# Group 15 Ionisation Energies

I had a little confusion regarding ionisation energies.

I get that in general, the second ionisation energy > first ionisation energy, due to increased effective nuclear charge on the second electron.
However, in the case of an element of group 15, say phosphorus, shouldn't it be the other way around because of the half-filled orbital configuration ([Ne]3s23p3, for phosphorus)? Won't it be harder to ionise the first electron because of increased stability?

• You're right in saying that the ionisation energy of phosphorus should be greater than that of sulphur due to its half-filled p subshell. This is one of the many exceptions... There's also the second group alkaline earth metals whose ionisation energy is greater compared to that of the thirteenth group elements, due to a fully filled s subshell... – AbhigyanC Nov 10 '17 at 18:28
• Periodic trends are just that, trends. They give you a general idea of what to expect, but each case needs to be examined individually. – Michael Lautman Nov 10 '17 at 22:16

The first ionisation energy corresponds to the following reaction: $$\ce{P(g) -> P+ (g) + e-}\tag{1}$$ The second ionisation energy corresponds to this reaction: $$\ce{P+(g) -> P^2+ (g) + e-}\tag{2}$$