Isotopes Practice Questions with Answers

These questions cover everything a typical course expects about isotopes: identifying protons, neutrons and electrons, reading isotope notation, explaining why isotopes behave the same chemically, and calculating relative atomic mass. Try each question before looking at the answers, which are all at the end with full working.

If you need a refresher first, read isotope notation and relative atomic mass calculations.

Useful reminders

• Atomic number (Z) = number of protons.
• Mass number (A) = protons + neutrons.
• Neutrons = A − Z.
• In a neutral atom, electrons = protons.
• Relative atomic mass = Σ(isotope mass × abundance) ÷ 100, when abundances are percentages.

Section A: particles in isotopes

Q1. Complete the table.

Isotope	Protons	Neutrons	Electrons
¹²₆C
¹⁴₆C
³⁵₁₇Cl
³⁷₁₇Cl

Q2. An atom has 26 protons and 30 neutrons. (a) Which element is it? (b) Write its isotope notation.

Q3. How many neutrons are in an atom of uranium-235? (Uranium has atomic number 92.)

Q4. An ion has 20 protons, 20 neutrons and 18 electrons. Identify it and give its charge.

Section B: understanding isotopes

Q5. Define the term isotope.

Q6. Carbon-12 and carbon-14 react with oxygen in exactly the same way. Explain why.

Q7. State one physical property that differs between isotopes of the same element, and explain why.

Q8. Which of these pairs are isotopes of each other? Explain your answer. (a) ⁴⁰₁₈Ar and ⁴⁰₂₀Ca (b) ¹H and ²H (c) ¹⁶O and ¹⁶O²⁻

Section C: relative atomic mass

Q9. Chlorine consists of 75.78% chlorine-35 (mass 34.969 u) and 24.22% chlorine-37 (mass 36.966 u). Calculate the relative atomic mass of chlorine to 2 decimal places.

Q10. Boron has two isotopes: boron-10 (19.9%) and boron-11 (80.1%). Using mass numbers as isotope masses, calculate the relative atomic mass of boron to 2 decimal places.

Q11. Magnesium has three stable isotopes:

Isotope	Mass (u)	Abundance (%)
²⁴Mg	23.985	78.99
²⁵Mg	24.986	10.00
²⁶Mg	25.983	11.01

Calculate the relative atomic mass of magnesium to 2 decimal places.

Section D: working backwards

Q12. Copper has a relative atomic mass of 63.55. It has two isotopes, copper-63 (mass 62.93 u) and copper-65 (mass 64.93 u). Calculate the percentage abundance of each isotope.

Q13. Bromine’s relative atomic mass is 79.90. Its isotopes are bromine-79 (78.92 u) and bromine-81 (80.92 u). What are their approximate abundances? What does this suggest about a mass spectrum of bromine atoms?

Q14. Lithium’s relative atomic mass is 6.94. Without calculating, decide which isotope, lithium-6 or lithium-7, is more abundant, and explain your reasoning.

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Answer key

A1.

Isotope	Protons	Neutrons	Electrons
¹²₆C	6	12 − 6 = 6	6
¹⁴₆C	6	14 − 6 = 8	6
³⁵₁₇Cl	17	35 − 17 = 18	17
³⁷₁₇Cl	17	37 − 17 = 20	17

Notice that isotopes of the same element differ only in neutrons.

A2.

(a) Atomic number 26 is iron (Fe). (b) Mass number = 26 + 30 = 56, so ⁵⁶₂₆Fe, also written iron-56.

A3.

Neutrons = 235 − 92 = 143.

A4.

20 protons means calcium. It has 2 fewer electrons than protons, so its charge is 2+: Ca²⁺ (specifically ⁴⁰Ca²⁺).

A5.

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and therefore different mass numbers.

A6.

Chemical reactions involve electrons, especially valence electrons. Both isotopes have 6 protons and 6 electrons, arranged identically (1s² 2s² 2p²), so they form the same bonds and react the same way. The extra neutrons are in the nucleus and don’t affect how electrons behave in bonding. (For more, see how valence electrons determine reactivity.)

A7.

Any one of:

• Mass / density. Heavier isotopes have more neutrons, so their atoms (and compounds) are heavier. Heavy water, D₂O, is about 11% denser than ordinary water.
• Rate of diffusion. Lighter isotopes diffuse slightly faster.
• Nuclear stability. Some isotopes are radioactive (carbon-14) while others are stable (carbon-12).

