LEARNING OBJECT
How can we count atoms and molecules?
Chemistry 10th grade
LEARNING UNIT
What is everything around
us made of?
S/K
SKILL 1: Establishes quantitative relationships between
the mass of a chemical compound and the number of
atoms, molecules, valence electrons, chemical bonds,
etc.
SKILL 2: Investigates and speaks about how Avogadro’s
number was determined.
SKILL 3: Explains the effect of natural abundance on the
molar mass of an element.
SKILL 4: Explains the reasons why the molar mass in
the periodic table is used to calculate the molar mass
of an ion.
SKILL 5: Investigates in which everyday activities mass
measurement is used to count units of homologous
objects.
Language
English
Socio cultural context of
the LO
Curricular axis
Colombia
Standard competencies
Background Knowledge
English Review topic
Vocabulary box
I can relate the structure of organic and inorganic
molecules to their physical and chemical properties and
their ability for chemical change
1. Explain the structure of atoms based on different
theories;
2. Explain nuclear energy generation based on the
alteration of the atomic structure;
3. Explain the relation between atomic structure and
atomic connections.
Particles and subatomic particles, Atomic theory, Atomic
structure, Atomic mass unit (amu), Molecule, Elements
and Periodic Table, Mass unit, Atomic mass, Molar mass,
Isotopes, Chemical bonds, Avogadro’s number,
Electronegativity, Lewis structure, Electronic
distribution.
How much/how many
Abound (verb): to exist in large numbers.
Compound (noun): a mixture of two or more different
parts or elements.
Explode (verb): to burst violently and usually with a
loud noise, or to cause this to happen.
Homologous (adjective): similar in position, structure
or purpose.
Percentage (noun): an amount of something, often
expressed as a number out of 100.
Weight (noun): a quality of an object that is a measure
of the force by which the earth attracts it, or an object
considered as having this quality.
Source:
http://dictionary.cambridge.org/us/dictionary/english/
Subscript (noun): A subscript or superscript is a
number, figure, symbol, or indicator that is smaller than
their normal line of type and is set slightly below or above
it.
Source:
https://en.wikipedia.org/wiki/Subscript_and_superscript
NAME:
GRADE:
INTRODUCTION:
All matter is made up of atoms, which are tiny particles that cannot be seen with
the naked eye, but that form elements when they join. Elements join to make
compounds, and these join until they form mass.
So, how many atoms are things really made of?
Jean Perrin asked himself this same question many years ago. Through
experiments, he determined that everything has the same number of molecules
in a mole of substance. He called this number Avogadro’s number.
Avogadro’s number is equal to 6.023*1023, a number so big, that it could be
compared to the number of cups of water needed to empty the ocean, or to the
number of carefully piled up soda cans needed to cover the Earth, until they
reach a height of 320 km…
Now…do you dare to count the molecules in your pencil?
1.
2.
3.
4.
OBJECTIVES
To analyze the relationship between the mass and the amount of matter in a
chemical compound.
To identify the quantitative relationships among atoms, molecules, valence
electrons, ions, and chemical bonds, and how they allow the formation of
compounds.
To explain how Avogadro’s number was determined to calculate the number of
atoms.
To acknowledge the chemical explanations of mass and matter that elucidate
our surroundings.
Activity 1 and 2:
Introduction
Skills:
- Establishes quantitative relationships between the mass of a chemical
compound and the number of atoms, molecules, valence electrons, chemical
bonds, etc.
- Investigates and speaks about how Avogadro’s number was determined.
- Explains the effect of natural abundance on the molar mass of an element.
- Explains the reasons why the molar mass in the periodic table is used to
calculate the molar mass of an ion.
The atom is defined as the basic unit of an element, which can generate a
chemical combination. It has an internal structure made up of even smaller
particles, called subatomic particles, which are the electrons, protons and
neutrons.
1. Electrons (e-) have negative charge and they move in orbit around the
nucleus, from a certain distance.
2. Protons (p+) have positive charge and they are in the nucleus.
3. Neutrons (n) have no charge, are neutral, and are also in the nucleus.
Image
adapted
http://adictamente.blogspot.com.co/2015_02_27_archive.html
from:
The number of protons in an atom is the atomic number, represented by Z. In
neutral atoms, the number of protons is equal to the number of electrons, so Z
represents the number of electrons too.
The mass number, represented by A, is the sum of the protons and neutrons
present in the nucleus of an element’s atom.
