL
TRY TO LOSE AN ELECTRON TO ANOTHER ATOM.
DANA: GOOD. THESE OUTER ELECTRONS ARE CALLED “VALENCE ELECTRONS.”
JULIEN: VALENCE ELECTRONS. GOT IT!
DANA: IN SOME CASES, WHEN TWO ATOMS INTERACT, AN
ELECTRON FROM EACH BECOMES ATTRACTED TO THE OTHER
ATOM. THESE ELECTRONS MOVE INTO A CENTRAL AREA BECAUSE THEY’RE ATTRACTED TO BOTH NUCLEI. BOTH ATOMS
20
SHARE THESE VALENCE ELECTRONS, FORMING WHAT’S
CALLED A “COVALENT BOND.” THE TWO NUCLEI ARE CONNECTED BY THE ELECTRONS THEY SHARE.
JULIEN: AND THE ATOM IS NOW MORE STABLE?
DANA: YES. SUBSTANCES THAT FORM THIS WAY ARE CALLED
“MOLECULAR COMPOUNDS.” AND YOU KNOW ONE COMMON
EXAMPLE.
JULIEN: WATER. THE HYDROGEN AND OXYGEN ATOMS SHARE
VALENCE ELECTRONS.
DANA: RIGHT, TO FORM A COVALENT BOND. HYDROGEN ATOMS HAVE ONE ELECTRON IN THEIR OUTER LAYER, BUT NEED
EITHER TWO OR NONE TO BE STABLE. THE OXYGEN ATOM HAS
SIX ELECTRONS IN THE OUTER LEVEL, BUT NEEDS A TOTAL OF
EIGHT TO BE STABLE. SO EACH HYDROGEN ATOM SHARES ONE
ELECTRON WITH THE OXYGEN ATOM. THE RESULT IS A MORE
STABLE ARRANGEMENT — THE WATER MOLECULE. THE
SHARED ELECTRONS MAKE WATER A “POLAR MOLECULE,”
BECAUSE THERE’S A NEGATIVELY CHARGED SIDE AND A POSITIVELY CHARGED SIDE.
JULIEN: WHY DOES EACH SIDE OF THE WATER MOLECULE
HAVE A DIFFERENT CHARGE?
DANA: THERE ARE THE SAME NUMBER OF POSITIVE AND
NEGATIVE CHARGES IN THE WATER MOLECULE, BUT YOU CAN
SEE THAT THE SHAPE ISN’T SYMMETRICAL.
JULIEN: SO THE HYDROGEN ATOMS ARE OVER HERE, AND THE
OXYGEN ATOM IS OVER HERE.
DANA: RIGHT. SO THE WATER MOLECULE HAS A POSITIVE
CHARGE OVER HERE ON THE HYDROGEN SIDE, AND A NEGATIVE CHARGE OVER HERE ON THE OXYGEN SIDE.
JULIEN: BECAUSE THE ELECTRONS AREN’T SHARED EQUALLY.
DANA: THAT’S IT. THE ELECTRONS, WHICH ARE NEGATIVE,
SPEND MORE TIME CIRCLING AROUND THE OXYGEN ATOM.
THIS LEAVES A SURPLUS OF POSITIVE CHARGE AROUND THE
TWO HYDROGEN ATOMS. SO THE WATER MOLECULE HAS ONE
SIDE WITH A SLIGHT NEGATIVE CHARGE, AND ONE SIDE WITH
A SLIGHT POSITIVE CHARGE. NOW CAN YOU FIGURE OUT WHY
WATER WAS ATTRACTED TO BOTH OF OUR CHARGED RODS?
JULIEN: BECAUSE WHEN YOU HOLD A POSITIVELY CHARGED
21
ROD CLOSE TO A STREAM OF WATER, THE NEGATIVE SIDES OF
THE WATER MOLECULES — THE OXYGEN SIDES — ARE ATTRACTED TO IT.
DANA: AND WHY WAS THE WATER ATTRACTED TO THE NEGATIVELY CHARGED ROD?
JULIEN: WHEN YOU HOLD A NEGATIVELY CHARGED ROD
CLOSE TO THE WATER, THE POSITIVELY CHARGED SIDES — THE
HYDROGEN ATOMS — ARE ATTRACTED TO IT.
