At the end of the article, you will able to describe electron affinity trend. Let’s get started. We have learned that when a neutral gaseous atom is to lose an electron, the ionization energy is required. In case the atom is to gain an electron from outside atom or ion to form negative ion (or anion). The energy which is involved in doing so is called electron gain affinity.
The amount of energy released when an extra electron is added to a neutral gaseous atom to form a monovalent anion is called as electron affinity or first electron affinity (EA). For example:
A(g)+e → A– Energy (EA1)
- It is measured as the capacity of an addition of an extra electron to an atom. It is an exothermic process for, EA1→∆H = negative.
- EA1 is an endothermic process because both (electrons) have the same charge with a lot of repulsions. Thus, the addition of an electron to A– requires energy to remove the force of repulsion. ∆H = + Ve, A–+e–+ Energy → A2-
- Its values are expressed either in electron volt (eV) or kJ mol-1.
Electron Affinity Trend
Electron affinity trend varies along period and group. Electron Gain affinity generally becomes more negative down a group and more negative along a period, though there are certain exceptions as well. Remember, Elements with symmetrical configurations and noble gas have positive electron gain affinity.
In a period – When we move from left to right along a period, the atomic size decreases and nuclear charge increases. Thus the incoming electron is attached strongly. Hence the value of E.A. increases in a period from left to right.
Exception – N, Ne, alkaline earth metals and noble gases. In Be, Ne, alkaline alkaline earth metals and noble gases due to fully filled orbitals and in N due to Half filled orbitals.
Group – As we move from top to bottom in a group the value of E.A decreases because the atomic size increases and also the nuclear charge but effect of increasing atomic size is greater than that of an effect of nuclear charge. Thus, the force of attraction between the incoming electron and nucleus decreases.
Exceptions – The E.A. of 2nd period is lower than E.A. of 3rd-period elements.
Reason In the 2nd-period elements incoming electron enters into the 2p orbital whereas in 3rd-period elements incoming electron enters into the 3p orbital. Former will experience more repulsion. Hence, E.A. (F)<E.A.(Cl).
Note Halogens have the highest E.A.
- Small size and high nuclear charge.
- To gain a noble gas configuration by accepting only one electron [ns2np5→ns2np6].
This is all about the basics of Electron Affinity Trend.