Unit 1.4 Atom, Element, Molecule, Compound
What would the philosophers do?
People enjoy getting together to discuss things,
whether it is how your favorite sports team is doing,
what the best new movie is, the current politics, or
any number of other topics. Often the question is
raised about who is right and who is wrong. If the
football game is to be played this coming weekend,
all we can do is offer opinions as to its
outcome. The game has not been played yet, so we
don’t know who will actually win.
The ancient Greek philosophers did a lot of discussing, with part of their conversations concerning the
physical world and its composition. There were different opinions about what made up matter. Some felt one
thing was true while others believed another set of ideas. Since these scholars did not have laboratories and
had not developed the idea of the experiment, they were left to debate. Whoever could offer the best argument
was considered right. However, often the best argument had little to do with reality.
One of the on-going debates had to do with
sand. The question posed was: into how small of
pieces can you divide a grain of sand? The
prevailing thought at the time, pushed by
Aristotle, was that the grain of sand could be
divided indefinitely, that you could always get a
smaller particle by dividing a larger one and
there was no limit to how small the resulting
particle could be.
Since Aristotle was such an influential philosopher, very few people disagreed with him. However, there
were some philosophers who believed that there was a limit to how small a grain of sand could be
divided. One of these philosophers was Democritus (~460-~370 B.C.), often referred to as the “laughing
philosopher” because of his emphasis on cheerfulness. He taught that there were substances
called atoms and that these atoms made up all material things. The atoms were unchangeable,
indestructible, and always existed.
The word “atom” comes from the Greek atomos and
means “indivisible.” The atomists of the time
(Democritus being one of the leading atomists) believed
there were two realities that made up the physical
world: atoms and void. There were an infinite number of
atoms, but different types of atoms had different sizes
and shapes. The void was the empty space in which the
atoms moved and collided with one another. When these
atoms collided with one another, they might repel each
other or they might connect in clusters, held together
by tiny hooks and barbs on the surface of the atoms.
Aristotle disagreed with Democritus and offered his own idea of the composition of matter. According
to Aristotle, everything was composed of four elements: earth, air, fire, and water. The theory of
Democritus explained things better, but Aristotle was more influential, so his ideas prevailed. We had to
wait almost two thousand years before scientists came around to seeing the atom as Democritus did.
How right was Democritus?
It is very interesting that Democritus had the basic idea of atoms, even though he had no experimental
evidence to support his thinking. We now know more about how atoms hold together in “clusters”
(compounds), but the basic concept existed over two thousand years ago. We also know that atoms can
be further subdivided, but there is still a lower limit to how small we can break up that grain of sand.
Who is Sherlock Holmes?
The famous fictional British detective Sherlock
Holmes was often said to make the statement
“elementary, my dear Watson.” In reality, the
closest he ever came to that line was an exchange
with Watson in the short story “The Crooked
Man.” Holmes shows shrewd insight into Watson’s
activities of the day. When asked how he knew what
Watson was doing, Holmes simply replies
“Elementary.” Regardless of exactly how he put it,
Sherlock was simply referring to what the Free
Dictionary defines as “relating to, or constituting
the basic, essential, or fundamental part.”
Elements and Atoms
An element is the simplest form of matter that has a unique set of properties. An atom is the smallest
particle of an element that retains its identity. Examples of well-known elements include oxygen, iron,
and gold (see the figure below). Elements cannot be broken down into a simpler substance. Likewise, one
element cannot be chemically converted into a different element. Today there are over 100 elements
known to man.
A
B
C
Chemical elements are the simplest substances. A) Oxygen tank for people to breath, B) Iron skillet for
cooking, C) gold bar for monetary purposes.
Some elements have been known for centuries (gold, silver, iron, copper, among others) while others have
been created in the lab only within the last several years. Most elements do not exist as pure element in
nature. They are so reactive that they can be found only in combination with other materials.
Several of the elements are very valuable while others are quite inexpensive. Gold is currently worth
almost $1700 per ounce. Aluminum, on the other hand, only sells for about 90 cents per pound,
considerably lower than gold. Copper is worth somewhat more, selling for approximately $3.50 pound.
Platinum is very valuable at about $1650 an ounce, not quite as expensive as gold.
