Download Compounds have different properties from the elements that make

Chapter 6, Section 1
Key Concept: Elements combine to form compounds.
BEFORE, you learned
• Atoms make up everything
on Earth
• Atoms react with different
atoms to form compounds
• How compounds differ from the
NOW, you will learn
* How compounds differ from the
elements that make them
*How a chemical formula represents
the ratio of atoms in a compound
* How the same atoms can form different
compounds
Compounds have different properties from the elements
that make them.
If you think about all of the different substances around you, it is
clear that they cannot all be elements. In fact, while there are just
over 100 elements, there are millions of different substances. Most
substances are compounds. A compound is a substance made of atoms
of two or more different elements. Just as the 26 letters in the alphabet
can form thousands of words, the elements in the periodic table can
form millions of compounds.
The atoms of different elements are held together in compounds
by chemical bonds. Chemical bonds can hold atoms together in large
networks or in small groups. Bonds help determine the properties of
a compound.
The properties of a compound depend not only on which atoms
the compound contains, but also on how the atoms are arranged.
Atoms of carbon and hydrogen, for example, can combine to form
many thousands of different compounds. These compounds include
natural gas, components of automobile gasoline, the hard waxes in
candles, and many plastics. Each of these compounds has a certain
number of carbon and hydrogen atoms arranged in a specific way.
The properties of compounds are often very different from the
properties of the elements that make them. For example, water is made
from two atoms of hydrogen bonded to one atom of oxygen. At room
temperature, hydrogen and oxygen are both colorless, odorless gases,
and they remain gases down to extremely low temperatures.Water,
however, is a liquid at temperatures up to 100°C (212°F) and a solid
below 0°C (32°F). Sugar is a compound composed of atoms of carbon,
hydrogen, and oxygen. Its properties, however, are unlike those of
carbon, hydrogen, or oxygen.
Calcium
+
Chlorine
=
Calcium Chloride
The picture above shows what happens when the elements
calcium and chlorine combine to form the compound calcium chloride.
Calcium is a soft, silvery metallic solid. Chlorine is a greenish-yellow
gas that is extremely reactive and poisonous to humans. Calcium
chloride, however, is a nonpoisonous white solid. People who live in
cold climates often use calcium chloride to melt the ice that forms on
streets in the wintertime.
How do the properties of a compound compare with the properties of the
elements that make it?
check your reading
Atoms combine in predictable numbers.
A given compound always contains atoms of elements in a specific
ratio. For example, the compound ammonia always has three hydrogen
atoms for every nitrogen atom—a 3 to 1 ratio of hydrogen to nitrogen.
This same 3:1 ratio holds for every sample of ammonia, under all
physical conditions. A substance with a different ratio of hydrogen
to nitrogen atoms is not ammonia. For example, hydrazoic acid also
contains atoms of hydrogen and nitrogen but in a ratio of one hydrogen
atom to three nitrogen atoms, or 1:3.
Chemical Formulas
Remember that atoms of elements can be represented by their chemical
symbols, as given in the periodic table. A chemical formula uses these chemical
symbols to represent the atoms of the elements and their ratios in a chemical
compound.
Carbon dioxide is a compound consisting of one atom of carbon
attached by chemical bonds to two atoms of oxygen. Here is how you
would write the chemical formula for carbon dioxide:
• Find the symbols for carbon (C) and oxygen (O) on the periodic
table. Write these symbols side by side.
• To indicate that there are two oxygen atoms for every carbon
atom, place the subscript 2 to the right of the oxygen atom’s
symbol. A is a number written to the right of a chemical symbol and
slightly below it.
• Because there is only one atom of carbon in carbon dioxide, you
need no subscript for carbon. The subscript 1 is never used. The
chemical formula for carbon dioxide is, therefore,
CO2
The chemical formula shows one carbon atom bonded to two
oxygen atoms.
Compound name
Hydrogen chloride
Water
Atoms
Hydrogen (1)
Chlorine (1)
Hydrogen (2)
Oxygen (1)
Atomic ratio
1:1
Chemical Formula
HCl
2:1
H 2O
Ammonia
Nitrogen (1)
Hydrogen (3)
1:3
NH3
Methane
Carbon (1)
Hydrogen (4)
1:4
CH4
Propane
Carbon (3)
Hydrogen (8)
3:8
C3H8
The chart above shows the names, atoms, ratios, and chemical
formulas for several chemical compounds. The subscripts for each
compound indicate the number of atoms that combine to make that
compound. Notice how hydrogen combines with different atoms in
different ratios. Notice in particular that methane and propane are
made of atoms of the same elements, carbon and hydrogen, only in
different ratios. This example shows why it’s important to pay attention
to ratios when writing chemical formulas.
check your reading
Why is the ratio of atoms in a chemical formula so important?
Same Elements, Different Compounds
Even before chemists devised a way to write chemical formulas, they
realized that different compounds could be composed of atoms of
the same elements. Nitrogen and oxygen, for example, form several
compounds. One compound consists of one atom of nitrogen attached
to one atom of oxygen. This compound’s formula is NO. A second
compound has one atom of nitrogen attached to two atoms of oxygen,
so its formula is NO2. A third compound has two nitrogen atoms
attached to one oxygen atom; its formula is N2O. The properties of
these compounds are different, even though they are made of atoms
of the same elements.
Chemical formulas show the ratios of atoms in a chemical compound.
Chemical Formulas
Compound Nam
e
Water (H2O)
Chemical Formula
Hydrogen Peroxide (H2O2)
C CC
There are many other examples of atoms of the same elements
forming different compounds. The photographs above show two bottles
filled with clear, colorless liquids. You might use the liquid in the first
bottle to cool off after a soccer game. The bottle contains water, which
is a compound made from two atoms of hydrogen and one atom of
oxygen (H2O). You could not survive for long without water.
You definitely would not want to drink the liquid in the second
bottle, although this liquid resembles water. This bottle also contains a
compound of hydrogen and oxygen, hydrogen peroxide, but hydrogen
peroxide has two hydrogen and two oxygen atoms (H2O2). Hydrogen
peroxide is commonly used to kill bacteria on skin. One way to tell
these two compounds apart is to test them using a potato. A drop of
hydrogen peroxide on a raw potato will bubble; a drop of water on the
potato will not.
The difference between the two compounds is greater than the
labels or their appearance would indicate. The hydrogen peroxide that
you buy at a drugstore is a mixture of hydrogen peroxide and water. In
its concentrated form, hydrogen peroxide is a thick, syrupy liquid that
boils at 150°C (302°F). Hydrogen peroxide can even be used as a fuel.
check your
reading What are the chemical formulas for water and hydrogen peroxide?
Questions for Chapter 6, Section 1
KEY CONCEPTS
1. How do the properties of compounds often compare with the
properties of the elements that make them?
2. How many atoms are in the compound represented by the formula
C12H22O11?
3. How can millions of compounds be made from the atoms of about
100 elements?
CRITICAL THINKING
4. Apply If a chemical formula has no subscripts, what can you
conclude about the ratio of the atoms in it?
5. Infer How might you distinguish between hydrogen peroxide and
water?
CHALLENGE
6. Analyze A chemist analyzes two compounds and finds that they
both contain only carbon and oxygen. The two compounds, however,
have different properties. How can two compounds made from the
same elements be different?