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THINK ABOUT IT

Sodium and chlorine atoms combine to form sodium chloride, NaCl, when sodium transfers a valence electron to chlorine. There are no other compounds that form between sodium and chlorine. For example, NaCl₄ and Na₃Cl are not possible compounds. Why is NaCl the compound that forms, rather than NaCl₄ or Na₃Cl?

How can valence electrons be used to predict chemical formulas?

To answer this question, you will explore

EXPLORING THE TOPIC

Ionic Compounds

There are millions of different compounds on the planet. Each compound is a result of a combination of atoms and has a chemical formula. However, some combinations of atoms are simply not possible. Valence electrons are the key to figuring out chemical formulas and to determining which compounds are possible.

COMPOUNDS BETWEEN METALS AND NONMETALS

Atoms combine with other atoms to achieve the stability of the noble gases. One way that atoms accomplish this is to transfer valence electrons to other atoms. When atoms gain or lose electrons, the resulting atoms are no longer neutral. They become ions with charges. The compounds that form in this way are called ionic compounds. Ionic compounds, like the noble gases, are very stable.

Ionic compounds form between metal atoms and nonmetal atoms. The metal atoms, on the left side of the periodic table, tend to give up electrons and become cations, which have positive charges. The nonmetal atoms, on the right side of the periodic table, tend to accept electrons and become anions, which have negative charges. Because the cation and anion have opposite charges, they are strongly attracted to one another.

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FORMULAS OF SIMPLE IONIC COMPOUNDS

The table below contains data on three simple ionic compounds. These compounds are all composed of a metal atom and a nonmetal atom in a 1:1 ratio. Pay particular attention to the total charge and the number of valence electrons on each of the atoms that combine.

The Rule of Zero Charge

You can see a distinct pattern to the ions that form in ionic compounds. Every time a metal atom and a nonmetal atom bond, they form a compound with an overall zero charge. This is known as the rule of zero charge.

The charges on the metal cations and nonmetal anions add up to zero. The rule of zero charge is useful in predicting the chemical formulas for ionic compounds, which form between a metal and a nonmetal.

PREDICTING IONIC BONDING USING THE PERIODIC TABLE

You may have noticed that the atoms that combine to form ionic compounds come from very predictable places on the periodic table. Sodium, with one valence electron, can combine with chlorine, fluorine, or bromine, each with seven valence electrons. In fact, any Group 1A element can combine with any Group 7A element to form an ionic compound.

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The drawing illustrates how you can use the periodic table to decide which atoms will combine in a 1:1 ratio to form an ionic compound. There are dozens of possible combinations. For example, an atom in Group 3A can form a compound with an atom in Group 5A.

More Complex Ionic Compounds

All of the compounds discussed in this lesson so far have a metal-to-nonmetal ratio of 1:1. However, it is also possible to have compounds with different ratios. For example, one magnesium atom can combine with two chlorine atoms to form magnesium chloride, MgCl₂. Before bonding, each magnesium atom has two valence electrons. These two electrons are transferred to two different chlorine atoms, each of which needs one more electron to have the same number of electrons as the noble gas argon, Ar.

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Notice in this table that the atoms combine in such a way that the resulting compound has a zero charge.

You can see that sometimes more than one atom of a particular element must combine to result in a compound with a zero charge. So, ionic compounds in ratios of 1:2, 2:1, or even 2:3 may result.

LESSON SUMMARY

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How can valence electrons be used to predict chemical formulas?

Metal atoms and nonmetal atoms combine to form ionic compounds through electron transfer. The metal atoms give up electrons. They form ions with a positive charge, called cations. The nonmetal atoms accept electrons and form ions with a negative charge, called anions. You can use the positions of the elements on the periodic table to predict the ionic compounds they can form. When an ionic compound forms, the total charge on the atoms adds up to zero. This is known as the rule of zero charge.