Answer & solution
Answer: (a) K, L, M, N, O….
Correct Answer: Option 1 — K, L, M, N, O….
In Niels Bohr's model of the atom (1913), electrons revolve around the nucleus only in certain fixed, discrete paths called orbits, shells or energy levels. These shells are numbered n = 1, 2, 3, 4, 5 … and are named by the letters K, L, M, N, O … in that order.
An electron moving in one of these shells does not radiate energy, so it never spirals into the nucleus — this was Bohr's correction of the main defect of Rutherford's model. Energy is absorbed or emitted only when an electron jumps from one shell to another, and the amount is a fixed quantum, E₂ − E₁.
The maximum number of electrons a shell can hold is given by the Bohr–Bury rule, 2n² — so K = 2, L = 8, M = 18, N = 32 — while the outermost shell can never hold more than 8 electrons.
❌ Why the other options are incorrect
✗ W, X, Y, Z, A…. — these letters are not used for shells at all; X, Y, Z are ordinary algebraic symbols in chemistry, not shell names.
✗ R, S T, U, V…. — no shell carries these letters; s, p, d, f (small letters) are the names of sub-shells/orbitals, which is a different idea altogether.
✗ A, B, C, D, E…. — shell naming begins at K, not at A; the sequence was chosen historically from X-ray spectroscopy, where the innermost series was labelled the K-series.
📌 SSC & Railway Exam Facts
• Shells: K (n=1), L (n=2), M (n=3), N (n=4), O (n=5); capacity by 2n² = 2, 8, 18, 32, 50.
• The outermost (valence) shell can hold a maximum of 8 electrons — the octet rule.
• Valency is decided by the number of electrons in the outermost shell.
• Bohr's model corrected Rutherford's defect: electrons in a fixed orbit do not radiate energy.
• Niels Bohr received the Nobel Prize in Physics, 1922; sub-shells are named s, p, d, f.