Electron Affinity
Another property that influences an
atom’s chemical behaviour is their
ability to accept one or more
electrons
This property is called electron
affinty
electron affinty: the negative of the
energy change that occurs when an
electron is accepted by an atom in
the gaseous state to form an anion
X(g) + e  X(g)
Electron affinity is positive if the
reaction is exothermic (releases energy)
and negative if the reaction is
endothermic (absorbs energy)
F(g) + e  F(g)
H = 328 kJ/mol
The electron affinity for fluorine is
assigned as +328 kJ/mol.
Ionization energy measures the ease
with which an atom loses an electron,
whereas electron affinity measures
the ease with which an atom gains an
electron.
When the addition of an electron
makes the atom more stable, energy
is given off
A large positive electron affinity
means that the negative ion is very
stable (that is, the atom has a great
tendency to accept an electron)
The halogens have the greatest
electron affinity because they are one
electron short of a filled p-block.
When the addition of an electron
makes the atom less stable, energy
must be put in
If the added electron must be placed
into a higher energy level than the
other valence electrons, then the
element is made less stable
Trend: Moving left to right across a
period, the electron affinity increases
(more energy given off  becoming
more stable)
Properties of Metals
Metals tend to have low ionization
energies and therefore tend to form
cations relatively easily
Most metal oxides are ionic solids that
are basic
(metal oxide + water → metal hydroxide)
Na2O(s) + H2O(l) → 2 NaOH(aq)
Properties of Nonmetals
Because of their large electron
affinities, nonmetals tend to gain
electrons when they react with
metals
Most nonmetal oxides are acidic
(nonmetal oxide + water → acid)
CO2(g) + H2O(g) → H2CO3(aq)