Ions

Ionic v metallic bonding
In the last module you studied metals and metallic bonding. In metallic bonding many identical (elemental) or similar (alloyed) atoms each contribute loosely held outer electrons to an electron pool or sea. Atoms that lose an electron become positively charged. Charged atomic particles are called ions. Electrons are negatively charged. Since particles with like charges repel each other, the negative electrons are fairly evenly distributed among the positively charged metallic ions so that the crystal as a whole is stable.

The main idea to get hold of is this: the electrons in the electron pool are no longer under the influence of a particular nucleus, as they would be in a neutral atom. They are shared. This arrangement was seen in the the metals module to lead to many of the unique properties of metals.

What's the difference between an atom and an ion?
The shell model of the atom provides an explanation for why metals tend to lose electrons and become positively charged. Atoms of most metals have only one or two electrons in their highest energy level. Being in the highest energy level, these electrons are relatively loosely held. Thus metals tend to lose electrons and become positively charged.

A sodium atom has 11 positively charged protons and 11 negatively charged electrons, giving it a net charge of zero. If it loses an electron it has only 10 negative charges;

11 - 10 = +1.

Compare this to fluorine. (Note the spelling: u before o.)

Fluorine can either lose seven electrons or gain one to achieve a complete outer energy level. Tearing away seven electrons takes vast amounts of energy. Far easier to find a passing sodium atom floating around and to accept from it a loosely held electron. If sodium donates (gives the influence over) one electron to fluorine, both have full outer energy levels. And both have charge. Sodium, now +11 (protons) and -10 (electrons), has a +1 charge and is written Na⁺. It is an ion with a positive charge, called a cation. (Remember '+' in the middle of ca+ion. Sorry.)

Fluorine, now +9 and -10, has a -1 charge and is written F^-. Negatively charged ions are called anions. As opposites attract, Na⁺and F^-will tend to join chemically on the basis of their opposite charges. So Na⁺+ F^- ⇒ NaF.

Considering the size of individual atoms and ions, a single fluorine and a single sodium are unlikely to randomly collide in the great, wide universe. Useful chemical reactions tend to involve vast numbers of atoms or ions. Gazillions of sodiums combine with gazillions of fluorines. Thus NaF does not really represent a single sodium linked to a single fluorine, but ratio of one sodium:one fluorine. The actual numbers of sodiums and fluorines will be gigantic.

But that, as they say, is another story - one that we look at in Reacting Quantities.

Next Page: Cations