Question

Arrange the elements of the second period in increasing order of their electron affinity. Name the elements which do not follow the trend in this period.

Answer 1

Hint: In the modern periodic table, the elements are arranged in an increasing order of their atomic numbers. We know that the energy released when one electron is added to a neutral gaseous atom in its ground state is known as electron affinity. When an electron is added to a neutral gaseous atom the atom forms an anion.


Complete solution:
We know that the energy released when one electron is added to a neutral gaseous atom in its ground state is known as electron affinity. When an electron is added to a neutral gaseous atom the atom forms an anion.
The elements of the second period of the periodic table are as follows:
Lithium $\left(\text{Li}\right)$, beryllium $\left(\text{Be}\right)$, boron $\left(\text{B}\right)$, carbon $\left(\text{C}\right)$, nitrogen $\left(\text{N}\right)$, oxygen $\left(\text{O}\right)$, fluorine $\left(\text{F}\right)$ and neon $\left(\text{Ne}\right)$.
In the periodic table, the trend in electron affinity is that the electron affinity increases as we move from left to right across a period.
Thus, according to the trend the increasing order of electron affinity should be as follows:
$\text{Li}<\text{Be}<\text{B}<\text{C}<\text{N}<\text{O}<\text{F}<\text{Ne}$
But if we see the electronic configuration of beryllium i.e. $1s^{2}2s^2$ we can see that the valence orbital of beryllium is completely filled. Thus, the electron cannot be added to the beryllium atom. Thus, the electron affinity of beryllium is very low.
Similarly, the electronic configuration of neon is $1s^{2}2s^{2}2p^6$ which has a completely filled valence p-orbital which results in lower electron affinity.
The electronic configuration of nitrogen is $1s^{2}2s^{2}2p^3$ which has a half-filled valence p-orbital which results in lower electron affinity.
Thus, the increasing order of electron affinity is as follows:
$$\text{Ne}<\text{N}<\text{Be}<\text{Li}<\text{B}<\text{C}<\text{O}<\text{F}$$

The elements that do not follow the trend are beryllium $\left(\text{Be}\right)$, nitrogen $\left(\text{N}\right)$ and neon $\left(\text{Ne}\right)$.


Note: In the periodic table, as we move from left to right across a period the atomic size decreases. As the atomic size decreases the electron affinity increases. This is because as the atomic size decreases the effective nuclear charge experienced by the valence electrons increases and it becomes easier to add an electron.