Isotope Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Isotope.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

Atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different mass numbers.

Same element, slightly different weight. Chemically identical, but different mass.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Isotope starts by naming the element, charge, and relevant protons, neutrons, or electrons.

Common stuck point: Students often know a formula related to isotope but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?

Worked Examples

Example 1

easy

Carbon has three naturally occurring isotopes:

{}^{12}\text{C}

{}^{13}\text{C}

, and

{}^{14}\text{C}

. How many protons and neutrons does each have?

Carbon-14: 6 protons, 8 neutrons — same protons as C-12 and C-13

Answer

{}^{12}\text{C: 6p, 6n}\quad {}^{13}\text{C: 6p, 7n}\quad {}^{14}\text{C: 6p, 8n}

First step

Isotopes of the same element have the same atomic number but different mass numbers. All carbon isotopes have

Z = 6

, so each has 6 protons.

Full solution

2
Use the formula $N = A - Z$ to find neutrons for each isotope: ${}^{12}\text{C}$ : $12 - 6 = 6$ neutrons; ${}^{13}\text{C}$ : $13 - 6 = 7$ neutrons; ${}^{14}\text{C}$ : $14 - 6 = 8$ neutrons.
3
Summary: ${}^{12}\text{C}$ (6n), ${}^{13}\text{C}$ (7n), ${}^{14}\text{C}$ (8n) — same element, different neutron counts.

Isotopes are atoms of the same element (same number of protons) that differ in the number of neutrons. They have identical chemical properties but different masses.

Example 2

medium

Boron has two stable isotopes:

{}^{10}\text{B}

(19.9%) and

{}^{11}\text{B}

(80.1%). Calculate the average atomic mass of boron.

Example 3

medium

Chlorine has two stable isotopes:

{}^{35}\text{Cl}

(75.78%) and

{}^{37}\text{Cl}

(24.22%). Calculate the average atomic mass.

Example 4

hard

An element has two isotopes with masses 84.91 amu and 86.91 amu. Its average atomic mass is 85.47 amu. Find the percent abundance of each.

Example 5

challenge

Lithium has isotopes Li-6 and Li-7 with average atomic mass 6.94 amu. Find the abundance of each.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

hard

Copper has two stable isotopes:

{}^{63}\text{Cu}

and

{}^{65}\text{Cu}

. The average atomic mass of copper is

63.55

amu. Calculate the percent abundance of each isotope.

Example 2

medium

Chlorine-35 and chlorine-37 are both chlorine. What is different about them, and what stays the same?

Chlorine-37: 17 protons, 20 neutrons — same protons as Cl-35 but 2 more neutrons

Example 3

easy

What are isotopes?

Example 4

easy

Carbon-12 and carbon-14 have the same number of which particle?

Carbon-12: 6 protons — the proton count is the same in every carbon isotope

Example 5

easy

Do isotopes of the same element have the same chemical properties?

Example 6

easy

Carbon-12 has 6 neutrons. How many neutrons does carbon-13 have?

Carbon-13: 6 protons and 7 neutrons

Example 7

easy

Are all isotopes radioactive?

Example 8

easy

How do isotopes differ from ions?

Example 9

easy

Two atoms have 8 protons each but 8 and 10 neutrons. Are they isotopes?

Oxygen-16: 8 protons, 8 neutrons

Example 10

easy

Hydrogen-1, hydrogen-2, and hydrogen-3 all have how many protons?

Hydrogen-2 (deuterium): 1 proton — all hydrogen isotopes share this single proton

Example 11

medium

Chlorine has two isotopes, Cl-35 and Cl-37. How many neutrons does each have if

Z = 17

Chlorine-35: 17 protons, 18 neutrons

Example 12

medium

Why is the atomic mass of an element usually not a whole number?

Example 13

medium

An element has isotopes of mass number 10 (20%) and 11 (80%). Find the average atomic mass.

Example 14

medium

Carbon-14 is radioactive but carbon-12 is stable. What differs between them, and what stays the same?

Carbon-14: 6 protons (same as C-12) but 8 neutrons (vs. 6 in C-12)

Example 15

medium

Are

\text{O}^{2-}

and oxygen-18 the same kind of variation from a neutral O-16 atom? Explain.

Example 16

medium

Element X has isotopes X-63 and X-65 with

Z=29

. Which isotope is heavier and by how many neutrons?

Copper-63: 29 protons, 34 neutrons

Example 17

medium

Uranium-235 and uranium-238 both have

Z=92

. How many more neutrons does U-238 have?

Example 18

medium

Oxygen-16 and oxygen-18 both have 8 protons. How many neutrons does each have?

Oxygen-16: 8 protons, 8 neutrons — oxygen-18 has the same protons but 10 neutrons

Example 19

medium

Are hydrogen-1 and hydrogen-2 (deuterium) isotopes, ions, or different elements?

Example 20

challenge

Chlorine is 75% Cl-35 and 25% Cl-37. Compute its average atomic mass and explain why it matches the periodic table's 35.5.

Example 21

challenge

An element has only two isotopes, mass numbers 69 and 71, with average atomic mass 69.8. Find the percent abundance of each.

Example 22

challenge

Why does carbon dating rely on carbon-14 decaying while carbon-12 does not, even though both are carbon?

Example 23

easy

{}^{16}\text{O}

has 8 protons and 8 neutrons. How many neutrons does

{}^{18}\text{O}

have?

Example 24

medium

An element has two isotopes with masses 24 amu (78%) and 25 amu (10%) and 26 amu (12%). Calculate the average atomic mass.

Example 25

medium

Tritium is

{}^{3}\text{H}

. How many protons, neutrons, and electrons does neutral tritium have?

Example 26

medium

An atom has 19 protons and 21 neutrons. Write its isotope notation.

Example 27

medium

Uranium-235 has 92 protons. How many neutrons does it have?

Example 28

medium

Compare U-235 and U-238: how do they differ?

Example 29

medium

How many protons and neutrons in

{}^{238}_{92}\text{U}

Example 30

medium

Which has more mass: a deuterium atom or a tritium atom?

Example 31

medium

Silicon has 3 stable isotopes: Si-28 (92.2%), Si-29 (4.7%), Si-30 (3.1%). Estimate average atomic mass.

Example 32

hard

Gallium has isotopes Ga-69 (60.1%) and Ga-71 (39.9%). Calculate average atomic mass.

Example 33

medium

Magnesium-24, Mg-25, Mg-26 differ in what?

Example 34

medium

An atom has 6 protons and 6 neutrons. Write its isotope notation.

← Back to Isotope Practice Mode

Related Concepts

Neutron Mass Number Radioactivity

Background Knowledge

These ideas may be useful before you work through the harder examples.

neutronmass number