Q.Electronic configurations of four elements A, B, C and D are given below:
(A) 1s^2 2s^2 2p^6
(B) 1s^2 2s^2 2p^4
(C) 1s^2 2s^2 2p^6 3s^1
(D) 1s^2 2s^2 2p^5
Which of the following is the correct order of increasing tendency to gain electron:

Concept: Electron Gain Tendency and Periodic Trends

The tendency to gain an electron reflects electron affinity—how readily an atom accepts an additional electron. This depends on nuclear charge, atomic size, and electronic configuration stability.

Analysis of each element:

• A ($1s^2 2s^2 2p^6$): Noble gas (Ne), completely filled octet. Extremely stable; gaining an electron disrupts this stability. Virtually no tendency to gain electrons.

• C ($1s^2 2s^2 2p^6 3s^1$): Alkali metal (Na), one valence electron. Prefers to lose an electron to achieve noble gas configuration rather than gain seven. Very low tendency to gain electrons.

• B ($1s^2 2s^2 2p^4$): Oxygen group (O), needs two electrons to complete octet. Moderate tendency to gain electrons. …

The question asks us to rank these elements by their tendency to gain an electron, which is fundamentally about electron affinity and how badly an atom "wants" an extra electron to achieve stability.

The driving principle: atoms gain electrons most readily when doing so brings them closer to a stable, filled-shell configuration (typically a noble-gas structure with eight valence electrons, the octet rule). The closer an atom is to completing its valence shell, the stronger its pull on an incoming electron.

Let me first identify what each element is:

Now let's rank them step by step:

1. Element A (Neon) already has a complete octet—a perfectly stable noble-gas configuration. Adding an electron would force it into a new, higher-energy shell ($3s$) and actually destabilize the atom. Noble gases have essentially zero (or even negative) electron affinity. A has the lowest tendency to gain an electron.

2. Element C (Sodium) has one electron beyond a noble-gas core. It strongly prefers to lose that single $3s^1$ electron to return to the stable $1s^2 2s^2 2p^6$ configuration. Gaining an electron would give it $3s^2$, still far from the next octet (which requires six more electrons to reach $3s^2 3p^6$). Metals like sodium have very low electron affinity. C has the second-lowest tendency.

3. Element B (Oxygen) has six valence electrons and needs two more to complete its octet. Gaining one electron gives $2p^5$, which is closer to stability but not yet complete. Oxygen does have a reasonably high electron affinity (the first electron is readily accepted), but it's not as desperate as an element that's just one electron away. …