Electron Affinity: Definition, Trends and Factors Affecting It

The change in energy (kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to form a negative ion is defined as electron affinity. In other words, the probability of a neutral atom gaining an electron. When an electron is added to a neutral gaseous atom to form a negative ion, the potential energy change of the atom is measured. As a result, the more negative the electron affinity, the more advantageous the electron addition process. Not all elements produce stable negative ions, in which case the electron affinity is either zero or even positive. Let’s discuss electron affinity in depth along with some important questions.

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What is Electron Affinity?

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The change in energy (in kJ/mol) of a neutral atom (in the gaseous phase) when an electron is added to form a negative ion is defined as electron affinity. When an atom loses or gains energy through chemical reactions that result in the loss or gain of electrons, the atom's energy is defined. A chemical process that releases energy is referred to as an exothermic reaction, while one that absorbs energy is referred to as an endothermic reaction.

Electron Affinity

Exothermic reactions produce negative energy, which is denoted by a negative sign, whereas endothermic reactions produce positive energy, which is denoted by a positive sign. Energy is released when an electron is added to a neutral atom (first electron affinity); consequently, first electron affinities are negative. However, adding an electron to a negative ion requires more energy (i.e., second electron affinity), which outweighs any energy released during the electron attachment process, resulting in positive second electron affinities.

First Electron Affinity (negative energy due to released energy):

X (g) + e^- → X^- (g)

Second Electron Affinity (positive energy because energy required exceeds energy gained):

X − (g) + e^-→ X^2- (g)

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Periodic Trends

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• Electron affinity is the amount of energy released when an electron is added to a neutral atom to form an anion. The affinities of electrons are difficult to quantify.
• Electron affinity increases from left to right over time. The overall trend occurs as a result of increased nuclear attraction over time.
• As one moves down the group, the electron affinity should decrease because the electron is added further away from the atom. Less tightly bound, and thus closer to a free electron in energy. 
• Electron Affinity= 1/Atomic Size
• The electron affinity is obtained indirectly through the Born-Haber cycle rather than directly.
• As we move from left to right over a period, the atomic size decreases due to an increase in the nuclear force, and thus the electron gain enthalpy increases. Moving down a group in the periodic table causes the atomic size to increase, resulting in a decrease in the value of electron gain enthalpy.

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Factors Affecting Electron Affinity

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The following are the general factors that influence electron affinity:

1. Electron Affinity= 1/Atomic Size
2. Electron Affinity= Effective Nuclear Charge
3. Electron Affinity= 1/Screening Effect
4. Reactivity of non-metals= Electron Affinity
5. The oxidizing power of element= Electron Affinity

Atomic size: If the atomic size is small, the electron gain enthalpy will be greater because the effective nuclear forces in the smaller atoms are stronger and the electrons will be held firmly.

Nuclear charge: The greater the nuclear charge, the greater the value for electron gain enthalpy, because increasing the nuclear charge increases the effective nuclear force on valence electrons.

In general, electron affinity tends to follow the following patterns:

Halogens > Oxygen family > Carbon family > Nitrogen family > Metals of group 1 & 13 > Metals of group 2

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Electron Affinity of Halogens

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• The amount of energy required to remove an electron from a gaseous atom is referred to as the ionization potential.
• When energy is supplied to remove an atom, it implies that energy will be released if an extra electron is added to the atom. The amount of energy released when a neutral atom in its gaseous state accepts an electron and is converted into a negatively charged ion is known as electron affinity.

X (g) + e- → X- (g) + E.A.

• In this case, X is any element in its gaseous state, and E.A. is the electron affinity. Electron affinity is measured in electron volts per atom or kilojoules per mole. It is denoted by a minus sign [-] for example, Cl (g) + e- → Cl- (g) + 349 KJ/mol
• As a result, chlorine's electron affinity is – 349 KJ/mol.

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Non-Metals Versus Metals

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• To summarize the difference between metal and nonmetal electron affinity (Figure 2):
• Metals prefer to lose valence electrons to form cations in order to have a fully stable octet. They absorb energy (are endothermic) in order to lose electrons. Nonmetals have a higher electron affinity than metals.
  In order to have a fully stable octet, nonmetals prefer to gain electrons and form anions. They expel energy (exothermic) in order to gain electrons and form an anion; thus, nonmetals have a higher electron affinity than metals.

Figure 2: A plot of the Periodic Variation of Electron Affinity with Atomic Number for the Periodic Table's First Six Rows. It is important to note that electron affinities can be both negative and positive.

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Things to Remember

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• Electron affinity is defined as the change in energy (in kJ/mol) of a neutral atom (in the gaseous phase) when an electron is added to form a negative ion.
• Exothermic reactions produce negative energy, which is denoted by a negative sign, whereas endothermic reactions produce positive energy, which is denoted by a positive sign.
• Electron Affinity= 1/Atomic Size
• First Electron Affinity (negative energy due to energy released): X (g) + e- → X- (g)
• Second Electron Affinity (positive energy because the energy required exceeds energy gained): X − (g) + e-→ X2- (g)
• The ionization potential is the amount of energy required to remove an electron from a gaseous atom.

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