Ionization Energy:  The tendency of an atom to lose electrons & form a (+) ion.

        Ionization Energy is the energy required to remove 1 electron from a single atom in the gaseous state.

Electron Affinity:   The tendency of an atom to gain electrons & form a (-) ion.

        Electron Affinity is the energy released when an electron is added to a single atom in the gaseous state.

Example:    Group IA elements (alkali metals) tend to lose 1 valence electron, thus forming ions with a +1 charge.

                                 Alkali Metals therefore have:

                                    1.  Small ionization energies.  Why?  - Because their electrons are easily lost.

                                    2.  Small electron affinity.  Why?  - Because electrons electrons are not easily gained.

                                    3.  Net Result:  Cation formation is favored.

Example:    Group VIIA elements (the halogens) tend to gain 1 valence electron, thus forming ions with a -1charge.

                                 Halogens therefore have:

                                    1.  Large ionization energies.  Why?  - Because their electrons are not easily lost.

                                    2.  Large electron affinity.  Why?  - Because they easily gain electrons.

                                    3.  Net Result:  Anion formation is favored.