Free Atomic Structure MCQs with Answers

21. According to the n plus l rule, which orbital is filled first?

• A. 3d
• B. 4s
• C. 4p
• D. 4d

Explanation: For 4s the sum is 4 plus 0, that is 4, while for 3d it is 3 plus 2, that is 5, so the lower sum fills first and 4s is occupied before 3d. When two orbitals have the same sum, the one with the lower n fills first. This ordering explains why potassium and calcium come before the transition metals.

22. The electronic configuration of the chloride ion, given that chlorine has atomic number 17, is

• A. 1s2 2s2 2p6 3s2 3p5
• B. 1s2 2s2 2p6 3s2 3p6
• C. 1s2 2s2 2p6 3s2 3p4
• D. 1s2 2s2 2p6 3s1

Explanation: Chlorine gains one electron to form the chloride ion, taking its total to 18 and giving it the argon configuration with a complete octet. That stability is exactly why the ion forms so readily. Writing the neutral atom's configuration instead is the standard slip.

23. Bohr's model explained the hydrogen spectrum by proposing that

• A. electrons can occupy any orbit
• B. electrons occupy fixed energy levels and emit a photon when moving to a lower one
• C. the nucleus emits light directly
• D. electrons spiral into the nucleus

Explanation: Restricting electrons to discrete levels means only certain energy differences are available, so only certain frequencies appear, which is exactly what a line spectrum shows. Classical physics predicted a continuous spectrum and an electron spiralling into the nucleus within a fraction of a second. The photon energy equals the difference between the two levels.

24. The lines of the Balmer series in the hydrogen spectrum arise from transitions ending at

• A. n equals 1
• B. n equals 2
• C. n equals 3
• D. n equals 4

Explanation: Transitions to the second level release energies corresponding to visible light, which is why the Balmer lines are the ones seen through a spectroscope. Transitions to the ground state form the ultraviolet Lyman series and those to the third level the infrared Paschen series. The series is identified by the level the electron lands on, not the one it starts from.

25. Planck's constant has the value

• A. 6.63 times 10 to the minus 34 J s
• B. 3.0 times 10 to the 8 J s
• C. 1.6 times 10 to the minus 19 J s
• D. 9.1 times 10 to the minus 31 J s

Explanation: The extreme smallness of Planck's constant is why quantisation is invisible in everyday life and only matters at atomic scale. The other values given are the speed of light, the electronic charge and the electron mass respectively, all of which appear alongside it in the same formulae. Knowing which constant is which prevents easy marks being dropped.

26. The energy of a quantum of radiation is directly proportional to its

• A. wavelength
• B. frequency
• C. speed
• D. amplitude

Explanation: Energy equals Planck's constant multiplied by frequency, so higher frequency means more energetic quanta, and since frequency and wavelength are inversely related, energy falls as wavelength rises. This is why ultraviolet is damaging and infrared is not. Amplitude governs the number of photons, that is the intensity, rather than the energy of each.

27. The proton was discovered through experiments on

• A. cathode rays
• B. positive rays produced in a discharge tube with a perforated cathode
• C. radioactive decay of uranium
• D. the photoelectric effect

Explanation: Goldstein observed rays travelling towards the cathode and passing through holes in it, and the lightest positive particles, obtained when hydrogen filled the tube, were identified as protons. Cathode rays led to the electron instead, which is the pairing this question tests. The charge to mass ratio of these positive rays depended on the gas used, unlike that of cathode rays.

28. Unlike cathode rays, the properties of positive rays depend on the gas in the discharge tube because

• A. positive rays are electromagnetic waves
• B. the positive ions formed are the ionised atoms of that particular gas
• C. positive rays travel faster
• D. the cathode is perforated

Explanation: Positive rays are the residue of atoms that have lost electrons, so their mass differs from gas to gas, whereas cathode rays are electrons and are identical whatever the source. This difference was itself strong evidence that the electron is a universal constituent of all matter. The lightest positive particle, from hydrogen, was named the proton.

29. The magnetic quantum number m for an electron in a 2p subshell can take the values

• A. 0 only
• B. minus 1, 0 and plus 1
• C. minus 2 to plus 2
• D. plus 1 only

Explanation: The magnetic quantum number runs from minus l to plus l, and for a p subshell l equals 1, giving three values that correspond to the three perpendicular orientations of the p orbital. A d subshell with l equal to 2 would give five values. The number of values is always 2l plus 1.

30. Which set of quantum numbers is not permitted?

• A. n equals 2, l equals 1, m equals 0
• B. n equals 3, l equals 2, m equals minus 2
• C. n equals 2, l equals 2, m equals 1
• D. n equals 1, l equals 0, m equals 0

Explanation: The azimuthal quantum number can never equal or exceed n, so l equal to 2 is impossible when n is 2, which would describe a non existent 2d orbital. The other three sets satisfy every restriction. Checking l against n first, then m against l, catches almost every invalid set.