The maximum number of electrons that can be accommodated in the $n$-th shell of an atom is:
A$n^2$
B$n$
C$2n$
D$2n^2$
Answer & Solution
Correct answer: D. $2n^2$
Bohr-Bury rule. Each shell of principal quantum number $n$ contains $n^2$ orbitals, and each orbital holds at most 2 electrons (Pauli). So shell capacity is $2n^2$.
Quick check: K shell ($n=1$) holds 2, L ($n=2$) holds 8, M ($n=3$) holds 18, N ($n=4$) holds 32. All powers-of-two-friendly numbers.
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