DANA: RIGHT! YOU’VE SEEN A FLOATING PAPER CLIP. THE
ASYMMETRICAL STRUCTURE OF THE WATER MOLECULE
MEANS THAT THERE ARE STRONG ATTRACTIVE OR COHESIVE
FORCES BETWEEN WATER MOLECULES. EACH WATER MOLECULE IS HELD TIGHTLY IN PLACE BY THE POSITIVE AND NEGATIVE SIDES OF SURROUNDING WATER MOLECULES. THESE
STRONG ATTRACTIVE FORCES ARE GIVEN A SPECIAL NAME —
“HYDROGEN BONDS.” AND THESE HYDROGEN BONDS ARE
THE KEY TO WATER’S UNUSUAL CHARACTERISTICS.
JULIEN: ARE THEY LIKE THE BONDS BETWEEN THE HYDROGEN AND OXYGEN ATOMS WHICH HOLD THE WATER MOLECULE TOGETHER?
DANA: NOT QUITE. THEY’RE AN ATTRACTIVE FORCE BETWEEN
WATER MOLECULES.
JULIEN: COHESION?
DANA: THAT’S IT. SO THIS IS WHAT MAKES WATER SPECIAL:
THE ASYMMETRICAL STRUCTURE OF THE WATER MOLECULE
MEANS THAT IT’S NEGATIVELY CHARGED OVER HERE BY THE
LARGER OXYGEN ATOM, AND POSITIVELY CHARGED OVER
HERE BY THE SMALLER HYDROGEN ATOMS.
JULIEN: THIS SHAPE ALLOWS FOR STRONG ATTRACTIVE
FORCES BETWEEN THE MOLECULES, OR HYDROGEN BONDS,
HOLDING WATER MOLECULES TOGETHER.
DANA: IT TAKES A HUGE AMOUNT OF ENERGY TO OVERCOME
THE FORCE OF THESE HYDROGEN BONDS. THAT’S WHY WATER HAS “HIGH” FREEZING AND BOILING POINTS, “HIGH” SURFACE TENSION, A “HIGH” SPECIFIC HEAT CAPACITY. IT ALSO
EXPLAINS WHY ICE FLOATS ON TOP OF LIQUID WATER. THAT’S
THE DENSITY ANOMALY. SO ALL THE SPECIAL CHARACTERISTICS OF WATER RELATE TO THE SHAPE AND POLAR NATURE
22
OF THIS MOLECULE.
SARAH: NORMALLY, WHEN YOU FREEZE SOMETHING, IT BECOMES MORE DENSE...IT CONTRACTS...TAKES UP LESS SPACE.
FOR EXAMPLE, THIS LIQUID COOKING FAT IS LESS DENSE THAN
THE FROZEN COOKING FAT. SEE THE LITTLE CONCAVE DIP IN
THE JAR? SO WHY IS WATER DIFFERENT? IT EXPANDS WHEN IT
FREEZES. HERE’S THE INSIDE STORY. AS WATER BEGINS TO
COOL, THE MOLECULES SLOW DOWN AND MOVE CLOSER TOGETHER — IT CONTRACTS. THEN, THE POLAR NATURE IN EVERY WATER MOLECULE TAKES OVER. LIKE CHARGES REPEL,
CREATING SPACES THAT OTHER WATER MOLECULES CAN’T
ENTER BECAUSE OF THE ELECTRIC REPULSION. DURING
FREEZING, WATER MOLECULES ARRANGE THEMSELVES TO
FORM THIS HEXAGONAL LATTICE. SO FROZEN WATER TAKES
UP MORE SPACE THAN LIQUID WATER.
DARREN: THROUGHOUT THE WINTER, WATER FREEZES AND
THAWS, EXPANDING AND CONTRACTING. THE REST OF THE
YEAR, ROAD CREWS ARE BUSY REPAIRING THE DAMAGE
CAUSED BY ICE. THIS IS THE SHAPE OF WATER. TWO HYDROGEN ATOMS AND AN OXYGEN ATOM, ARRANGED AT AN ANGLE
OF 104.45 DEGREES, TO FORM A WATER MOLECULE.
STEPHANIE: AND THIS IS THE SHAPE OF WATER...AN EXTRAORDINARY AND BEAUTIFUL SUBSTANCE THAT KEEPS US ALIVE.
