“The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms forming 1 mole of single charged ions”.
M(g) M+(g) + 1e–
The energy needed is dependent on the force with which the outer electron is held. This in turn is a function of the charge at the nucleus and the radius of the atom.
Down a group
Descending a group, the nuclear charge increases, but so does the atomic radius. These two factors would tend to cancel each other out, but the ionisation energy decreases on descending a group. This is explained by inter-electron repulsions increasing as the number of shells increases. It is sometimes called the shielding effect.
Across a period
We have already seen that the nuclear charge increases across a period and the atomic radius decreases. We would expect a steady increase in the force needed to remove an electron (the ionisation).
However, when we graph the first ionisation energies for the first 20 elements:
There is a general trend upwards across a period, but there are two points of inflection (changes of direction) in both period 2 and 3. In period 2 the first point of inflection is between element number 4 and 5, beryllium and boron.
There can be no doubt that the nuclear charge increases from beryllium to boron therefore the only possible reasons for boron requiring less energy to dislodge an electron are either inter-electron repulsion, or the electron is further from the nucleus that previously thought.
We now know that boron loses an electron from a ‘p’ sup-shell, which is more diffuse and further from the nucleus.
The point of inflection between nitrogen and oxygen is explained by inter-electron repulsion between two electrons now paired in a ‘p’ orbital.