Electron Affinity

Introduction

Electron affinity is a change in the energy of an atom. When a neutral atom adds electrons to its outer shell it releases energy and carries a negative charge. An element gains electrons to stabilise its octet.

Energy is released when an element accepts or loses an electron. In a chemical reaction when an element accepts an electron to form a compound, it releases the energy, it's called an exothermic reaction. The energy is released to attract the electron by a nucleus from another element that's why the energy is released in an exothermic reaction. When an element loses an electron it absorbs energy, so-called it an endothermic process. When an atom losses electrons atom gains the energy.

What is electron Affinity?

In a chemical reaction, energy is released or gained by a molecule. This release and gain of energy happen by gaining or losing electrons. The atoms which accept electrons are releases energy and these reactions are known as exothermic reactions. On the other hand, the atoms which lose an electron in a chemical reaction absorbs energy and are known as endothermic reaction. The affinity of electrons is the ability to accept an electron. when a neutral gaseous atom accepts the electron, it will charge with a negative ion. the first electron affinity is always negative, and the second electron affinity is always positive. it is difficult to measure the electron affinity of an atom. It is measured by the release of energy of ionic compounds. The electron affinity is also measured by an atom's tendency to act as an oxidising or reducing agent. It is measured in kilojoules/ Mole. Electron affinity is symbolised by Ea.

First Electron Affinity:

When a neutral atom accepts an electron, the energy is released and the atom gets a negative charge. The first electron affinity is always negative as more energy is required to pull an electron by the nucleus.

$$\mathrm{X(g)\:+\:e\:\rightarrow\:X(g)}$$

The electron affinity is exhibited best by groups 16 and 17 elements in the periodic table. These non-metals have more ability to accept electrons from other elements than to release an electron to complete their octet.

Example − The electron affinity for the first electron in fluorine and chlorine atom is -328 kJ/Mol and -349 KJ/Mol respectively.Metals have less electron affinity as it is easy for metals to lose an electron from the outer shell than to gain an electron. Metals have less pull to their outer shell electrons, losing an electron is an endothermic process. The electron affinity by metals and non- metals is listed below.

Metals have less electron affinity as it is easy for metals to lose an electron from the outer shell than to gain an electron. Metals have less pull to their outer shell electrons, losing an electron is an endothermic process. The electron affinity by metals and non- metals is listed below.

Metals	Electron Affinity	Non-Metals	Electron Affinity
𝐿𝑖	-60 KJ/Mol	F	-328 KJ/ Mol
𝑁𝑎	-53 KJ/Mol	Cl	-349 KJ/ Mol
K	- 48 KJ/Mol	Br	-324 KJ/ Mol

• Electron affinity decreases when we move down in a group in the table.

• Electron affinity increases when we move left to right in the period.

• Metals form cation by losing an electron.

• Non-metals form anions by accepting an electron.

Second Electron Affinity

Electron affinity is seen in the group-16 elements like oxygen, sulphur, and selenium. The energy required to add an electron to negatively charged elements (anion). These elements form anions with (-2) ions are known as second electron affinity.

The energy needed to add an electron to 1 mole of (-1) gaseous ion to form 1 mole of the gaseous ion with -2 ions.

$$\mathrm{X(g)\:+\:e\:\rightarrow\:X^{-2}(g)}$$

Example −

We can understand that by the example of sulphur ion,

$$\mathrm{S(g)\:+\:e\:\rightarrow\:S^{-2}(g)}$$

$$\mathrm{S^{-1}(g)\:+\:e\:\rightarrow\:S^{-2}(g)}$$

Electron affinity for sulphur in the equation is -200 kJ/Mol.

In Oxygen the electron affinity −

$$\mathrm{O(g)\:+\:e\:\rightarrow\:O^{-1}(g)\:EA\:=\:142KJ/Mol}$$

$$\mathrm{O^{-1}(g)\:+\:e\:\rightarrow\:O^{-2}(g)\:EA\:=\:+844KJ/Mol}$$

The second electron affinity is higher than the first electron affinity in oxygen molecules because of the electron-electron repulsion in the negatively charged ion of oxygen.

