How would you explain why the electron affinity of nitrogen is lower (less negative) than those of both carbon and oxygen?

A few important aspects about the sign used to express electron affinities.

Electron affinity is directly related to change in energy by the equation

#"E"."A". = -Delta"E"#

This means that if energy is released when an atom is added to the atom, i.e. #Delta"E"# is negative, the electron affinity will be positive.

Likewise, if energy is required to add an electron to an atom, i.e. #Delta"E"# is positive, then electron affinity will be negative.

#stackrel("---------------------------------------------------------------------------------------------------------")#

The key to why the electron affinity of nitrogen is actually negative lies with two factors

• effective nuclear charge
• electron configuration

As you know, electron affinity tells you how much energy is relesed (hence the negative sign) when one mole of electrons is added to one mole of atoms in the gaseous state.

#"X" + "e"^(-) -> "X"^(-) + color(blue)("released energy")#

Now, for elements that share a period, atomic size decreases as you move from left to right. This happens because effective nuclear charge, which is a measure of what the net positive charge felt by the electrons is, increases.

This implies that the atomic size of carbon will be a little bigger than that of nitrogen, which in turn will be a little bigger than that of oxygen.

I nessence, electron affinity tells you how "bad" an atom wants an incoming electron.

It goes without saying that the higher the effective nuclear charge, the more attracted to the nucleus an incoming electron will be, and the more negative the electron affinity of the atom will be.

Now, what happens when you need energy to add an electron to an atom?

#"Y" + "e"^(-) + color(red)("needed energy") -> "Y"^(-)#

In this case, electron affinity will be positive, but the value listed will be negative.

Take a look at the electron configurations of carbon, nitrogen, and oxygen

In carbon's case, an incoming electron would be added to the empty #2p_z# orbital. This mens that it will encounter no repulsion from other electrons, since the orbital is empty.

On the other hand, in nitrogen's case, an incoming electron will be added to a #p# orbital that already contains an electron.

This means that the incoming electron will experience significant repulsion compared with when it's added to an empty orbital.

As a result, you need energy to add an electron to nitrogen, and hence its electron affinity is actually negative.

So why isn't this the case for oxygen, since an incoming electron would be added to an orbital that already contains an electron?

Because the effective nuclear charge overpowers this repulsion, and energy is being released when an electron is being added to oxygen, hence the electron affinity will be positive.