This is represented as follows:
A = Mass number
A = Neutrons + Protons
Z = Atomic number
Z = Protons
A
X = Element’s symbol
Z
Image
adapted
from:
http://educativa.catedu.es/44700165/aula/archivos/repositorio/1000/1162/html/22_n
mero_atmico_y_nmero_msico_istopos.html
For sodium, for example:
A = Mass number
A = Neutrons + Protons
A = 12 + 11
Z = Atomic number
Z = Protons
Z = 11
23
Na = Sodyum’s symbol
11
Image adapted from http://www.eis.uva.es/~qgintro/atom/tutorial-05.html
Some atoms of a given element do not have the same mass. Atoms that have
the same atomic number but a different mass number are known as isotopes.
For example, Carbon has three isotopes, remaining the same element, as is
shown in the following figure.
Carbon
Carbon
Carbon
Image modified from http://iiquimica.blogspot.com.co/2006/03/trminos-enteora-atmica.html
The frequency of production of the isotopes of each element in nature, called
natural abundance, can be mathematically calculated and is expressed in
percentages. The following is the example of the element Silver, which has two
isotopes.
Silver
Natural abundance = 56%
Silver
Natural abundance = 44%
Image modified from http://www.eis.uva.es/~qgintro/atom/tutorial-05.html
his percentage does not appear on the periodic table, but it is necessary to
calculate the unit of atomic weight, known as “amu”, of each element. The amu
is calculated from the sum of all the isotopes there are of an element, as follows.
We will continue working with the element Silver (Ag). An information table:
=∑
atomic mass of isotope 1
% natural abundance 1
+
atomic mass of isotope 1
% natural abundance 2
100
100
1. How many isotopes of Silver are there? = 2
107
Ag and
109
Ag
2. Is the abundance of each isotope in nature known? = Yes
107
Ag = 56% and
109
Ag = 44%
3. Is the atomic mass of each isotope known? = Yes
For Ag1 = 107 and Ag2 = 109
The question would be: how many atomic mass units (amu) does Silver have?
=∑
atomic mass of isotope 1
% natural abundance 1
100
=∑
107
100
=∑
=∑
107
56 %
+
109
44
100
0,56
59,92
+
+
47,96
= ∑ 107.88 A.M.U.
109
0,44
+
atomic mass of isotope 1
% natural abundance
100
Exercise
taken
and
modified
http://www.eis.uva.es/~qgintro/atom/tutorial-05.html
from
Talking about amu is the same as talking about molecular weight, but the latter
has units of grams/mole. So, in the case of Silver, 107.8 amu is the same as
107.8 g/mole.This measurement, and Avogadro’s number, which is 6.023*1023
atoms/mole or particles/mole, allow us to calculate the quantity of atoms,
molecules or particles.
6.23 *1023 atoms represent the amount of matter that a mole contains. So, the
mole and Avogadro’s number will always equal the molecular weight of an
element.
Example 1:
How many atoms are there in 25g of Oxygen?
1. What is the molecular weight of oxygen? = 16g/mole or 16 amu
2. How many atoms are there in 16g/mole of oxygen? = 6.023*1023
atoms. *Rememeber that 6.023*1023 atoms is the amount there is in the
molecular weight of each element.
3. These problems are solved through cross multiplication or rule of three:
16 g/mole
25 g
6.023 x 10
X
23
23
(25 g) * (6.023 x 10
X=
16 g/mole
atoms
atoms)
23
= 9.410 * 10
Oxygen atoms
Example 2:
How many atoms are there in 3 moles of Hydrogen?
1. What is the molecular weight of Hydrogen? = 1g/mole or 1 amu
2. How many atoms are there in 1g/mole of Hydrogen? = 6.023*1023
atoms.
*Remember that 6.023*1023 atoms is the quantity there is in the
molecular weight of each element.
3. How many moles are there in 1g/mole of Hydrogen? = 1 mole
*Remember that 1 mole is equal to the molecular weight of
each element.
three.
4. These problems will be solved through cross multiplication or rule of
5. The next steps should be followed:
1. The moles are converted to grams:
1 mole
1 g/mole
3 mole
X
X=
(3mole) * (1g/mole)
= 3gr/Hydrogen atoms
1mole
2. The result is converted to atoms, which is what we need to find out.
1 g/mole
3 g/mole
6.023 x 10
23
atoms
X
23
(3mole) * (6.023 x 10
atoms)
X=
= 18,069 Hidrogen atoms
1mole
When two different elements react, compounds are formed. The molecular
weight of compounds can also be found. To calculate this weight, all you need
to know is the molecular weight of each separate element, which is on the
periodic table. Then, you add the two numbers as follows:
Individual molecular
weight
1 g/mole
Hydrogen
35g/mole
Chlorine
Molecular weight of
the compound
36g/mole
Hydrochloric acid.
