Free Atomic Structure MCQs with Answers

11. The line spectrum of hydrogen provides direct evidence that

• A. electrons can have any energy value
• B. electron energies inside an atom are quantised
• C. the nucleus contains neutrons
• D. hydrogen atoms contain more than one electron

Explanation: A hot gas emits light only at particular wavelengths, producing separate bright lines instead of a continuous band, and each line corresponds to one fixed energy difference between two levels. If electrons could take any energy the spectrum would be continuous. Nothing in the spectrum speaks to the neutron, and hydrogen has exactly one electron.

12. Positive rays were used in the discovery of the proton. These rays were produced from

• A. the cathode of the discharge tube
• B. the residual gas atoms losing electrons in a discharge tube
• C. the radioactive decay of uranium
• D. the anode metal evaporating

Explanation: Goldstein observed that cathode rays travelling through a perforated cathode knocked electrons off the residual gas atoms, leaving positive ions that streamed towards the cathode as canal rays. Their charge to mass ratio depended on which gas was in the tube, and it was largest for hydrogen, identifying the proton. Cathode rays themselves are electrons and come from the cathode.

13. Which orbital is filled immediately after the 4s orbital according to the Aufbau principle?

• A. 4p
• B. 3d
• C. 4d
• D. 5s

Explanation: Orbitals fill in order of increasing n plus l, and where that sum ties the lower n goes first. For 3d the sum is 5 and for 4p it is also 5, but 3d has the lower principal quantum number and so fills first, which is why the first transition series follows calcium. The 4p orbitals are filled only after the 3d subshell is complete at zinc.

14. The principal quantum number n determines

• A. the shape of the orbital
• B. the main energy level and the average distance of the electron from the nucleus
• C. the orientation of the orbital in space
• D. the direction of electron spin

Explanation: The value of n fixes the shell, so a larger n means a higher energy and an electron held further out on average. Shape is set by the azimuthal quantum number l, orientation by the magnetic quantum number m, and spin by s. Knowing which of the four quantum numbers controls which property answers a large family of questions.

15. For n equals 3, the possible values of the azimuthal quantum number l are

• A. 0 only
• B. 0 and 1
• C. 0, 1 and 2
• D. 1, 2 and 3

Explanation: The azimuthal quantum number runs from 0 up to n minus 1, so for the third shell it takes the values 0, 1 and 2, corresponding to the 3s, 3p and 3d subshells. It never equals n itself, which rules out the last option. This is why the first shell has only an s subshell.

16. The maximum number of electrons that can occupy the third shell is

• A. 8
• B. 18
• C. 32
• D. 2

Explanation: Shell capacity is 2n squared, so for n equal to 3 the maximum is 2 multiplied by 9, that is 18, made up of 2 in 3s, 6 in 3p and 10 in 3d. The figure 8 is the octet that fills only the s and p subshells, which is why it is the tempting answer. The fourth shell holds 32 by the same formula.

17. The number of orbitals in a d subshell is

• A. 3
• B. 5
• C. 7
• D. 1

Explanation: The number of orbitals in a subshell is 2l plus 1, and for a d subshell l equals 2, giving five orbitals that together hold up to ten electrons. A p subshell has three orbitals and an f subshell seven. Each orbital holds a maximum of two electrons with opposite spins.

18. A node in an orbital is a region where

• A. the electron density is maximum
• B. the probability of finding the electron is zero
• C. the nucleus is located
• D. two orbitals overlap

Explanation: The wave function passes through zero at a node, so the electron is never found there, and the number of nodes increases with the principal quantum number. A 2s orbital has one radial node while a 1s has none. Nodes are a direct consequence of treating the electron as a wave.

19. Hund's rule states that within a subshell, electrons

• A. pair up in the lowest orbital first
• B. occupy separate orbitals singly with parallel spins before any pairing occurs
• C. always have opposite spins
• D. fill the highest energy orbital first

Explanation: Spreading out into empty orbitals minimises electron electron repulsion, so nitrogen's three 2p electrons occupy the three separate p orbitals rather than crowding two into one. This is why so many transition metal ions are paramagnetic. Pairing only begins once every orbital in the subshell holds one electron.

20. The Pauli exclusion principle states that

• A. no two electrons in an atom can have the same set of all four quantum numbers
• B. electrons always pair up
• C. orbitals fill in order of increasing energy
• D. electrons occupy the nucleus

Explanation: Because the first three quantum numbers define an orbital, the principle limits any orbital to two electrons, which must then differ in spin. This single rule is what gives every shell its fixed capacity and ultimately why the periodic table has the shape it does. The filling order is the separate aufbau principle.