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Classification of Elements and Periodicity in Properties

Electron Gain Enthalpy

How many of you are aware of what electrons are? Too trivial a question, isn’t it? But, what is electron gain enthalpy? Now, that’s a tough one! Isn’t it? Well, not anymore! In this chapter, we will look at the concept of electron gain enthalpy and discuss it in greater detail.

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What is Electron Gain Enthalpy?

Electron gain enthalpy of an element is the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous negative Ion i.e. anion. We can denote it by Δ egH. Greater the amount of energy released in the above process, higher is the electron gain enthalpy of the element.

The electron gain enthalpy of an element is a measure of the firmness or strength with which an extra electron is bound to it. It is measured in electron volts per atom or kJ per mole. It can be an endothermic or exothermic reaction when you add an electron to the atom.

Electron Gain Enthalpy

Some Facts about Electron Gain Enthalpy

  • Energy is released when an electron is added to the atom. Therefore, the electron gain enthalpy is negative.
  • The electron gain enthalpy for halogens is highly negative because they can acquire the nearest stable noble gas configuration by accepting an extra electron.
  • Noble gases have large positive electron gain enthalpy. This is because the extra electron is placed in the next higher principal quantum energy levels. Thus, a highly unstable electronic configuration is produced.

Now, let us look at some of the factors that determine the electron gain enthalpy.

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Factors affecting Electron Gain Enthalpy 

1) Atomic Size

As the size of the atom increases, the distance between the nucleus and the last shell which receives the incoming electrons increases. This decreases the force of attraction between the nucleus and the incoming electron. Hence, the electron gain enthalpy becomes less negative.

2) Nuclear charge

As the nuclear charge increases, the force of attraction between the nucleus and the incoming electron increases. Hence, the enthalpy becomes more negative.

3) Electronic configuration

Elements with exactly half filled or completely filled orbitals are very stable. You have to supply energy to add an electron. Hence, their electron gain enthalpy has large positive values. The electron gain enthalpy becomes less negative in going from top to bottom in a group. It becomes more negative in going from left to right in a period.

Variation of Electron Gain Enthalpy within a Group

  • The electron gain enthalpy becomes less negative as we move down a group.
  • As we move down a group, both the atomic size and the nuclear charge increases. But the effect of the increase in atomic size is much more pronounced than the nuclear charge.
  • With the increase in atomic size, the attraction of the nucleus for the incoming electron decreases. Hence, the electron gain enthalpy becomes less negative.
  • Chlorine has the most negative electron gain enthalpy.

Variation along a Period

  • Electron gain enthalpy becomes more and more negative from left to right in a period.
  • As we move across a period from left to right the atomic size decreases and the nuclear charge increases. Both these factors tend to increase the attraction by the nucleus for the incoming electron. Hence, electron gain enthalpy becomes more and more negative in a period from left to right.
  • Halogens have the most negative electron gain enthalpy. As we move from chlorine to iodine, the electron gain enthalpies become less and less negative due to the corresponding increase in their atomic radii.
  • As the distance of the nucleus from the subshell which receives the additional electron increases, the force with which it is attracted by the nucleus decreases and hence the electron gain enthalpy become less negative as we move down the group from Cl —-> Br —-> I

The Electron Gain Enthalpy of Fluorine is Less Negative than that of Chlorine

This is due to its small size. As a result of its small size, the electron-electron repulsion in the relatively compact to 2p subshell are comparatively large. Hence the incoming electron is not accepted with the same ease as is the case with chlorine.

The Electron Gain Enthalpy of Noble Gases is Positive

The atoms of these elements have completely filled subshell. As a result, there is no room in their valence orbitals and the additional electron has to be placed in an orbital of next higher shell. As a result, energy has to be supplied to add on additional electrons.

A Solved Question for You

Q: Explain why the electron gain enthalpies of some of the elements of 2nd period i.e. O and F are less negative than the corresponding elements of the third period.

Ans: The elements of the second period have smallest atomic size among the elements in their respective group. As a result, there are considerable electron-electron repulsions within the atom itself and hence the additional electron is not accepted with the same ease as is the case with the remaining elements in the same group.

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