Electron affinity (E.A) is the amount of energy released when an electron is added to a neutral gaseous atom or molecule to form a negative ion. Specifically, it is the tendency of an atom to attract electrons.
It has a negative value for most of the atoms which means that energy is released upon the addition of an electron to an atom. However 2nd E.A is always positive due to the repulsion between the anion and the incoming electron.
It is also known as the electron gain enthalpy which is the amount of energy released when one mole of a gaseous neutral atom is converted to its anions by the addition of electrons.
E.A is expressed in electron volts, kJ/mole, and Kcal/mole. The value of electron affinity cannot be determined directly. However, it can be determined by the Born-Haber cycle.
- Where E1 and E2 stand for first and second electron affinity.
First and second Electron Affinities
Adding the first electron to a neutral atom is an exothermic process. So the first electron affinity values are always negative. However, when a second electron is added to a uni-negative ion. The negative ion repels the incoming electron. So, energy is absorbed in this process. Hence, it is an endothermic process.
For example, E.A for O-1 is -142 KJ., and for O2- is +844 KJ. Here, the value of electron affinity decreases, and the addition of electrons becomes more and more difficult.
Factors affecting Electron Affinity
There are the following factors that affect electron affinity:
The relationship between E.A and atomic size is given below:
When the atomic size or atomic radius is large, the E.A will be smaller. This is because of the smaller effective nuclear charge.
Effective nuclear charge
The relation between effective nuclear charge and electron affinity is as follows:
The electron affinity is directly proportional to the effective nuclear charge. This is because the increase in nuclear charge attracts the electrons firmly.
The relationship between the shielding effect and E.A is given below:
The shielding effect increases with increasing atomic size. Hence, it decreases the attraction between the outer electron. Thus, However, E.A decreases with increasing screening effect.
Periodic Trends of Electron Affinity
Along a period
As we move from left to right in a period, E.A increases. This is because of an increase in effective nuclear charge and a decrease in size in a period.
The trend is as follows:
The order of E.A in the second period is as follows:
The order of electron affinities in the third period is given as:
Note that, beryllium and magnesium have zero values of electron affinities. This is due to the presence of completely filled s orbital in them. However, in the fifth group, the atom has a half-filled orbital and E.A is also low.
Down the group
In moving from top to bottom in a group the value of electron affinities decreases. This is because of the increase in size and decrease in the effective nuclear charge.
Exceptions to the trends of Electron Affinity
There are the following exceptions in the trends:
ELECTRON AFFINITY OF HALOGENS
The halogens do not follow the regular trend of E.A. Chlorine having greater electron affinity as compared to fluorine. It is expected, for instance, that the electron affinity of fluorine is greater due to the smaller size however, on the addition of another electron to an already crowded region, the incoming electron experiences repulsion, which decreases the tendency of the nucleus to attract that electron. Therefore, fluorine shows anomalous behavior.
The order of electron affinities of halogens is as follows:
Electron affinity of noble gases
The electron affinities of noble gases are zero. This is because of the presence of completely filled valence shell. Hence, there is no possibility of adding an extra electron.
What is E.A and its trend?
Electron affinity is the amount of energy released when an electron is added to the atom of the element. In general, it increases along the period whereas decreases down the group.
What are electron affinity and electronegativity?
E.A. is the tendency of an atom to attract a single electron and electronegativity, on the other hand, is the ability of an atom to attract a shared pair of electrons.
Which element has the highest affinity?
Chlorine has the highest E.A. As fluorine has a very small structure so more energy is required to overcome the repulsion and lesser energy is released with the addition of electrons.