Atomic structure MCQs
MDCAT · Chemistry · 8 questions
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PMDC MDCAT syllabus: Atomic structure
Know who found what: Thomson the electron, Rutherford the nucleus (gold-foil experiment), Chadwick the neutron (1932), and Bohr the fixed energy levels. Isotopes share an atomic number (protons) but differ in neutrons. Four quantum numbers describe an electron: principal n (shell and size), azimuthal l (subshell and shape: s, p, d, f for l = 0 to 3), magnetic m (orientation; 2l + 1 orbitals, so p has 3 and d has 5) and spin s (+½ or −½). A shell holds at most 2n² electrons. Three filling rules: Aufbau (lower n + l first; for equal n + l, lower n first, so 4s before 3d before 4p), Pauli (no two electrons share all four quantum numbers, so at most two per orbital with opposite spins) and Hund (fill degenerate orbitals singly first). Learn the exceptions chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹), which favour half-filled and filled d subshells.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
The maximum number of electrons that the third shell (n = 3) can hold is:
- A. 8
- B. 18
- C. 32
- D. 9
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Answer: B. 18
A shell can hold at most 2n² electrons. For n = 3 that is 2 × 3² = 18 (3s² 3p⁶ 3d¹⁰).
Open this question on its own pageQ2 · Parcha original · Sample
How many orbitals are there in a d-subshell?
- A. 3
- B. 7
- C. 1
- D. 5
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Answer: D. 5
For a d-subshell, l = 2, and the magnetic quantum number takes 2l + 1 = 5 values, so there are five d orbitals holding up to 10 electrons.
Q3 · Parcha original · Sample
The neutron was discovered by:
- A. J. J. Thomson
- B. Ernest Rutherford
- C. James Chadwick
- D. Niels Bohr
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Answer: C. James Chadwick
James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles. Thomson discovered the electron, and Rutherford's gold-foil experiment revealed the nucleus.
Q4 · Parcha original · Sample
Which quantum number describes the orientation of an orbital in space?
- A. Magnetic quantum number
- B. Principal quantum number
- C. Azimuthal quantum number
- D. Spin quantum number
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Answer: A. Magnetic quantum number
The magnetic quantum number (m) gives the orientation of an orbital. The principal number (n) gives the shell and size, the azimuthal number (l) the subshell and shape, and the spin number (s) the electron's spin.
Q5 · Parcha original · Sample
The ground-state electronic configuration of chromium (Z = 24) is:
- A. [Ar] 3d⁴ 4s²
- B. [Ar] 3d⁶
- C. [Ar] 3d⁵ 4s¹
- D. [Ar] 4s² 4p⁴
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Answer: C. [Ar] 3d⁵ 4s¹
Chromium is an exception to the simple filling order: a half-filled 3d subshell (3d⁵) with 4s¹ is more stable than 3d⁴ 4s². Copper is a similar exception ([Ar] 3d¹⁰ 4s¹).
Q6 · Parcha original · Sample
Isotopes of an element have the same number of:
- A. Neutrons
- B. Protons
- C. Nucleons
- D. Mass units
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Answer: B. Protons
Isotopes have the same atomic number (protons) but different numbers of neutrons, so their mass numbers differ. For example, carbon-12 and carbon-14 both have 6 protons.
Q7 · Parcha original · Sample
"No two electrons in an atom can have the same set of four quantum numbers." This is:
- A. Hund's rule
- B. The Aufbau principle
- C. Heisenberg's uncertainty principle
- D. Pauli's exclusion principle
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Answer: D. Pauli's exclusion principle
Pauli's exclusion principle means an orbital holds at most two electrons, with opposite spins. Hund's rule is about filling degenerate orbitals singly first; the Aufbau principle is about filling lower-energy orbitals first.
Q8 · Parcha original · Sample
According to the Aufbau (n + l) rule, which subshell fills immediately after 4s?
- A. 4p
- B. 3d
- C. 4d
- D. 5s
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Answer: B. 3d
Subshells fill in order of increasing n + l. 4s has n + l = 4; 3d has 3 + 2 = 5 and 4p has 4 + 1 = 5. With equal n + l, the lower n fills first, so 3d comes before 4p.