The subscripts in the formulas indicate the number of atoms of each element
that are present. So, to determine the atomic mass, it is necessary to multiply
the mass of each element times the subscript, like this:
2 (55g/mole) (16g/mole) 3
Molecular weight of each element
Fe = 55g/mole
Iron
O3 = 16g/mole
Oxygen
Subscript = number of present
atoms of that element
2 (55g/mole) = 110
3 (16g/mole) = 48
110 + 48 = 158 g/mole Fe O
1) Find the molecular weight of each element. In this case, Fe = 55g/mole and
O = 16g/mole.
2) Identify if there are atoms in the formula. In this example, Fe = 2 and O = 3.
3) Multiply the number of atoms of each element times its molecular weight.
4) Add the products of the previous step to know the weight of the compound.
2
3
Exercises: Activity One and Two
1. Carry out the following exercises:
- Avogadro’s number is equal to_
the number of_
or_
, and with it you can calculate
that are in an element.
- How many atoms are there in 1 mole of Lead?
- How many molecules of Na are there in 6 grams of Na?
- What is the mass, expressed in grams, of a Sodium atom?
- How many atoms are there in 145 g of CaCO3?
2. Select the correct answer for the following question:
An amu is the same as…
a) The molecular weight of an element.
b) The quantity of subatomic particles in an atom.
c) The percentage of isotopes of an element
d) The molecular mass of a compound
3. Calculate the molar mass for each case. For this, you must have access to a
periodic table, to see the molar masses. After you find the answers, explain in
one sentence why the molar mass that appears on the periodic table is used to
calculate the molar mass of each ion.
Elements may show isotopes when there are changes in the quantity of the
neutrons they have, but not in the number of protons. These isotopes have
percentages of abundance in nature. With this fraction, the atomic mass units
(amu) of an element are established.
4. Complete the following table:
ISOTOP
E
% OF ABUNDANCE
Z
A
P
+
N
E-
56
Fe
6
54
Fe
92
57
Fe
2
2
6
2
6
2
6
56
5
4
56
3
1
Calculate the atomic mass unit. Do so using the formula of natural abundance.
5. Carry out the following exercise. According to the result you obtain, explain
in maximum two sentences how natural abundance is related to the molar mass
of an element.
The average atomic mass of chromium is 52.003 amu. This element has two
isotopes: one with an atomic mass of 52.323 amu and an abundance of 68.43%.
Determine the atomic mass of the other isotope, considering that its abundance
is 31.7%.
Did you know?
The measuring of radioactive isotopes of arsenic in the hair of the remains of
Napoleon allowed to find out that he had not died by poisoning.
Activity three
The student will investigate and summarize in his/her own words, in a paragraph
of maximum 5 sentences, how mass measurement is used to count units of
homologous objects. He/she shall answer this question based on the following
problems:
1. In a ball factory, there are 1250 balls that need to be packed in boxes that do
not exceed 30 kg. How many balls must be packed in each box, and how many
boxes would be required? 41 balls and 30 boxes.
2. Four cups weigh the same as five glasses. If each cup weighs 0.115 kg, how
much does each glass weigh? 92 g.
Abstract
Matter is composed of molecules, which are made up of elements that are
composed of atoms, the little units that form all the things that exist in the
universe. Atoms contain subatomic particles like electrons (e-), protons (p) and
neutrons (n), which determine the atomic number and the mass number of each
element.
Electrons are the most unstable particles of the atom because of their location.
They move among orbitals that can have contact with other orbitals, and they
are attracted to their own nucleus as well as to others. Neutrons and protons
tend to make the stable structure of the atom. Both of these subatomic particles
are involved in the determination of the mass number, which can vary in
elements that are the same, forming isotopes. Isotopes have the same quantity
of protons, but their neutrons vary.
Isotopes have different percentages of presence on earth. This means that not
all of them abound in the same quantities, which is why measuring the natural
abundance is important to quantify the atomic mass unit of an element. The
atomic mass unit, known as amu, allows us to really know the atomic weight of
an element, since it is equivalent to it.
The amu is also related to the quantity of atoms that an element has; 6.023*1023
atoms, molecules or particles is a number of reference for this quantity. This
measure is related to the mole, which refers to the concentration of matter in
an element.
The quantity of atoms present in a reaction should be taken into account to
calculate the atomic weight of a compound, since each element has an individual
weight and the number of atoms increases the molecular weight of compounds.
Homework
Knowing how scientists and mathematicians arrived to discoveries that became
laws in different sciences is very important. For this reason, the students will
investigate how, when and why Avogadro’s number started being used, taking
into account its relationship with atoms.
Then, in groups of 5, they will discuss the results of their individual investigations
and together they will write a conclusion that will be shared with the rest of the
class. The conclusion will respond to the questions: why is this measure
important for chemistry? And, how did it affect the evolution of this science?