Small differences in melting and boiling points also exist for compounds of different isotopes.

A8.

(a) Not isotopes. They have the same mass number but different atomic numbers (18 vs 20), so they are different elements. (Nuclei with the same mass number are called isobars.) (b) Isotopes. Both are hydrogen (Z = 1); ¹H has no neutrons and ²H (deuterium) has one. (c) Not isotopes. Both have 8 protons and 8 neutrons. They differ only in electrons, so ¹⁶O²⁻ is an ion of the same isotope.

A9.

Ar = (34.969 × 75.78 + 36.966 × 24.22) ÷ 100 = (2649.95 + 895.32) ÷ 100 = 3545.27 ÷ 100 = 35.45

This is why the periodic table shows 35.45 for chlorine, not a whole number.

A10.

Ar = (10 × 19.9 + 11 × 80.1) ÷ 100 = (199 + 881.1) ÷ 100 = 1080.1 ÷ 100 = 10.80

(Using exact isotope masses gives 10.81, the value on the periodic table.)

A11.

Ar = (23.985 × 78.99 + 24.986 × 10.00 + 25.983 × 11.01) ÷ 100 = (1894.58 + 249.86 + 286.07) ÷ 100 = 2430.51 ÷ 100 = 24.31 (to 2 d.p.)

The accepted value is 24.305.

A12.

Let the abundance of copper-63 be x%. Then copper-65 is (100 − x)%.

63.55 = [62.93x + 64.93(100 − x)] ÷ 100 6355 = 62.93x + 6493 − 64.93x 6355 − 6493 = −2.00x −138 = −2.00x x = 69.0%

So copper-63 ≈ 69% and copper-65 ≈ 31%. (Accepted values: 69.15% and 30.85%.)

A13.

Let bromine-79 be x%.

79.90 = [78.92x + 80.92(100 − x)] ÷ 100 7990 = 78.92x + 8092 − 80.92x −102 = −2.00x x = 51%

So bromine-79 ≈ 51% and bromine-81 ≈ 49%, almost equal amounts. A mass spectrum of bromine atoms would show two peaks of nearly equal height at m/z 79 and 81. (Br₂ molecules would show three peaks, at 158, 160 and 162, in a ratio of about 1:2:1.)

A14.

The relative atomic mass, 6.94, is much closer to 7 than to 6, so lithium-7 is far more abundant. (It makes up about 92.4% of natural lithium.) A weighted average always lies closer to the more abundant isotope.

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Why relative atomic masses are not whole numbers

Students often ask why the periodic table shows 35.45 for chlorine when every chlorine atom has a whole-number mass number. There are two reasons, and the questions above touch on both.

The first and biggest reason is mixing. A sample of chlorine contains two isotopes, and the value on the table is their weighted average. Because about three-quarters of chlorine atoms are chlorine-35, the average sits much nearer 35 than 37.

The second, smaller reason is that even a single isotope’s mass is not exactly a whole number of atomic mass units. Chlorine-35 has a mass of 34.969 u, not 35. Protons and neutrons each weigh slightly more than 1 u, and when they bind together in a nucleus a little mass is converted into binding energy. The result is that isotope masses differ from mass numbers by small amounts. Only carbon-12 is exactly 12 u, by definition.

So when an exam gives you mass numbers instead of exact isotope masses (as in Q10), your answer will be very slightly off from the accepted value. That’s expected, and examiners allow for it.

Common mistakes to watch for

• Using the atomic mass from the periodic table as the mass number. Mass numbers are whole numbers; the table shows weighted averages.
• Forgetting to divide by 100 when abundances are percentages.
• Mixing up isotopes and ions. Isotopes differ in neutrons, ions in electrons.
• Rounding too early in abundance calculations. Keep full values until the last step.

Key takeaways

• Isotopes have the same number of protons and electrons but different numbers of neutrons.
• They have the same chemical properties but slightly different physical and nuclear properties.
• Relative atomic mass is the abundance-weighted mean of the isotope masses.
• You can work backwards from a relative atomic mass to find abundances by setting one abundance as x.
• Want more? Try finding isotope abundance from atomic mass, or check any element’s isotopes on its page, for example chlorine.