be found farther from the nucleus as the number of filled energy shells increases. Thus, the atomic
radius will increase.
IONIZATION ENERGY
The ionization energy (IE), or ionization potential, is the energy required to completely remove an
electron from an atom or ion. Removing an electron from an atom always requires an input of
energy, since the electron is attracted to the positively charged nucleus. The closer and more tightly
bound an electron is to the nucleus, the more difficult it will be to remove, and the higher the
ionization energy will be. The first ionization energy is the energy required to remove one valence
electron from the parent atom; the second ionization energy is the energy needed to remove a
second valence electron from the ion with +1 charge to form the ion with +2 charge, and so on.
Successive ionization energies grow increasingly large; that is, the second ionization energy is
always greater than the first ionization energy. For example:
Ionization energy increases from left to right across a period as the atomic radius decreases. Moving
down a group, the ionization energy decreases as the atomic radius increases. Group I elements
have low ionization energies because the loss of an electron results in the formation of a stable
octet.
ELECTRON AFFINITY
Electron affinity is the energy that is released when an electron is added to a gaseous atom, and it
represents the ease with which the atom can accept an electron. The stronger the attractive pull of
the nucleus for electrons, the greater the electron affinity will be. A positive electron affinity value
represents energy release when an electron is added to an atom.
Generalizations can be made about the electron affinities of particular groups in the periodic table.
For example, the Group IIA elements, or alkaline earths, have low electron affinity values. These
elements are relatively stable because their s subshell is filled: They do not particularly “care” to
gain an extra electron, even though the process is still favorable. Group VIIA elements, or halogens,