WE KNOW WHAT IT LOOKS LIKE IN THE OCEAN, OR IN A
BATHTUB.\E BUT THE MOLECULAR STRUCTURE MAKES WATER SPECIAL.
DARREN: TWO HYDROGEN ATOMS EACH SHARE ONE ELECTRON WITH A CENTRAL OXYGEN ATOM. THE OXYGEN SIDE OF
THE MOLECULE IS SLIGHTLY
NEGATIVE, AND THE HYDROGEN SIDE IS SLIGHTLY POSITIVE.
THIS GIVES WATER ITS POLAR NATURE.
STEPHANIE: BECAUSE WATER IS A POLAR MOLECULE, IT’S ATTRACTED TO BOTH POSITIVE AND NEGATIVE CHARGES.
DARREN: IT’S ALSO ATTRACTED TO THE POSITIVE AND NEGATIVE SIDES OF NEIGHBORING WATER MOLECULES.
STEPHANIE: THIS FORCE BETWEEN NEIGHBORING MOLECULES
GIVES WATER ITS HIGH SURFACE TENSION.
DARREN: THE STRONG FORCE BETWEEN NEIGHBORING MOL23
ECULES MEANS THAT IT TAKES A LOT OF ENERGY TO MAKE
MOLECULES MOVE FASTER.
STEPHANIE: WHICH EXPLAINS WHY WATER HAS SUCH A HIGH
SPECIFIC HEAT CAPACITY.
DARREN: THE ATTRACTIVE FORCES BETWEEN WATER MOLECULES CAUSE WATER TO EXPAND AS IT FREEZES. LIKE
CHARGES REPEL IN THE CRYSTAL, SO THAT INSTEAD OF BECOMING MORE DENSE AS IT FREEZES, WATER BECOMES LESS
DENSE.
STEPHANIE: AND THIS MAKES IT POSSIBLE FOR ICE TO FLOAT
ON LIQUID WATER. AND THAT’S VERY UNUSUAL!
DARREN: THE ATTRACTIVE FORCES BETWEEN MOLECULES,
THE “HYDROGEN BONDS,” GIVE WATER ITS EXTRAORDINARY
CHARACTERISTICS.
JULIEN: WHAT ARE THESE, CINDY?
CINDY PASZKOWSKI: WELL, THIS LITTLE GUY IS A WOOD FROG.
THIS IS THE MOST NORTHERLY SPECIES OF AMPHIBIAN OR REPTILE IN NORTH AMERICA, AND A VERY COMMON SPECIES IN
ALBERTA.
JULIEN: I’VE SEEN THEM WHEN I’M HIKING BEFORE.
CINDY PASZKOWSKI: YEAH. THEY SPEND A LOT OF TIME IN
THE WOODS. AND, IN FACT, THEY EVEN HIBERNATE IN THE
WOODS. MANY OTHER AMPHIBIANS GO INTO THE SOIL AND
DIG DEEP HOLES IN THE SOIL IN ORDER TO OVERWINTER.
OTHER SPECIES, LIKE THIS SALAMANDER, MIGHT ACTUALLY
HIBERNATE AT THE BOTTOM OF PONDS. BUT THE WOOD FROG
ACTUALLY HIBERNATES ON THE FOREST FLOOR, JUST IN THE
LEAF LITTER.
JULIEN: DON’T THEY FREEZE?
CINDY PASZKOWSKI: WELL, ACTUALLY, SOME OF THE WATER
IN THE FROG’S BODY WOULD FREEZE, BUT IT’S THE WATER
OUTSIDE THE CELLS. BECAUSE WHEN THE FROGS ARE EXPOSED
TO FREEZING TEMPERATURES IN THE FALL, WHAT HAPPENS
IS THEY RAPIDLY ACCUMULATE THE SUGAR, GLUCOSE, IN
THEIR CELLS. AND THEN THE GLUCOSE PREVENTS THE WATER MOLECULES FROM FORMING HYDROGEN BONDS AND
KEEPS THEM FROM FREEZING. SO THE SUGAR SERVES AS A
NATURAL ANTIFREEZE.
