Electron affinity and periodic variation of electron affinity.

What is electron affinity ?

The amount of energy released when an electron is added to the valence shell of an isolated gaseous atom or an ion in its ground state, is called electron affinity.

The greater the energy released in the process of taking up the extra electron, the greater is the electron affinity.

The electron affinity of an atom measures the tightness with which it binds an additional electron to itself.

The electron affinities are expressed in eV per atom or Kj /mol.

Periodic variation of inductive effect.  

In general, electron affinity decreases in going from top to bottom in a group and increases in going from left to right along a period.

Because on moving down a group the atomic size increases and therefore the effective nuclear attraction for the electron decreases.

Consequently, the atom has a less tendency to attract additional electron towards itself, i.e., its electron affinity would decrease.

On the other hand, when we move from left to right along a period, the atomic size decreases and hence the force of attraction exerted by nucleus on the electrons increases. 

Consequently, the atom has a greater tendency to attract the additional electron, i.e., its electrons affinity increases. 

Why Be and Mg have electron affinity values equal to zero?

Electron affinity of Be and Mg are practically zero. This is attributed to the extra stability of the fully complete s-orbitals in Be (2s2) and Mg (3s2).

So, they have no affinity for extra electrons to gain. If, we force to add the extra electron to their fully filled outer most orbital, then energy will be supplied.

Consequently, the electron affinity values of beryllium and magnesium are zero or slightly positive.

Electron affinity of SF₅  is very high while that of  SF₆ is only modest.

Electronic configuration of sulfur atom is 1s2, 2s2, 2p6, 3s2, 3p4The total number of electron in its outermost shell is 6. As a result, it can show a maximum of hexa-covalence.

From the concept of hybridization we can see that the structure of SF₆ is octahedral. Octahedral structure is one of the most stable structure and has very little tendency to accept additional electron.

But in SF₅ hexa-covalence of S is not achieved. As a result, SF₅ shows a very high electron affinity.

Why the first electron affinity of oxygen is positive but second electron affinity is negative?

.According to definition, electron affinity is the amount of energy released when an electron is added to a gaseous isolated atom and the value is taken as positive.

In case of oxygen, when firstelectron is added to the outer most shell, energy is released, but in addition of second electron, energy must be supplied. So the second electron affinity value is negative.

It has been found that, the value of first electron affinity of oxygen is +140.9 kJ /mol while the second electron affinity is -770 kJ /mol.

Why halogens have high electron affinity?

It has been found that halogens have high electrons affinities. This can be explained on their electronic configuration. 

The general electronic configuration of halogen atoms are ns² ns⁵. Hence, halogens elements have a strong tendency to gain an additional electron to change into the more stable electronic configuration ns² ns⁶, like inert gas.

Why the electron affinity of fluorine (F) is less  than chlorine (Cl) ?

The electronegativity of fluorine is greater than chlorine. So the electron affinity of fluorine should be greatest than chlorine. But the actual order is reversed.

The size of F atom is less than Cl atom. When an additional electron is added to the 2p sub shell of F, the electronic charge density increases which produces strong electron-electron repulsion.

But in case of Cl atom, when an additional electron is added to the 3p sub shell of Cl atom, the charge density is less increases due to its big size. 

So, electron-electron repulsion is less for the Cl atom. For the above reason the electron affinity of F atom is less than Cl atom.

Why the electron affinity of noble gases  are zero ?

The electronic configuration of noble metal elements are, s2 p6, which is more stable electronic configuration.

Since the valence shell of noble metal atoms are completely filled, so they have no chances for the addition of an extra electron. Hence the electron affinities of noble metal elements are zero.

 Summary

• What is inductive effect?
• What is periodic variation of inductive effect?
• Why the E.A of alkaline earth metals and noble gases are zero?
• Why the first EA of oxygen is positive but the second EA is negative? 
• Why halogens have high electron affinity? 

Click here: How does IP depends of atomic size and nuclear charge.