IB Chemistry Quiz: Understand The Nuclear Atom

20 questions · exam conditions

0:00

Understand The Nuclear AtomQuestion 1 of 20

An atom has 16 neutrons, and its most stable ion has a charge of 3- and contains 18 electrons. What is the mass number of this atom?

← Back to quizzes

IB Chemistry Quiz

IB Chemistry Quiz: Understand The Nuclear Atom

Practice Understand The Nuclear Atom in IB Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Understand The Nuclear Atom, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

An atom has 16 neutrons, and its most stable ion has a charge of 3- and contains 18 electrons. What is the mass number of this atom?

29
31 (correct answer)
33
34

Explanation: The ion has a 3- charge, meaning it has gained 3 electrons to reach 18. The neutral atom therefore has 18 - 3 = 15 electrons. In a neutral atom, the number of electrons equals the number of protons, so the atomic number (Z) is 15. The mass number (A) is the sum of protons and neutrons: A = 15 (protons) + 16 (neutrons) = 31.

Question 2

An isotope of element X has a mass number of 58 and contains 30 neutrons. An isotope of element Y has a mass number of 60 and contains 32 neutrons. What is the relationship between neutral atoms of X and Y?

They are isobars of different elements.
They are isotopes of the same element. (correct answer)
They are in the same period but different groups.
They have different numbers of protons but the same chemical properties.

Explanation: For element X: Atomic number (Z) = Mass number (A) - Number of neutrons (n) = 58 - 30 = 28. For element Y: Atomic number (Z) = 60 - 32 = 28. Since both X and Y have an atomic number of 28, they are both atoms of the element Nickel (Ni). Atoms with the same number of protons but different numbers of neutrons (and thus different mass numbers) are isotopes of the same element.

Question 3

A sample of boron is composed of two isotopes, $^{10}\text{B}$ and $^{11}\text{B}$ . If the relative atomic mass of the sample is 10.80, what is the percentage abundance of the $^{10}\text{B}$ isotope?

20.0% (correct answer)
80.0%
25.0%
50.0%

Explanation: Let the fractional abundance of

^{10}\text{B}

be x. Then the fractional abundance of

^{11}\text{B}

is (1-x). The relative atomic mass is the weighted average:

10.80 = (10 \times x) + (11 \times (1-x))

. Solving for x:

10.80 = 10x + 11 - 11x

;

10.80 = 11 - x

;

x = 11 - 10.80 = 0.20

. The percentage abundance is 0.20 × 100 = 20.0%.

Question 4

Gallium has a relative atomic mass of 69.72 and consists of two stable isotopes, $^{69}\text{Ga}$ and $^{71}\text{Ga}$ . A second element, Z, also has only two stable isotopes, $^{68}\text{Z}$ and $^{70}\text{Z}$ . If the percentage abundance of $^{68}\text{Z}$ is the same as the percentage abundance of $^{69}\text{Ga}$ , what is the relative atomic mass of element Z?

68.72 (correct answer)
69.00
69.28
69.72

Explanation: First, find the abundance of

^{69}\text{Ga}

. Let its fractional abundance be x. Then

69.72 = 69x + 71(1-x)

. This gives

69.72 = 71 - 2x

, so

2x = 1.28

and

x = 0.64

. The abundance of

^{69}\text{Ga}

is 64%. The problem states the abundance of

^{68}\text{Z}

is the same, so its abundance is 64%. The abundance of

^{70}\text{Z}

must be 100% - 64% = 36%. The relative atomic mass of Z is the weighted average:

A_r(Z) = (68 \times 0.64) + (70 \times 0.36) = 43.52 + 25.20 = 68.72

Question 5

Which pair of species has the same number of neutrons? (Atomic numbers: Ar=18, K=19, Ca=20)

$^{40}\text{Ar}$ and $^{40}\text{Ca}^{2+}$
$^{39}\text{K}^{+}$ and $^{40}\text{Ca}^{2+}$ (correct answer)
$^{39}\text{K}$ and $^{40}\text{K}$
$^{41}\text{Ca}$ and $^{39}\text{K}$

Explanation: To find the number of neutrons, subtract the atomic number (Z) from the mass number (A). A:

^{40}\text{Ar}

has 40-18=22 neutrons;

^{40}\text{Ca}^{2+}

has 40-20=20 neutrons. B:

^{39}\text{K}^{+}

has 39-19=20 neutrons;

^{40}\text{Ca}^{2+}

has 40-20=20 neutrons. They have the same number of neutrons. C: These are isotopes of potassium with different numbers of neutrons (20 and 21, respectively). D:

^{41}\text{Ca}

has 41-20=21 neutrons;

^{39}\text{K}

has 39-19=20 neutrons.

