Arrange elements ‘A’, ‘B’, 'C', and 'D' in the ascending order of their atomic numbers based on the description given below.
(A) A has 3 electrons in O shell.
(B) B is an element with 5 electrons in the fifth shell.
(C) C is a halogen with 5 shells.
(D) D attains a pseudo inert gas configuration (18 electrons in the outermost shell) after losing 2 electrons from N shell.

The correct option is C

DABC

The atomic number of an element is the charge number of the nucleus of that atom. Thus, atomic number = number of protons. And is a neutral atom, the number of protons is always equal to the number of electrons.

Therefore, the Atomic number of an element = number of electrons

Shells and the number of electrons they can accept-

1. The electrons revolve around the nucleus of an atom in definite orbits and these orbits are referred as shells. The shells are named as K, L, M, N, O and so on.
2. The general formula to determine the maximum number of electrons in each shell is 2n².
3. The number of electrons accommodated by each shell is-

K - 2 electrons, L- 8 electrons, M- 18 electrons, N- 32 electrons, O- 50 electrons and so on.

The explanation for the correct option:

1. A has 3 electrons in O shell-

Its electron distribution is shown as

$$\begin{gathered} \mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N}\mathrm{O} \\ 2818323 \end{gathered}$$

Hence, the total number of electrons in A = 63.

2. B is an element with 5 electrons in the fifth shell- The fifth shell is O shell.

Its electronic distribution is shown as-

$$\begin{gathered} \mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N}\mathrm{O} \\ 2818325 \end{gathered}$$

Hence, the total number of electrons in B = 65.

3. C is a halogen with 5 shells.- Halogens belong to the 17th group and they have 7 electrons in their valence shell. Thus, the 5th shell will have 7 electrons.

Its electronic distribution is shown as-

$$\begin{gathered} \mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N}\mathrm{O} \\ 2818327 \end{gathered}$$

Hence, the total number of electrons in C = 67.

4.D attains a pseudo inert gas configuration (18 electrons in the outermost shell) after losing 2 electrons from N shell-

The electronic distribution of D is shown as-

$$\begin{gathered} \mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N} \\ 28182 \end{gathered}$$

Hence, the total number of electrons in D = 30.

5. Thus, the elements are arranged in ascending order of their atomic numbers as: D<A<B<C (DABC).

Conclusion: The correct option is c).