The number of maximum electrons in N Shell is

Correct — D, 32. The maximum number of electrons a shell can hold is 2n2, where n is the shell number. The N shell is the fourth shell (K=1, L=2, M=3, N=4), so its capacity is 2 x 4 squared = 2 x 16 = 32 electrons.

Why the others are wrong

• (a)
  2 — 2 is the capacity of the first (K) shell, where n = 1 gives 2 x 1 squared = 2.
• (b)
  8 — 8 is the capacity of the second (L) shell, where n = 2 gives 2 x 2 squared = 8.
• (c)
  18 — 18 is the capacity of the third (M) shell, where n = 3 gives 2 x 3 squared = 18.

Electron shells are labelled K, L, M, N for the principal quantum numbers n = 1, 2, 3, 4. Each shell holds at most 2n2 electrons, so the capacities run 2, 8, 18 and 32. The outermost shell of a stable neutral atom, however, never holds more than 8 (the octet rule).

Key facts

• The maximum number of electrons in a shell equals 2n2.
• K, L, M and N shells correspond to n = 1, 2, 3, 4.
• Their capacities are 2, 8, 18 and 32 electrons respectively.
• The outermost shell holds at most 8 electrons in a stable neutral atom (octet rule).

Common traps

• Reading 'N shell' as the first shell instead of the fourth.
• Forgetting to square n in the 2n2 formula.

• practice — not a real PYQ

  The maximum number of electrons that the M shell can hold is

  • (a)2
  • (b)8
  • (c)18
  • (d)32
  Answer(c) 18 — from 2 x 3 squared for n = 3.
• practice — not a real PYQ

  The rule giving the maximum number of electrons in a shell is

  • (a)n squared
  • (b)2n
  • (c)2n squared
  • (d)n + 2
  Answer(c) 2n squared, where n is the shell number.