Question 6

Naturally occurring chlorine has a relative atomic mass of 35.45 and consists of isotopes $^{35}\text{Cl}$ and $^{37}\text{Cl}$ . A specific sample is enriched such that the mole ratio of $^{35}\text{Cl}$ to $^{37}\text{Cl}$ is 1:1. What is the relative atomic mass of this enriched sample?

35.45
36.55
35.50
36.00 (correct answer)

Explanation: A 1:1 mole ratio means the sample contains 50%

^{35}\text{Cl}

and 50%

^{37}\text{Cl}

. The relative atomic mass is the weighted average of the isotopic masses. For this sample,

A_r = (35 \times 0.50) + (37 \times 0.50) = 17.5 + 18.5 = 36.00

. The natural abundance does not need to be calculated but is approximately 77.5%

^{35}\text{Cl}

and 22.5%

^{37}\text{Cl}

, which results in the natural

A_r

of 35.45.

Question 7

What is the total number of neutrons in a 3.40 g sample of ammonia gas ( $\text{NH}_3$ )? (Assume nitrogen is $^{14}\text{N}$ and hydrogen is $^{1}\text{H}$ . $N_A = 6.02 \times 10^{23} \text{ mol}^{-1}$ )

$8.43 \times 10^{23}$ (correct answer)
$1.20 \times 10^{24}$
$1.40$
$2.05 \times 10^{24}$

Explanation:

Molar mass of $\text{NH}_3 = 14.0 + 3(1.0) = 17.0 \text{ g mol}^{-1}$ . 2. Moles of $\text{NH}_3 = \frac{\text{mass}}{\text{molar mass}} = \frac{3.40 \text{ g}}{17.0 \text{ g mol}^{-1}} = 0.200 \text{ mol}$ . 3. Neutrons per molecule: $^{14}\text{N}$ has 14-7=7 neutrons. $^{1}\text{H}$ has 1-1=0 neutrons. Total per molecule = 7 + 3(0) = 7 neutrons. 4. Total neutrons = moles $\times$ Avogadro constant $\times$ neutrons per molecule = $0.200 \text{ mol} \times 6.02 \times 10^{23} \text{ mol}^{-1} \times 7 = 8.428 \times 10^{23}$ .

Question 8

When comparing the chemical reactions of propan-1-ol ( $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ ) and its deuterated isotopologue ( $\text{CH}_3\text{CH}_2\text{CH}_2\text{OD}$ ), which statement is most accurate? (D is the isotope $^{2}\text{H}$ )

The deuterated compound will react significantly faster in all reactions due to the greater mass of deuterium.
The deuterated compound is non-polar while propan-1-ol is polar, leading to major differences in reactivity.
The two compounds will form different organic products when oxidized due to their different atomic compositions.
The two compounds are expected to have nearly identical chemical properties as they have the same electron configuration. (correct answer)

Explanation: Isotopes have the same number of protons and electrons, leading to identical electron configurations. Since chemical reactions are governed by electron interactions, isotopes of an element exhibit virtually identical chemical properties. While there can be small differences in reaction rates (the kinetic isotope effect), for the purposes of IB Chemistry, their chemical behavior is considered the same. Both molecules are polar and would form the same oxidation products.

Question 9

An ion X³⁻ contains 50 neutrons and has a total of 36 electrons. In which group and period of the periodic table is element X found?

Group 15, Period 4 (correct answer)
Group 17, Period 4
Group 16, Period 3
Group 15, Period 3

Explanation: The ion has a 3- charge and 36 electrons. The neutral atom of X must have 36 - 3 = 33 electrons, and therefore 33 protons. The atomic number Z=33 identifies the element as Arsenic (As). The electron configuration of As is [Ar] 4s² 3d¹⁰ 4p³. The highest principal energy level (n) is 4, so it is in Period 4. As a p-block element, its group number is 10 + (number of valence s and p electrons) = 10 + (2 + 3) = 15. Thus, it is in Group 15, Period 4.

