Ionization energy decreases moving down a group for the same reason atomic size
increases: electrons add new shells creating extra shielding that supersedes the addition of
protons. The atomic radius increases, as does the energy of the valence electrons. This
means it takes less energy to remove an electron, which is what ionization energy
measures.
Electron Affinity
An atom's electron affinity is the energy change in an atom when that atom gains an
electron. The sign of the electron affinity can be confusing. When an atom gains an
electron and becomes more stable, its potential energy decreases: upon gaining an
electron the atom gives off energy and the electron affinity is negative. When an atom
becomes less stable upon gaining an electron, its potential energy increases, which
implies that the atom gains energy as it acquires the electron. In such a case, the atom's
electron affinity is positive. An atom with a negative electron affinity is far more likely
to gain electrons.
Electron Affinities Across a Period
Electron affinities becoming increasingly negative from left to right. Just as in ionization
energy, this trend conforms to and helps explain the octet rule. The octet rule states that
atoms with close to full valence shells will tend to gain electrons. Such atoms are located
on the right of the periodic table and have very negative electron affinities, meaning they
give off a great deal of energy upon gaining an electron and become more stable. Be
careful, though: the nobel gases, located in the extreme right hand column of the periodic
table do not conform to this trend. Noble gases have full valence shells, are very stable,
and do not want to add more electrons: noble gas electron affinities are positive.
Similarly, atoms with full subshells also have more positive electron affinities (are less
attractive of electrons) than the elements around them.
Electron Affinities Down a Group
Electron affinities change little moving down a group, though they do generally become
slightly more positive (less attractive toward electrons). The biggest exception to this
rule are the third period elements, which often have more negative electron affinities than
the corresponding elements in the second period. For this reason, Chlorine, Cl, (group
VIIa and period 3) has the most negative electron affinity.
Electronegativity refers to the ability of an atom to attract the electrons of another
atom to it when those two atoms are associated through a bond. Electronegativity is
based on an atom's ionization energy and electron affinity. For that reason,
electronegativity follows similar trends as its two constituent measures.
Electronegativity generally increases moving across a period and decreases moving down
a group. Flourine (F), in group VIIa and period 2, is the most powerfully electronegative
of the elements. Electronegativity plays a very large role in the processes of Chemical
Bonding.
