Electron affinity is the energy released when an electron is added to a neutral, gaseous, and isolated atom in its ground state to form a monovalent anion.
It is a quantitative measurement of the energy change caused by adding a new electron to a neutral atom or molecule in the gaseous state. So, electron affinity is a measure of how strongly an incoming electron attracts the nucleus. As a result, the greater the attraction, the more energy is released. The higher an atom’s affinity for electrons, the higher will be its electron affinity value. It is denoted by the symbol Ea and is usually expressed in kJ/mol units.
The reaction that occurs when an atom accepts an electron can be expressed as follows:
X + e− → X− + energy
Additionally, electron affinity can be defined as the amount of energy required to remove an electron from a single negatively charged ion:
X− + energy → X + e−
However, electron affinities are much smaller than ionization energies because removing electrons from an anion is easier than removing electrons from a neutral atom. Electron affinities are more difficult to measure and usually less accurate than ionization energies. Fluorine and oxygen have large and negative electron affinities, whereas metals have small and positive affinities.
First electron affinity
The first electron affinity is the amount of energy released when one mole of gaseous atoms gains an electron to form one mole of gaseous -1 ions. As the energy is released in this process, the first electron affinity is always negative. It is an exothermic process.
X(g) + e- —> X⁻ (g)
Second electron affinity
It is the amount of energy required to add one electron to each ion in one mole of gaseous 1-ions to produce one mole of gaseous 2-ions. This is usually found in oxygen and sulfur.
X⁻(g) + e- —> X²⁻ (g)
Second electron affinity, unlike first electron affinity, is an endothermic process. An anion is formed when an electron is added to an atom. The addition of one or more electrons to an anion causes inter-electronic repulsion between the anion’s electron and the added electron. So, more energy is required to force (push) the electron against repulsion. As a result, second, third, and so on electron affinities are always positive.
Factors affecting electron affinity
1. Effective nuclear charge
The higher the effective nuclear charge on the valence shells the greater will be the attraction of the incoming electron. Consequently, they have high electron affinity values.
E.A α Z eff
2. Atomic radius
As the size of atom increases, the distance between the nucleus and the electron increases which results in a small force of attraction. Therefore, the value of electron affinity will be small. So, small atoms have high electron affinity and large atoms have low electron affinity. Hence with the decrease in atomic radius, the E.A. of the element increases.
E.A α 1/ r
3. Electronic configuration
The more stable an atom’s configuration, the less likely it is to accept an electron. As a result, its electron affinity will be reduced. Electrons with half-filled and full-filled electronic configurations are stable. So, such elements have either positive or almost zero electron affinities.
Periodic variation of electron affinity
A. In period
On moving left to right in the periodic table, the atomic size decreases, and the effective nuclear charge increases. So, electron affinity increases on going from left to right.
According to the definition, the first electron affinity of atoms should be negative. On moving from left to right in the period E. A. also increases. However, in some places, the electron affinity of the element becomes positive or zero.
For example, the electron affinity of Be and Mg are positive. This can be explained on the basis of an electronic configuration.
Be (2S2) + e– —> Be – (2s2 2p1)
Mg (3S2) + e– —> Mg – (3s2 3p1)
Be and Mg have fully filled electronic configurations (i.e., s2). The addition of an electron on Be and Mg results in the formation of a less stable electronic configuration (i.e., s2 p1). Furthermore, the energy has to supply to add the extra electron on this higher energy p-orbital. So, the first electron affinity of Be and Mg is positive. Similarly, the first electron affinity of nitrogen is also positive.
B. In group
The atomic radius increases on descending the group. So, on moving from top to bottom in the group the electron affinity of elements decreases.
I. The most stable octet configuration is that of noble gas (except He which has a doublet configuration). As a result, noble gases do not tend to gain electrons and form anions. As a result, the noble gases have zero electron affinity and no change in electron affinity from top to bottom.
II. When it comes to halogens, E.A. Fluorine E.A. of fluorine should be greater than that of chlorine but in fact, the E.A. of chlorine is greater than that of fluorine.
As fluorine has a small size, when an extra electron is added to it, it experiences strong electronic repulsion between the already crowded valance electrons and entering electrons. Because of this repulsion, the F-atom emits less energy in the formation of F– ion from the F atom. While in another hand, chlorine has a larger atomic size so it does not experience high electronic repulsion when an electron is added to it. As a result, higher energy is released in the formation of Cl– ion. The lower affinity of fluorine than chlorine can further be explained by the enthalpy data.
F (g) + e– —> F –(g) , H = -333 kJ/mole
Cl (g) + e– —> Cl –(g) , H = -349 kJ/mole
Experimental determination of electron affinity
The electron affinity of elements cannot be determined directly, but these values are indirectly determined from Born-Haber Cycle.
- Lee J. D. (1977). A new concise inorganic chemistry (3d ed.). Van Nostrand Reinhold. Retrieved December 15 2022 from https://archive.org/details/newconciseinorga00leej.