Question 10

Two atoms, P and R, undergo nuclear processes. Atom P has 26 protons and 30 neutrons initially. After beta-minus decay, the resulting nucleus captures 2 electrons to form an ion. Atom R starts with the same mass number as the final form of P but has 2 fewer neutrons. How many protons does atom R contain?

25 protons in atom R's nuclear structure
27 protons in atom R's nuclear structure
29 protons in atom R's nuclear structure (correct answer)
31 protons in atom R's nuclear structure

Explanation: Atom P starts with 26 protons and 30 neutrons (mass number = 56). After beta-minus decay, a neutron converts to a proton, so the new nucleus has 27 protons and 29 neutrons (mass number still = 56). Atom R has the same mass number (56) but 2 fewer neutrons than P's final form, so R has 29 - 2 = 27 neutrons. Since mass number = protons + neutrons: 56 = protons + 27, so R has 56 - 27 = 29 protons.

Question 11

Two isotopes of the same element have mass numbers that differ by 4. If the lighter isotope has 18 neutrons and the heavier isotope has a neutron-to-proton ratio of 1.22, how many protons does this element have?

16 protons
18 protons (correct answer)
20 protons
22 protons

Explanation: Let the number of protons be P (same for both isotopes). Light isotope: mass number = P + 18. Heavy isotope: mass number = P + 18 + 4 = P + 22, and neutrons = 22. For the heavy isotope, neutron/proton ratio = 22/P = 1.22, so 22 = 1.22P, giving P = 22/1.22 = 18.03 ≈ 18. Check: light isotope has 18 protons + 18 neutrons = mass 36; heavy isotope has 18 protons + 22 neutrons = mass 40 (differs by 4 ✓). Neutron/proton ratio = 22/18 = 1.22 ✓.

Question 12

Consider the nuclear equation: $^{238}$ U → $^{234}$ Th + X. After this decay process occurs, a thorium atom gains 2 electrons to achieve a stable electron configuration. What is the neutron-to-proton ratio in the resulting thorium ion?

1.54 neutrons per proton in the ionic species
1.61 neutrons per proton in the ionic species (correct answer)
2.54 neutrons per proton in the ionic species
2.61 neutrons per proton in the ionic species

Explanation: The decay shows

^{238}

U (92 protons) →

^{234}

Th + X, where X is an alpha particle.

^{234}

Th has mass number 234 and atomic number 90 (thorium), so it has 90 protons and 234 - 90 = 144 neutrons. When the thorium atom gains 2 electrons, it becomes Th²⁻, but the nuclear composition doesn't change. The neutron-to-proton ratio remains 144/90 = 1.60 ≈ 1.61.

Question 13

An element exists as two naturally occurring isotopes. Isotope 1 has a mass of 62.930 u and accounts for 69.17% abundance. Isotope 2 has a mass of 64.928 u and accounts for 30.83% abundance. Based on this isotopic data, how many neutrons are present in the nucleus of the more abundant isotope?

29 neutrons in the predominant nuclear isotope
34 neutrons in the predominant nuclear isotope (correct answer)
35 neutrons in the predominant nuclear isotope
36 neutrons in the predominant nuclear isotope

Explanation: First, calculate the average atomic mass: (0.6917 × 62.930) + (0.3083 × 64.928) = 43.52 + 20.02 = 63.54 u. This corresponds to copper (Cu, atomic number 29). The more abundant isotope has mass ≈ 63 u (rounded from 62.930). Since atomic number = 29 protons, neutrons = mass number - protons = 63 - 29 = 34 neutrons.

Question 14

Which property is expected to be significantly different between the isotopes uranium-235 and uranium-238?

The charge on their most stable ions under identical conditions.
The empirical formula of the oxide formed when they burn in excess oxygen.
The rate of diffusion of their gaseous hexafluoride compounds, $\text{UF}_6$ . (correct answer)
The number of valence electrons in a neutral atom of each isotope.

Explanation: Isotopes of an element have the same number of protons and electrons, resulting in identical electron configurations and thus nearly identical chemical properties (A, B, D). However, they have different numbers of neutrons and therefore different masses. Physical properties that depend on mass, such as density, boiling point, and rate of diffusion, will differ. According to Graham's Law of Effusion/Diffusion, the rate is inversely proportional to the square root of the molar mass. Since

^{235}\text{UF}_6

and

^{238}\text{UF}_6

have different masses, their rates of diffusion will be significantly different.

