have high electron affinities because the addition of an electron to the atom results in a completely
filled shell, which represents a stable electron configuration. Achieving the stable octet involves a
release of energy, and the strong attraction of the nucleus for the electron leads to a high energy
change. The Group VIII elements, or noble gases, have electron affinities on the order of zero since
they already possess a stable octet: Gaining an extra electron is really not that favorable and would
not result in the release of much energy. In general, we can expect electron affinity to decrease
down groups and increase across periods.
A crude way of describing the difference between ionization energy and electron affinity is that the
former tells us how attached the atom is to the electrons it already has, while the latter tells us how
the atom feels about gaining another electron.
A CLOSER LOOK
Cesium (Cs), at the bottom left of the periodic table, has the largest atomic radius of any
naturally occurring atom. It also has the lowest ionization energy, the lowest electron
affinity, and the lowest electronegativity of all stable neutral atoms. (Francium is not a
stable, naturally occurring element.)ELECTRONEGATIVITY
Electronegativity is a measure of the attraction an atom has for electrons in a chemical bond. The
greater the electronegativity of an atom, the greater its attraction for bonding electrons. It is related
to ionization energy and electron affinity: Elements with low ionization energies and low electron
affinities will have low electronegativities because their nuclei do not attract electrons strongly,
while elements with high ionization energies and high electron affinities will have high
electronegativities because of the strong pull the nucleus has on electrons. Therefore,
electronegativity increases from left to right across periods. In any group, the electronegativity
decreases as the atomic number increases, as a result of the increased distance between the valence
electrons and the nucleus, i.e., greater atomic radius.