Periodic Trends:
Electron Affinity
The electron affinity is a measure of the energy change when an electron
is added to a neutral atom to form a negative ion.
For example, when a neutral chlorine atom in the gaseous
form picks up an electron to form a Cl- ion,
it releases an energy of 349 kJ/mol or 3.6 eV/atom.
It is then said to have an electron affinity of -349 kJ/mol and this large
number indicates that it forms a stable negative ion.
Small numbers indicate that a less stable negative ion is formed. Groups
VIA and VIIA in the periodic table have the largest electron affinities.
Alkali earth elements (Group IIA) and noble gases (Group VIIIA) do not
form stable negative ions.
Remember: Electron Affinity is essentially the
opposite of Ionization Energy.
We are adding electrons to a ground atom- not
removing them.
Why are some elements at zero?
Remember: Electron Affinity is essentially the
opposite of Ionization Energy.
We are adding electrons to a ground atom- not
removing them.
Why are some elements at zero?
Periodic Trends: Electron Affinity
Why isn’t Flourine the most accepting of the extra electron?
The chart goes against the expected trend.
Why isn’t Flourine the highest
Electron Affinity?
• Electron affinities generally become smaller down a column of the
for two reasons.
– the electron being added to the atom is placed in larger orbitals, where it
spends less time near the nucleus of the atom- shielding
– Second, the number of electrons on an atom increases as we go down
a column, so the force of repulsion between the electron being added
and the electrons already present on a neutral atom becomes larger.
• Remember: the repulsion between the electron being
added to the atom and the electrons already present on the
atom depends on the size of the atom.
– In Groups VIA and VIIA, this repulsion is largest for the very smallest
atoms in these columns: oxygen and fluorine. Thus, these elements
have a smaller electron affinity than the elements below them.