ELECTRON AFFINITY

This page explains what electron affinity is, and then looks at the factors that affect its size. It assumes that you know about simple atomic orbitals, and can write electronic structures for simple atoms.

First electron affinity

Ionisation energies are always concerned with the formation of positive ions. Electron affinities are the negative ion equivalent, and their use is almost always confined to elements in groups 6 and 7 of the Periodic Table.

Defining first electron affinity

The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.

This is more easily seen in symbol terms.

X(g) + e^-    X^-(g)

It is the energy released (per mole of X) when this change happens.

First electron affinities have negative values. For example, the first electron affinity of chlorine is -349 kJ mol^-1. By convention, the negative sign shows a release of energy.

The first electron affinities of the group 7 elements

Is there a pattern?

Yes - as you go down the group, first electron affinities become less (in the sense that less energy is evolved when the negative ions are formed). Fluorine breaks that pattern, and will have to be accounted for separately.

The electron affinity is a measure of the attraction between the incoming electron and the nucleus - the stronger the attraction, the more energy is released.

The factors which affect this attraction are exactly the same as those relating to ionisation energies - nuclear charge, distance and screening.

The increased nuclear charge as you go down the group is offset by extra screening electrons. Each outer electron in effect feels a pull of 7+ from the centre of the atom, irrespective of which element you are talking about.

For example, a fluorine atom has an electronic structure of 1s²2s²2p_x²2p_y²2p_z¹. It has 9 protons in the nucleus.

The incoming electron enters the 2-level, and is screened from the nucleus by the two 1s² electrons. It therefore feels a net attraction from the nucleus of 7+ (9 protons less the 2 screening electrons).

By contrast, chlorine has the electronic structure 1s²2s²2p⁶3s²3p_x²3p_y²3p_z¹. It has 17 protons in the nucleus.

But again the incoming electron feels a net attraction from the nucleus of 7+ (17 protons less the 10 screening electrons in the first and second levels).

The over-riding factor is therefore the increased distance that the incoming electron finds itself from the nucleus as you go down the group. The greater the distance, the less the attraction and so the less energy is released as electron affinity.

Why is fluorine out of line?

The incoming electron is going to be closer to the nucleus in fluorine than in any other of these elements, so you would expect a high value of electron affinity.

However, because fluorine is such a small atom, you are putting the new electron into a region of space already crowded with electrons and there is a significant amount of repulsion. This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity.

A similar reversal of the expected trend happens between oxygen and sulphur in Group 6. The first electron affinity of oxygen (-142 kJ mol^-1) is smaller than that of sulphur (-200 kJ mol^-1) for exactly the same reason that fluorine's is smaller than chlorine's.

Comparing Group 6 and Group 7 values

As you might have noticed, the first electron affinity of oxygen (-142 kJ mol^-1) is less than that of fluorine (-328 kJ mol^-1). Similarly sulphur's (-200 kJ mol^-1) is less than chlorine's (-349 kJ mol^-1). Why?

It's simply that the Group 6 element has 1 less proton in the nucleus than its next door neighbour in Group 7. The amount of screening is the same in both.

That means that the net pull from the nucleus is less in Group 6 than in Group 7, and so the electron affinities are less.

First electron affinity and reactivity

The reactivity of the elements in group 7 falls as you go down the group - fluorine is the most reactive and iodine the least.

Often in their reactions these elements form their negative ions. At GCSE the impression is sometimes given that the fall in reactivity is because the incoming electron is held less strongly as you go down the group and so the negative ion is less likely to form. That explanation looks reasonable until you include fluorine!

An overall reaction will be made up of lots of different steps all involving energy changes, and you cannot safely try to explain a trend in terms of just one of those steps. Fluorine is much more reactive than chlorine (despite the lower electron affinity) because the energy released in other steps in its reactions more than makes up for the lower amount of energy released as electron affinity.

Second electron affinity

You are only ever likely to meet this with respect to the group 6 elements oxygen and sulphur which both form 2- ions.

Defining second electron affinity

The second electron affinity is the energy required to add an electron to each ion in 1 mole of gaseous 1- ions to produce 1 mole of gaseous 2- ions.

This is more easily seen in symbol terms.

X^-(g) + e^-    X^2-(g)

It is the energy needed to carry out this change per mole of X^-.

Why is energy needed to do this?

You are forcing an electron into an already negative ion. It's not going to go in willingly!

The positive sign shows that you have to put in energy to perform this change. The second electron affinity of oxygen is particularly high because the electron is being forced into a small, very electron-dense space.