24
DANA: WHEN YOU LOOK AT A SCENE LIKE THIS — WATER,
SMOOTH AS GLASS — REMEMBER WHAT YOU CAN’T SEE. BILLIONS OF MOLECULES MOVING AROUND, ELECTRONS ORBITING THE NUCLEUS, NEGATIVE AND POSITIVE FORCES PULLING
AT EACH OTHER, MOLECULES BREAKING FREE AND ESCAPING
INTO THE AIR. MAYBE IT’S JUST AS WELL WE CAN’T SEE IT. IN
EXTREME CLOSE-UP, OR SEEN FROM A DISTANCE, WATER’S
UNIQUE QUALITIES RESULT FROM THE SHAPE OF THE WATER
MOLECULE. IT’S SIMPLY SCIENCE!
25
1
Name___________________________________
PRE-TEST
Directions: Circle the letter indicating whether the following statements are either true ("T") or false
("F").
T
F
1.
Ice floating on liquid water is the result of what is known as the density anomaly.
T
F
2.
Insects walking on water and paper clips floating are the result of strong
attractive forces between the water molecules and the molecules of the insect leg
or paper clip.
T
F
3.
Water's freezing and boiling points are similar to other substances with
molecules of similar size.
T
F
4.
The heat of fusion is the amount of thermal energy which needs to be added to
one unit mass of a substance in order to change it from solid to liquid.
T
F
5.
In the equation Q = mc∆t the term "c" is the specific heat capacity of the mass
"m.”
T
F
6.
A 50 g mass of steel and 50 g of water left in the sun would be at the same
temperature after an hour.
T
F
7.
Water is a compound composed of two parts hydrogen and one part oxygen.
T
F
8.
All matter is made up of indivisible particles called atoms.
T
F
9.
Atoms contain positively and negatively charged particles.
T
F
10.
A molecule is a group of atoms bonded chemically by sharing electrons.
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GLOSSARY
Covalent bond – a chemical bond between atoms that results when those atoms share one or more
electrons
Electrolysis – a process in which an electric current is passed through a solution of an electrolyte, causing a chemical reaction to occur
Energy levels – distances from the nucleus of an atom at which electrons may be found
Hydrogen bond – relatively strong forces acting between molecules; hydrogen bonds result because of
the highly polar nature of some compounds containing hydrogen
Molecular compound – a compound in which the components are bonded covalently; they are composed of non-metal elements sharing electrons
Nucleus
1. biological – the organelle which acts as the control centre for the cell
2. chemical – the dense, positively charged, center of the atom, containing protons
and neutrons
Polar molecule – a molecule with one slightly positive and one slightly negative end, due to uneven
charge distribution, e.g., the water molecule
Valence electrons – electrons in the energy level farthest from the nucleus of an atom
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WHAT’S WATER MADE OF?
Electrolysis is the process of decomposing a substance by passing an electric current through it. Because pure water is not a good conductor, salt is added to improve the flow of electricity.
After decomposing the water and collecting the gases, we can see there is twice as much of one gas as
the other. Amadeo Avogadro (1776 - 1856) developed the theory that equal quantities of gases – e.g.,
oxygen, hydrogen, neon – occupy the same volume provided they are at the same temperature and
pressure. So our observation about volume allows us to make inferences about the number of molecules.
The tube with 20 mL of gas has twice as many molecules as the tube containing 10 mL of gas. The
chemical formula for water, H2O(l), tells us to expect two atoms of hydrogen for every atom of oxygen.
Based on this, we can predict that we have collected 20 mL of hydrogen gas and 10 mL of oxygen gas.
reservoir
water
oxygen
hydrogen
buret
electrodes
retort stand
power supply
insulated copper wires
Check your understanding of this segment by completing the following. Use the back of the sheet if
necessary.
1. Describe the burning splint test for hydrogen gas. What indicates the presence of hydrogen?
2. Describe the glowing splint test for oxygen gas. What indicates the presence of oxygen?
3. The chemical formula for ammonia is NH3(1). If ammonia undergoes electrolysis and 15 mL of nitrogen gas are collected, what volume of hydrogen gas would be produced?
4. Inflate a balloon and rub it against your hair or clothes. Hold it near a thin stream of water. Record
your observations.
5. Does the activity from question 4 allow you to decide if the balloon is positively or negatively charged?
Explain your answer.
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4
Name___________________________________
THE ATOMIC MODEL
Copy the chart below into your notes. Review the chart so you know what you need to record while
watching the video, “Water’s Structure.” Record the description of the atomic model proposed by
each of these scientists.
