Review of atomic ions

The nature of atom-bound electrons, as we have seen, makes the ionic forms of some atoms much more energetically stable—and thus more common—than the neutral form. For example, we saw that inert gas atoms don't tend to form ions because they have a full octet of electrons in their outer shell. Halides (F, Cl, Br, I, At), on the other hand, have one more electron than the next smaller inert gas (e.g. Na compared to Ne), and tend to form +1 ions by losing that "extra" electron.

In the atoms section I mentioned that the typical way an atom with more than an octet of electrons loses one (or more) is by coming into contact with one that has the opposite "need." For example, when sodium metal comes into contact with fluorine gas, the fluorine readily takes an electron from sodium, and both are left in a more energetically favorable state. What results is an ion pair with a neutral overall charge, Na⁺F^-, or just NaF.

Ionic compounds

Nature tends to neutralize charges, so ions of opposite charge tend to attract to make ionic compounds. Some examples are NaCl (table salt), KCl, CsF, and RbBr. Each is composed of a positive ion and a negative ion.

Formula unit and structure of NaCl

Notice that some of our ions are multiply charged. For example, the magnesium ion carries a +2 charge. Nature still tends to neutralize this charge, except that now we either need two -1 charged ions or one with a -2 charge. We can form many ionic compounds that satisfy this need. MgO consists of Mg²⁺ and O²-, and MgCl₂ is Mg²⁺ and two Cl^- ions.

About subscripts

In MgCl₂, our chemical abbreviation for "magnesium chloride", the subscript 2 means that there are two chlorine ions in the compound. This is a general feature of subscripts in chemical formulae. A subscript in a chemical formula always counts the number of the atom that precedes it.

Naming atomic ionic compounds

You probably noticed that we have naming conventions for ions. For atomic ionic compounds, it's pretty easy, just substitute the suffix "ide" for the last syllable of the anion and let the cation name stay the way it is. Generally, the subscripts don't play a role in naming, although, because chemistry is an old field, there are some exceptions. Here are some examples:

Molecular ions

Now to introduce another important class of ions, the molecular ions. So far in our study of chemistry, we haven't talked about molecules, and we really won't here. For now, let's just say that atoms can bind together in another way, quite different from the ionic bonding we've seen so far, and leave it at that. There are certain molecules that are much more stable after giving up one or more electrons or taking on one or more extra electrons - molecular ions.

Common molecular ions

CATIONS (+)

ANIONS (-)

Molecular ionic compounds

Ionic compounds are very important in chemistry. Here are some common ones formed by combining atomic and molecular ions. For historical reasons, some are known by a more common name, like the acids below. We will figure out the naming of acids later.

Practice problems

A. Using the names given, write the formula unit (chemical formula) for the ionic compound.

B. Write the name each of the following ionic compounds.