Question 15

A particle has a mass number of 32, contains 16 neutrons, and possesses 18 electrons. Which of the following correctly identifies this particle?

A neutral sulfur atom, $^{32}\text{S}$
A sulfide ion, $^{32}\text{S}^{2-}$ (correct answer)
A phosphide ion, $^{31}\text{P}^{3-}$
An isotope of a chloride ion, $^{34}\text{Cl}^{-}$

Explanation: The number of protons (atomic number, Z) is calculated by subtracting the number of neutrons from the mass number: Z = 32 - 16 = 16. The element with Z=16 is Sulfur (S). The particle has 16 protons and 18 electrons. The charge is the number of protons minus the number of electrons: Charge = 16 - 18 = -2. Therefore, the particle is the sulfide ion,

^{32}\text{S}^{2-}

Question 16

Which of the following statements about the fundamental subatomic particles is correct?

The mass of a neutron is slightly less than the mass of a proton.
Protons and electrons have charges of equal magnitude but opposite sign. (correct answer)
Protons and neutrons are found in the nucleus and have approximately equal charges.
The electron is the least massive particle and has no charge.

Explanation: The charge of a proton is +1 (relative) and an electron is -1 (relative), so their charges are of equal magnitude and opposite sign. Option A is incorrect; a neutron is slightly more massive than a proton. Option C is incorrect; neutrons have no charge. Option D is incorrect; electrons have a negative charge.

Question 17

Which statement correctly explains why the mass number of a specific isotope is always an integer, while the relative atomic mass of an element is often not an integer?

Mass number is a measured quantity that includes binding energy, while relative atomic mass is a simple count of nucleons.
Mass number is an approximation, while relative atomic mass is an exact measurement of the mass of the most common isotope.
Mass number is a count of nucleons in a single nucleus, whereas relative atomic mass is a weighted average of isotopic masses. (correct answer)
Mass number is based on the carbon-12 scale, while relative atomic mass is an absolute value based on the mass of a proton.

Explanation: The mass number (A) represents the total number of protons and neutrons (nucleons) in the nucleus of a single atom of a specific isotope. Since this is a count of particles, it is always an integer. The relative atomic mass (

A_r

) of an element is the weighted average of the masses of its naturally occurring isotopes. Because it is an average of different isotopic masses with non-integer abundances, it is typically not an integer.

Question 18

Consider two species, $^{A}_{Z}X$ and $^{B}_{Z+1}Y$ . If these two species have the same number of neutrons, what is the relationship between their mass numbers, A and B?

A = B + 1
A = B + 2
A = B
A = B - 1 (correct answer)

Explanation: The number of neutrons in a species is given by its mass number minus its atomic number. For species X: neutrons = A - Z. For species Y: neutrons = B - (Z+1) = B - Z - 1. If they have the same number of neutrons: A - Z = B - Z - 1. Adding Z to both sides: A = B - 1.

Question 19

An ion, X³⁺, has a mass number of 52 and contains 21 electrons. What is the composition of the nucleus of a neutral atom of X?

21 protons and 31 neutrons
24 protons and 21 neutrons
27 protons and 25 neutrons
24 protons and 28 neutrons (correct answer)

Explanation: The ion X³⁺ has a charge of +3, meaning it has 3 fewer electrons than protons. Since it has 21 electrons, the number of protons is 21 + 3 = 24. The atomic number (Z) is 24, which identifies the element as Chromium (Cr). The composition of the nucleus is determined by the number of protons and neutrons, which is the same for the ion and the neutral atom. The number of neutrons is the mass number minus the number of protons: 52 - 24 = 28. Therefore, the nucleus contains 24 protons and 28 neutrons.

Question 20

Which statement best defines the relative atomic mass, $A_r$ , of an element?

The total number of protons and neutrons in the most abundant isotope of the element.
The weighted average mass of an atom of the element compared to 1/12th the mass of an atom of carbon-12. (correct answer)
The mass of one mole of atoms of the element measured in grams, divided by the Avogadro constant.
The average number of nucleons in the atoms of a naturally occurring sample of the element.

Explanation: The relative atomic mass (

A_r

) is the weighted average mass of the naturally occurring isotopes of an element relative to a standard. The standard is 1/12th of the mass of a single neutral atom of carbon-12 in its ground state. Option A describes the mass number of an isotope. Option C describes the mass of a single atom in grams, not the relative mass. Option D is an imprecise description that omits the reference standard.