Factors Affecting Electron Affinity

There are a few factors that affect the electron affinity of a molecule these are the atomic size of the element, the nuclear charge on the molecules, and the electronic configuration of atoms.

• Effect of Atomic size − The atoms with smaller sizes have more electron affinity than the atoms with bigger sizes. The nucleus of smaller atoms has more attraction for electrons as compared to the bigger size. As the size of the atom increases the outer shell was far from the nucleus and the attraction for electrons in the outer shell will also decrease.

  Example − the electron affinity in Br than I i.e., -324 KJ/Mol and -285 KJ/Mol respectively.

• Nuclear Charge − The nuclear charge also affects the electron affinity. As the charge increases on atoms, the attraction in electrons also increases as a result the electron affinity also increases. If a molecule is already charged the electron repulsion increases and the pull from the nucleus also increases, so electron affinity in charged ions also increases.

  Example − The electron affinity in the first reaction of oxygen is less as compared to the second electron affinity.

$$\mathrm{O(g)\:+\:e\:\rightarrow\:O^{-1}(g)\:EA\:=\:142KJ/Mol}$$

$$\mathrm{O^{-1}(g)\:+\:e\:\rightarrow\:O^{-2}(g)\:EA\:=\:+844KJ/Mol}$$

• Shielding Effect − The electron affinity increases as we decrease the screening effect on the inner shell of an atom.

  $$\mathrm{Electron\:Affinity\:=\:\frac{1}{shielding\:Effect}}$$

• Electronic Configuration − The electronic configuration also affects the electron affinity. The elements with full octet have zero tendencies to accept electrons, hence the electron affinity will be zero in inert gases. The electronic configuration plays a vital role in electron affinity. Metals have less affinity for electrons as compared to non-metals because of the electronic configuration of metals.

Conclusion

Electron affinity is the ability to accept electrons in gaseous form and form an anion. The accepting of electrons releases energy hence called the exothermic process. The electron affinity decreases when we move up to down in groups and increases when moving left to right in a period. It is represented by the symbol EA, and it is measured in Kilojoule per mole (KJ/Mol). The first electron affinity is always less than the second electron affinity due to the electron-electron repulsion. Electron affinity is affected by the atomic size, electronic configuration, screening effect, and nuclear charge of elements.

FAQs

1. Why does fluorine have less electron affinity as compared to Chlorine?

The atomic size of the fluorine molecules is smaller than chlorine molecule, the outer shell of fluorine is already filled with electrons and the nucleus is quite closer to the outer shell, so the electron repulsion is higher than the force of attraction of the nucleus when we put an electron in the outer shell of the fluorine molecules as compared to chlorine molecule.

2. Why is the electron affinity of the sulphur molecule higher than the oxygen molecule?

As we move down the group the atomic size of the molecule increases and the electron affinity also decreases, but sulphur is an exception, as sulphur has a -200 KJ/Mol electron affinity higher than oxygen molecules which are -142KJ/Mol. Due to the smaller size of the sulphur, the outer shell is crowded with electrons and the nucleus is close to the outer shell, so the electron repulsion is higher than the nucleus attraction that’s why the electron affinity in sulphur is higher as compared to oxygen.

3. Is electron affinity positive or negative?

When an electron is added to an element it is negative, so the electron affinity at first is negative, but when we add an electron to a negative ion it can be positive or negative due to the electron repulsion.

4. Why is the electron affinity of noble gases zero?

The electron affinity of noble gases is zero because the octet of noble gases is complete, and they do not have an affinity for electrons. That’s why the electron affinity of noble gases is zero.

5. Why does group 17 exhibit high electron affinity?

The elements of the halogens group have high electron affinity as the size of the halogens is small and the outer shell has more electrons in the outer shell. To complete their octet, it is for a halogen to accept an electron rather than to lose seven electrons.