2.4 Compounds, Atoms, and Ions (7.1 pg 169-174

Compounds are formed when two or more atoms bond together; modern atomic theory suggests the tendency of atoms to bond is based on the arrangement of their e (protons/neutrons not involved.)

Ironically, to understand bonding we look to the family of elements that are totally un-reactive (Noble Gases). One theory of bonding assumes there is an underlying tendency for elements to acquire an outer electron shell similar to the nearest noble gas.

An ion = an atom that has either lost or gained electrons. We represent the charge of an ion with a the charge written as a superscript (above) to the right e.g. a Nitrogen atom that has gained 3 e is written as N3-. The type of charge (+ or -) is always present.

An atom may lose or gain e to become more stable An atom may lose or gain e to become more stable. The most stable atoms are those with full outer e shells (group 8 noble gases). The outermost electron shell on each atom is called the valence shell and the electrons that are in it are valence electrons.

Nitrogen has 5 valence electrons Nitrogen has 5 valence electrons. It would be easier for it to gain 3 more electrons than to lose 5, so it become an ion with a charge of 3-. This e configuration would match that of Neon, the nearest noble gas. The tendency to lose / gain e to get a full valence shell is called the Octet rule (elements want a full outer shell, which usually means a shell with 8e).

The diagram below shows how 2 atoms on either side of the Noble gas Neon form ions (Fig.4 p.171). Fluorine (F) has 7 valence electrons in is outer shell. It would be more stable with 8. It has the tendency to gain one e. Since electrons are negative, this gives F a negative charge, F-

Sodium (Na) has 1 electron in its outer shell Sodium (Na) has 1 electron in its outer shell. It would take too much energy to gain 7 more electrons. Instead, it has a tendency to lose an electron. Its outer shell is now the 2nd shell, and is ‘full’ with 8 e.

Both of these ions end up with electron shell configurations that are the same as the nearest noble gas, Neon.

There are two kinds of ions: Anions = A negative ION (remember the first letter of each); always have a negative charge. Cations= A positive ION (remember that CATS are positive); always have a positive charge An ion’s charge is easy to figure out. Add the number of protons to the number of electrons on the ion. Then write the charge in the upper right hand corner. E.g. Sulfur ion = 16p + 18e- = 16 + 18(-) = 2- = S2- Another method is described on TABLE 1 p.173 of your text; you may prefer this method of determining the charge of ions.

Please note: Metals form positive ions, non-metals form negative ions. Groups (chemical families) tend to have the same ion charge. Hydrogen is unique – it can become H+ or H- (no family).