Scientist
Model
Dalton
Billiard Ball
Thomson
Plum Pudding
Rutherford
Bohr
Description
•
•
All matter is made of atoms
The atom is solid and indivisible
Nuclear
Energy Level or
Bohr Model
Check your understanding of this segment by completing the following. Use the back of the sheet if
necessary.
6. What evidence led Thomson to introduce the Plum Pudding model?
7. Most of the alpha particles Rutherford shot at a thin gold foil went straight through. What led him to
propose that most matter is located in a very small part of the atom called the nucleus?
8. What is the major difference between Rutherford’s and Bohr’s model of the atom?
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5a
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ATOMS SHARING ELECTRONS
One way for two atoms to bond is by sharing electrons. This is called a covalent bond. The Bohr
model helps us to explain how these bonds form, and allows us to predict how many of these bonds
will form for a specific atom.
Three Principles
•Atoms tend toward stability.
•Atoms can share electrons to fill their outer energy level.
•Atoms are most stable when their outer energy level is filled (or empty).
Energy Levels
We count the energy levels in the Bohr model of the atom starting at the level closest to the nucleus.
The outer energy level, which contains electrons, is called the valence energy level.
Nearest the nucleus
is the first energy
level – a maximum of
two electrons can
occupy this level.
-
-
-
-
-
-
-
-
-
-
Ar
-
-
Next are the second
and third energy levels
– a maximum of eight
electrons can occupy
these energy levels.
-
Example
1.
-
-
-
-
Show how oxygen and hydrogen can both have full valence energy levels when water forms.
oxygen
atom
-
8p+
-
1p+
-
-
hydrogen
atom
The oxygen atom has six electrons in its valence energy level and requires two more to fill it.
Hydrogen has a single electron, and requires one more to fill the first energy level.
By sharing one electron with an oxygen
atom, a hydrogen atom
completes its valence
energy level.
Oxygen still requires
another electron to
fill its energy level.
© 1997 Alberta Education
-
-
-
-
8p+
-
-
-
- -
A second hydrogen
shares its electron.
Now, all three
atoms have filled
valence energy
levels.
-
1p+
1p+
-
-
-
-
8p+
-
-
- 1p+
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ATOMS SHARING ELECTRONS
Polar Molecules
Some covalently bonded molecules have sides which are slightly positive and slightly negative. This
can occur as a result of the distribution of electrons. The atoms share electrons, but they don’t share
them equally. We call molecules that have these slightly charged sides polar molecules. Water is a
polar molecule.
Positive Side
The hydrogen atoms are
sharing electrons with the
oxygen. That leaves their
positive nucleus exposed.
This side of water is
slightly positive.
Negative Side
H
O
Both hydrogen atoms are
on one side of the oxygen
atom. That makes this side of
the water molecule slightly
negative.
H
A glass full of water is a glass full of identical molecules. Each one of them is polar, so they interact
with one another. The positive side of one water molecule attracts the negative side of its neighbours.
This force of attraction is called a hydrogen bond. The hydrogen bond is responsible for many of
water’s unusual physical properties.
Why does it take so much energy to heat water? Because the forces between water molecules make it
difficult for them to move – and molecular motion is what we measure as temperature. Why does
water have a high heat of vaporization? Because the forces between the molecules must be overcome
before liquid water can change to gas. These attractive forces, hydrogen bonds, help explain water’s
other unusual properties as well.
Check your understanding of this segment by completing the following. Use the back of the sheet if
necessary.
9. The shape of a hydrogen sulfide molecule, H2S(g), a component of sour gas,
can be illustrated by its Bohr-model diagram. From this molecular shape
would you predict the hydrogen sulfide molecule to be a polar molecule?
10. Given the Bohr-model diagram for methane (CH4(g)), the major component
of natural gas, compare it to that of water. Predict whether methane molecules
are bonded to one another by hydrogen bonding.
11. Compare and contrast covalent bonds with hydrogen bonds.
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6
THE SHAPE OF WATER
Review the meaning of “polar molecule” and “hydrogen bonding” on Blackline Master #2: Glossary.
The bonding between neighboring molecules is key to understanding the unusual properties of water.
If we compare two substances made up of molecules that are similar in mass, but one undergoes hydrogen bonding and the other does not, we see the effects of the hydrogen bonds.
