IMAT - International Medical Admissions Atomic Structure and Periodicity Questions and Answers — Questions and Answers

Question 1: An atom of the isotope chlorine-37 (³⁷Cl) contains which of the following combinations of subatomic particles?

17 protons, 20 neutrons, 18 electrons
17 protons, 20 neutrons, 17 electrons (Correct answer)
20 protons, 17 neutrons, 20 electrons
17 protons, 37 neutrons, 17 electrons

Correct answer: 17 protons, 20 neutrons, 17 electrons

The superscript (37) is the mass number, which is the sum of protons and neutrons. The atomic number for Chlorine (Cl) from the periodic table is 17, which represents the number of protons. Therefore, the number of neutrons is the mass number minus the atomic number (37 - 17 = 20). In a neutral atom, the number of electrons is equal to the number of protons.

Question 2: Which of the following elements has the highest first ionisation energy?

Sodium (Na)
Oxygen (O)
Neon (Ne) (Correct answer)
Fluorine (F)

Correct answer: Neon (Ne)

First ionisation energy generally increases across a period (from left to right) and decreases down a group. Oxygen, Fluorine, and Neon are in Period 2, while Sodium is in Period 3, so Sodium has a relatively low ionisation energy. Among the Period 2 elements, Neon is a noble gas with a full, stable valence electron shell. This stable configuration makes it extremely difficult to remove an electron, giving it the highest first ionisation energy of the group.

Question 3: What is the correct ground-state electron configuration for the iron(III) ion, Fe³⁺?

[Ar] 4s²3d³
[Ar] 4s¹3d⁴
[Ar] 3d⁵ (Correct answer)
[Ar] 4s²3d⁹

Correct answer: [Ar] 3d⁵

The neutral iron atom (Fe, atomic number 26) has the electron configuration [Ar] 4s²3d⁶. To form a cation, electrons are first removed from the orbital with the highest principal quantum number (n). In this case, the two electrons from the n=4 shell (4s²) are removed first. To achieve a +3 charge, a third electron is removed from the 3d orbital. This results in the final configuration of [Ar] 3d⁵.

Question 4: An electron in an atom is described by a set of four quantum numbers. Which of the following sets of quantum numbers (n, l, m_l, m_s) is not permissible?

(3, 3, -2, -1/2) (Correct answer)
(4, 0, 0, -1/2)
(2, 1, 0, +1/2)
(3, 2, -1, +1/2)

Correct answer: (3, 3, -2, -1/2)

The rules for quantum numbers dictate the possible values for each. The principal quantum number (n) can be any positive integer. The angular momentum quantum number (l) can be any integer from 0 to n-1. The magnetic quantum number (m_l) can range from -l to +l. In option A, n=3 and l=3. This is not permissible because the value of l must be less than n. The maximum value for l when n=3 is 2.

Question 5: Which of the following species, which are all isoelectronic with argon, has the largest radius?

K⁺
S²⁻ (Correct answer)
Ar
Cl⁻

Correct answer: S²⁻

All the given species (K⁺, S²⁻, Ar, Cl⁻) are isoelectronic, meaning they have the same number of electrons (18). When comparing the size of isoelectronic species, the radius is determined by the nuclear charge (number of protons). The species with the fewest protons will have the largest radius because its electrons are held less tightly by the nucleus. Sulfur (S) has 16 protons, Chlorine (Cl) has 17, Argon (Ar) has 18, and Potassium (K) has 19. Therefore, S²⁻ has the weakest nuclear pull on the 18 electrons and is the largest species.

Question 6: Which of the following covalent bonds is the most polar?

C-H
F-H (Correct answer)
N-H
O-H

Correct answer: F-H

The polarity of a bond is determined by the difference in electronegativity (ΔEN) between the two atoms. A larger difference results in a more polar bond. Comparing the options, Fluorine (F) is the most electronegative element on the periodic table. The electronegativity difference between Fluorine (Pauling scale value ≈ 3.98) and Hydrogen (≈ 2.20) is the greatest among the given pairs, making the F-H bond the most polar.

IMAT - International Medical Admissions Atomic Structure and Periodicity Questions and Answers

An atom of the isotope chlorine-37 (³⁷Cl) contains which of the following combinations of subatomic particles?