7.2 Putting Atoms Together
In Chapter 6, you read that all substances are made of the
elements of the periodic table. The way that the atoms in a
substance are arranged determines the substance's properties.
For example, the carbon atoms in graphite are arranged in layers
that are only loosely held together (Figure 1). That is why
graphite is soft and brittle. These properties also allow some of
the graphite to slide off of your pencil and onto paper as you
write.
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Figure 1 Graphite is made entirely of carbon (C) atoms.
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Figure not reproduced.
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Molecules
Most substances are not made up of individual atoms. Instead,
they are made up of molecules. A molecule is a group of atoms
that are chemically joined together. For example, the air you
breathe contains many kinds of molecules. These molecules
include oxygen, carbon dioxide, and water vapour.
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- molecule: two or more atoms that are chemically joined
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Oxygen and Ozone
Oxygen molecules make up 21 °lo of air. Each oxygen molecule is
made up of two atoms of oxygen joined together (Figure 2(a)).
A chemical formula is used to describe the number and type of
atom in each substance. The chemical formula for an oxygen
molecule is O2. The subscript, 2, tells you that there are two
atoms of oxygen in each molecule of oxygen.
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- subscript: the small number that follows the symbol for an
element in a chemical formula and shows how many atoms of the
element are present
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Oxygen can also be found as O3 molecules. This type of oxygen
has a special name: ozone (Figure 2(b)). Because O2 and O3
molecules are put together differently, these substances have
different chemical and physical properties. Your body breathes in
O2 molecules to survive. O3, on the other hand, is toxic and highly
reactive. Inhaling ozone damages lung tissue.
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Figure 2 (a) An oxygen molecule (O2) consists of two oxygen
atoms joined together. (b) An ozone molecule (O3) consists of
three oxygen atoms.
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Figure not reproduced.
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DIG DEEPER!
Did You Know?
The Good and Bad of Ozone
Ozone high in the atmosphere protects Earth by absorbing
harmful UV radiation from the Sun. Near Earth's surface, ozone is
an irritating air pollutant in smog.
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Molecular Elements
Like an oxygen atom, a molecule of oxygen is also an element.
Oxygen is an element that occurs as molecules. There are two
different kinds of oxygen molecules, O2 and O3. Hydrogen (H2)
and phosphorus (P4) are other examples of elements that occur
as molecules (Figure 3).
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Figure 3 This diver is using a torch that burns hydrogen gas (H2)
with a flame so hot that the torch can be used effectively
underwater.
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Figure not reproduced.
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There are seven elements that form molecules consisting of two
atoms: H2, N2, O2, F2, Cl2, Br2, and I2. A molecule that contains
two atoms is sometimes called "diatomic." The prefix di- means
"two."
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DIG DEEPER!
Learning Tip
Understanding Molecular Models
In these molecular models, the white spheres represent hydrogen
atoms, the black spheres represent carbon atoms, the red
spheres represent oxygen atoms, and the blue spheres represent
nitrogen atoms.
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Compounds
Other than oxygen (O2) and nitrogen (N2) in air, it is rare to find
elements on their own. Most of the pure substances we come
across contain more than one element.
Water, for example, is made up of two different elements-hydrogen (H) and oxygen (O). A pure substance that contains
two or more elements is called a compound. The chemical
formula for the compound water is H2O. That means that each
molecule of water has two H atoms and one O atom. The formula
for water is read H-2-O. Note that the subscript 1 is not written.
Figure 4 shows models of three compounds found in the air.
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- compound: a pure substance made up of two or more
elements that are chemically joined
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Figure 4 (a) Water (H2O) is made of two hydrogen (H) atoms
and one oxygen (O) atom. (b) Carbon dioxide (CO2) is made of
one carbon (C) atom and two oxygen (O) atoms. (c) Nitric oxide
(NO) is made of one nitrogen (N) atom and one oxygen (O) atom.
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Figure not reproduced.
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Interpreting Formulas
A chemical formula tells you important information about a pure
substance.
- The symbols in the formula tell you the elements in the
substance.
- If there is only one symbol, the substance is an element.
- If there is more than one symbol, the substance is a compound.
- The subscripts tell you how many atoms of each element are in
each molecule of the substance.
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SAMPLE PROBLEM: Interpreting Chemical Formulas
Interpret the chemical formula for sodium carbonate (Na2CO3).
Step 1: Determine how many different elements are in the
formula.
- Sodium carbonate contains 3 elements: Na, C, and O.
Step 2: Determine how many atoms of each element are in the
formula.
- Na2CO3 is made up of Na2, C, and O3, so Na2CO3 has 2 atoms of
sodium (Na) 1 atom of carbon (C) and 3 atoms of oxygen (O).
Step 3: Determine the total number of atoms in the chemical
formula. - 2 Na atoms + 1 C atom + 3 O atoms = 6 atoms
There are 6 atoms in the formula for sodium carbonate.
Practice: Interpret the formulas of these common compounds:
(a) ammonia (NH3)
(b) butane (C4H10)
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7.2 Wrap Up
- Elements are pure substances that contain only one type of
atom. They are represented by the chemical symbols found in the
periodic table.
- Compounds are pure substances made up of two or more
elements.
- Atoms can chemically join together to form larger units called
molecules.
- Molecules may contain atoms of different elements or atoms of
the same element.
- Chemical formulas state the type and number of atoms in each
molecule of a pure substance.
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CHECK YOUR LEARNING
1. In your own words, explain how a compound and a molecule
are alike and how they are different. K/U, C
2. Classify each of the following as an element or a compound.
K/U
(a) O3
(b) KCl
(c) NO
(d) MgCl2
(e) Fe
(f) Cl2
3. Which part of Figure 5 represents the following? K/U
(a) a compound
(b) an element that exists as diatomic molecules
(c) individual atoms of an element
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Figure 5
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Figure not reproduced.
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4. (a) What information do the subscripts in the chemical
formulas O2 and O3 provide?
(b) How do the properties of these forms of oxygen differ? K/U
5. Interpret each of these chemical formulas. K/U
(a) F2
(b) C3H8
(c) KBr
(d) CaCO3
(e) HCI
(f) CH4
(g) AgNO3
(h) Fe2O3