What is the maximum number of electrons that can occupy the first shell of an atom?
- (a)2
- (b)4
- (c)8
- (d)16
Correct — A, 2. The maximum number of electrons a shell can hold is 2n², where n is the shell number. For the first shell — the K shell, n = 1 — this gives 2 × 1² = 2. The first shell contains only the 1s orbital, and an orbital holds at most two electrons, so the K shell is full with 2 electrons.
- (b)4 — The 2n² rule gives 2 for the first shell, not 4; no shell holds a maximum of 4 electrons.
- (c)8 — 8 is the capacity of the second shell (L shell, n = 2: 2 × 2² = 8), and also the octet limit of the outermost shell — not the first shell.
- (d)16 — 16 is not a shell capacity under the 2n² rule (n = 3 gives 18, n = 4 gives 32); the first shell holds only 2.
Electrons occupy shells (energy levels) around the nucleus, labelled K, L, M, N for n = 1, 2, 3, 4. The Bohr-Bury scheme fixes each shell's maximum capacity as 2n². The first shell is the smallest — a single s-orbital that holds two electrons.
Just apply 2n²: n = 1 gives 2. The common confusion is to jump to 8 (the octet), but 8 is the capacity of the second shell and the maximum for the outermost shell — not the first.
- Maximum electrons in a shell = 2n².
- K shell (n=1) = 2, L (n=2) = 8, M (n=3) = 18, N (n=4) = 32.
- The first shell has only the 1s orbital (2 electrons).
- The outermost shell can hold at most 8 electrons (octet rule).
The first (K) shell holds a maximum of 2 electrons — answer (a).
- Answering 8 by confusing the first shell with the octet.
- Assuming each shell simply adds 8 — capacities follow 2n² (2, 8, 18, 32).
Direct — 'maximum electrons in the K/L/M shell', or by asking for an element's electronic configuration.
In an atom, the order of filling up of the orbitals is governed by
- (a) Aufbau principle
- (b) Heisenberg’s uncertainty principle
- (c) Hund’s rule
- (d) Pauli’s exclusion principle
Answer(a) Aufbau principle
Same concept — how electrons are arranged in an atom. This NDA item tests a shell's capacity (2n² → 2 for the first shell); the UPSC item tests the rule (Aufbau) that governs the order in which those orbitals fill.
- practice — not a real PYQ
The maximum number of electrons that the second (L) shell of an atom can hold is:
- (a)2
- (b)8
- (c)18
- (d)32
Answer(b) 8 — the L shell (n = 2) holds 2 × 2² = 8 electrons.
- practice — not a real PYQ
According to the 2n² rule, the third (M) shell can hold a maximum of:
- (a)8
- (b)16
- (c)18
- (d)32
Answer(c) 18 — the M shell (n = 3) holds 2 × 3² = 18 electrons.