Untitled - Kelly Walsh High School

122 CHEMISTRY FOR THE UTTERLY CONFUSED
The IE increases from left to right across a period. This is because the effective
nuclear charge is increasing. This increase leads to a greater attraction, which
requires more energy to overcome. However, the trend is not linear. For example,
in the second period there are peaks at beryllium, Be, nitrogen, N, and
neon, Ne. (Refer to your textbook for a graph of the IE trends.) These peaks in
ionization energies correspond to stable electron configurations of the atoms.
For example, the electron configuration of beryllium is 1s 2 2s 2 . It has a filled
valence sublevel that provides additional stability. In nitrogen, the 2p sublevel
is half-filled. The electrons are in different 2p-orbitals. Being in different
orbitals, they are as widely separated as possible. This separation minimizes the
repulsion among the negative charges. This leads to an increase in stability. The
increased stability leads to an increase in the IE. In neon, the second energy
level is full, making the electron configuration particularly stable. High ionization
energies occur for stable electron configurations. The stable electron configurations
are filled shells and subshells, and half-filled subshells.
Ions, like atoms, have size. For ions, the term is ionic radii. For cations, the loss
of electrons results in a decrease in size, since (for the representative metals) an
entire energy level is usually lost. A sodium ion, Na , is smaller than a sodium
atom. The greater the number of electrons removed, the greater the decrease
in radius. This applies to any element and its cations as illustrated by the trend
in radii of Fe > Fe 2+ > Fe 3 .
8-3 Electron Affinity
Don’t Forget!
It is possible to add one or more electrons to an atom. This process yields an ion
with a negative charge. An ion with a negative charge is an anion. The energy
change accompanying the addition of an electron to a gaseous atom in its
ground state is the electron affinity (EA). The first EA may be endothermic,
exothermic, or even zero. The second EA is for the addition of a second electron.
Each EA, after the first, requires more energy. This is because the
approaching negative electron must overcome the repulsion of the negative ion
charge. Electron affinities are less reliable than ionization energies. The reason
for this is that EAs are more difficult to measure.
Atoms may gain electrons to form anions.