Answer: 4s.
- A 3d
- B 4p
- C 4d
- D 4s
Correct answer: D. 4s
Explanation: For 4s, (n + l) = 4 + 0 = 4, which is lower than 3d (3 + 2 = 5) and 4p (4 + 1 = 5), so 4s fills first.
Bohr's model: electrons occupy fixed circular shells (K, L, M) around the nucleus, with each shell holding up to 2n2 electrons.
Concept context
Explore how electrons are arranged inside an atom. Covers Bohr's model, quantum numbers, Heisenberg's uncertainty principle, shapes of s, p, d orbitals, and electronic configuration rules.