Evaluation
The following 9 questions aim to evaluate the performance of the student in this
learning object:
Mark the right answer:
1. What is the average atomic mass of lithium (Z=3)? Take into account that
two isotopes with mass numbers 6 and 7, and with percentages of 7,6% and
92,4% respectively, are known.
a) 7,92
b) 2.32
c) 5.92
d) 6,94
2. How many molecules are there in a mole of H2O?
a) 14.325*1023
b) 6.023*1022
c) 6,023*1023
d) None of the above
3. Natural copper is formed of isotopes Cu-63 and Cu-65. The most abundant is
the first one, with a natural abundance of 64,4%. Calculate the abundance of
the second one:
a) 63.9%
b) 35,6%
c) 4.8%
d) 25%
4. The atomic masses of lithium -6 and lithium -7 are 6.015 amu and 7.016 amu
respectively. Calculate the abundance of each isotope, taking into account that
the average atomic mass of lithium is 6.94 amu. With the given information, fill
in the following table:
Abundance Subtot
Isotop
Atomic
e
mass
al
Li -6
6.015amu
Li -7
7.016amu
Total
X+Y = 1
6.941
Based on the following text, answer questions 5 to 9:
The energy stored in the nuclei of atoms is called nuclear energy. If the energy
is extracted slowly and in a controlled way from the nuclei, it can be very useful.
This is the operation principle of a nuclear reactor. However, there’s another way
to free the energy from the nucleus, and it is in a very quick and violent way.
There would be so much energy that the process would be a very potent
explosion. The bomb detonated over Hiroshima was about a thousand times
more potent, since it freed as much energy as the explosion of 13 tons of
dynamite.
The energy that each nucleus of uranium liberates when a bomb explodes comes
from its fission into lighter nuclei. Each time this happens, two fragments of
approximately half the original mass are formed, plus two or three light particles
called neutrons. Neutrons, along with protons, are the usual components of
nuclei. Uranium has 92 protons and 143 neutrons. After a fission, some neutrons
flow free while others become part of the two fragments. This process of fission
happens spontaneously, but very slowly. To make use of it, be it in reactors or
bombs, one must “help” uranium break. This is achieved by shooting some
neutrons that break the uranium nuclei once they collide with them and thus
start the discharge of energy.
This is how a chain reaction starts and there are each time more fissions, since
each one of them produces more than one free neutron. There are mechanisms
that can make some neutrons “disappear”. For example, when a different
nucleus absorbs them or when they simply escape. For fissions to continue, there
must be a sufficient amount of neutrons at every moment, despite of their
losses. This condition, which depends on the type of nucleus which will undergo
fission (not only uranium, but other elements like plutonium work), on the shape
and size of the designed device, and on the material that surrounds the fuel,
defines a critical mass. A critical mass of fuel is the minimum amount that keeps
the chain reaction going, and a bomb needs a mass which is greater than the
critical. It is thought that the masses of a kilogram of fissionable uranium (called
235) would be enough to build a bomb, if an excellent design is used. A kilogram
of uranium has the size of a Ping-Pong ball.
Text
taken
from:
http://www.cubaeduca.cu/medias/cienciatodos/Libros_2/ciencia3/061/htm/sec
_4.htm
5. How many protons and neutrons does Uranium have?
6. What is fission?
7. Which is the subatomic particle that generates energy?
8. Which are the elements that can be used for the liberation of nuclear energy?
9. How many atoms are in each element of the following compound?
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0at%C3%B3mica
Glossary:
•
Atom: Basic unit of all matter; structure that defines all elements and that
has well-defined chemical properties.
•
Chemical compound: A substance formed by the union of two or more
elements of the periodic table.
•
Electron: Stable elemental particle that is negatively charged and one of
the fundamental components of the atom.
•
Element: Matter that is made up of atoms of the same type.
•
Ion: Atom that has an electric charge, whether it’s positive or negative.
•
Isotope: An atom that has the same number of protons in the nucleus,
but a different number of neutrons.
•
Mass: Measure that indicates the quantity of matter in a body.
•
Molar Mass: Refers to the mass in a mole of certain substance, expressed
in grams.
•
Matter: Anything that takes up space, has a property called mass and
possesses inertia.
•
Mole: The quantity of matter that contains 6.02 x 1023 particles, atoms,
molecules, ions, subatomic particles, etc.
•
Neutron: A subatomic particle that has no charge and is contained in the
atomic nucleus.
•
Atomic number: Indicates the quantity of protons that are present in the
nucleus of an atom.
•
Atomic orbital: A zone of space where there’s a high probability (greater
than 90%) of finding an electron.
•
Proton: Positively charged particle that is in the atomic nucleus.
•
Amu: The atomic mass unit is used to measure atomic mass and relative
atomic mass.