Enjoy an old song about the elements sung by Harvard lecturer Tom Lehrer. There were only 102 known
elements at the time he wrote this song.
http://www.youtube.com/watch?v=K8eT-k4f7_8
What do you need to start building a house?
When building a house, you start with a blueprint of what the
house will look like. The plan states how many windows and
what kind, how many doors and what style, how many rooms
and what type (bedroom, kitchen, other). The blueprint
shows how the different pieces will go together to make the
house. As long as the blueprint is followed and exactly the
same items are used, the houses will be identical.
Compounds
A compound is a substance that contains two or more elements chemically combined in a fixed proportion.
A molecule is the smallest unit of a compound consisting of that fixed proportion of atoms. One molecule
of salt, sodium chloride - NaCl, consists of one atom of sodium and one atom of chlorine bonded together.
The elements carbon and hydrogen combine to form many different compounds. One of the simplest is
called methane, in which there are always four times as many hydrogen particles as carbon
particles. Methane is a pure substance because it always has the same composition. However, it is not an
element because it can be broken down into simpler substances – carbon and hydrogen.
Recall that the components of a mixture can be separated from one another by physical means. This is
not true for a compound. Table salt. Sodium chloride, is a compound consisting of equal parts of the
elements sodium and chlorine. Salt cannot be separated into its two elements by filtering, distillation, or
any other physical process. Salt and other compounds can only be decomposed into their elements by a
chemical process. A chemical change is a change that produces matter with a different
composition. Many compounds can be decomposed into their elements by heating. When sugar is heated,
it decomposes to carbon and water. Water is still a compound, but one which cannot be broken down into
hydrogen and oxygen by heating. Instead, the passage of an electrical current through water will
produce hydrogen and oxygen gases.
The properties of compounds are generally very different than the properties of the elements from
which the compound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air
or water. Chlorine is a deadly gas. The compound sodium chloride is a white solid which is essential for
all living things.
A
B
C
A) Sodium is so reactive it must be stored under oil, B)Chlorine is poisonous yellow-green gas, C) Salt crystals,
a compound of sodium and chlorine
Summary
Aristotle and others believed that a grain of sand could be divided indefinitely.
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Democritus believed there was a lower limit to the dividing of a grain of sand.
An element is the simplest form of matter that has a unique set of properties.
One element cannot be chemically converted to another element.
An atom is the smallest particle of an element.
A compound is a substance that contains two or more elements chemically combined in a fixed
proportion.
A molecule is one unit of a compound.
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A chemical change is a change that produces matter with a different composition.
Review
1.
What was the major contribution Democritus made to the thinking of his day?
2.
List characteristics of atoms according to Democritus.
3.
What is an element?
4.
How many elements did we know about when Tom Lehrer wrote his song?
5.
What is the smallest particle of an element?
6.
What is a compound?
7.
How is a compound different from an element?
8.
How is a molecule different from an atom?
9.
What is a chemical change?
Answers
1. The idea that all material is made from small particles he called atoms.
2. There were an infinite number of kinds of atoms of different sizes and shapes. They had hooks on
them allowing for attachment to other atoms.
3. Simplest form of matter with a unique set of properties, made from one type of atom.
4. Only 102, Today, we know of 118.
5. The smallest particle of an element is one atom.
6. A substance made of 2 or more elements in definite proportions chemically combined.
7. A compound is made of2 or more elements, types of atoms, chemically combined. An element is one
type of atom.
8. A molecule is the smallest particle of a compound, 2 or more elements, types of atoms, chemically
combined. An atom is the smallest particle of an element.
9. A chemical change is a change that produces matter with a different composition.
Practice
Select an element from this list:
http://www.lenntech.com/periodic/name/alphabetic.htm
Answer the following questions:
1.
What is the name of the element?
2.
When was it discovered?
3.
Who discovered it?
4.
What is one use for this element?
Watch the video at the link below to answer the following questions:
http://www.youtube.com/watch?v=9YPvwsDeLUo
1.
What lists all the elements we know about?
2.
What is the compound made out of two hydrogen atoms and one oxygen atom?
3.
What is the compound made out of one carbon atom and two oxygen atoms?