Water (H20)
shape of the molecule
*
Methane (CH4)
v-shaped
symmetric
relative mass*
18
16
hydrogen bonding
yes
no
freezing point
0°C
-183°C
boiling point
100°C
-162°C
Relative mass compares the mass of equal numbers of molecules. For example, if a sample of
water had a mass of 18 grams, then an equal number of methane molecules would have a mass of
16 grams.
Consider the data shown above. Water, which has unusually high freezing and boiling point temperatures, has hydrogen bonding. Methane, whose molecules have a similar mass to that of water,
has no hydrogen bonding and very low freezing and boiling points.
Check your understanding of this segment by completing the following. Use the back of the sheet if
necessary.
12. Predict how the shape of the water molecule contributes to hydrogen bonding.
13. Water forms a crystal lattice when it freezes. What is it about this lattice that makes ice less dense
than liquid water?
14. Write a hypothesis for this question: Does methane produce a crystal lattice when it freezes?
15. Dr. Paszkowski described how, in wood frogs, glucose stops ice from forming in their cells during
the winter. How does the glucose do this?
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7a
Name___________________________________
POST-TEST
MULTIPLE CHOICE
Directions: Decide which of the choices best completes the statement or answers the question, then
circle the letter that corresponds to your choice. (3 marks each)
1.
The water molecule is
a.
b.
c.
d.
2.
In the modern model of the atom the valence electrons are
a.
b.
c.
d.
3.
by
Bohr
Dalton
Thomson
Democritus
J.J. Thomson is credited with the identification of the
a.
b.
c.
d.
5.
furthest from the nucleus
may be shared with other atoms
may be transferred to other atoms
all of the above
The suggestion that all matter was composed of tiny particles called atoms was first proposed
a.
b.
c.
d.
4.
ionic
polar
diatomic
symmetric
electron
nucleus
proton
atom
Strong cohesive forces between water molecules are due to
a.
b.
c.
d.
ionic bonds
covalent bonds
hydrogen bonds
polar covalent bonds
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7b
Name___________________________________
POST-TEST
LONG ANSWER
Directions: Answer the following questions in the spaces provided. Use the back of the sheet if
necessary.
1.
The results of the Hoffman apparatus demonstration indicate that _____________________ change
took place. (3 marks)
2.
What evidence in the Hoffman apparatus demonstration indicates the ratio of water's components? (6 marks)
3.
Describe the tests used to identify the gases collected using the Hoffman apparatus. Which
properties of oxygen and hydrogen make these tests valid? (8marks)
4.
In the electrolysis of water electric current provides the energy required to ______________ the
bonds in the water molecule. (3 marks)
5.
A balloon acquired an electrostatic charge by being rubbed on a head of hair. Describe a procedure which would allow you to determine whether the charge on the balloon is positive or negative. (6
marks)
6.
The "Plum Pudding" of the atom, which held that most of the atom was made up of positive
matter and negative particles were distributed throughout, was proposed by
______________________________________. (3 marks)
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7c
Name___________________________________
POST-TEST
7.
The most prominent features of the Rutherford model of the atom are: (6 marks)
_________________________________________________________________ and
_________________________________________________________________.
8.
Describe the changes Niels Bohr suggested for Rutherford's model of the atom. (6 marks)
9.
Modern atomic theory holds that atoms are most stable when their valence level contains
___________________________________ electrons. (3 marks)
10.
Covalent bonds are formed when two or more atoms _____________________________. (3 marks)
11.
Most substances contract when they solidify. Water expands when it freezes. Explain this anomalous behavior. (6 marks)
12.
A fine stream of water from a tap is attracted to an electrically charged rod regardless of the sign
of the charge (negative or positive). This is evidence that water molecules
________________________________________________________________. (3 marks)
13.
Describe the shape the water molecule is considered to have, and how this influences the properties of water. (8 marks)
14.
Molecular compounds are formed when _________________________________________
______________________________________________________________. (5 marks)
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7d
POST-TEST
15.
Compare covalent bonds and hydrogen bonds as they apply to water. (6 marks)
16.
Wood frogs hibernate under the snow on the forest floor during the winter. Describe the mechanism which prevents the water in the frog's cells from freezing. (6 marks)
17.
Sketch a Bohr-model diagram of a water molecule. (4 marks)
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