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?

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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?