Isotopes of an element have different numbers of what?

neutrons. The proton number is fixed for an element; the neutron number can vary.

What do carbon-12 and carbon-14 both have?

6 protons. They are both carbon, so both have proton number 6.

An element is 20% an isotope of mass 10 and 80% an isotope of mass 11. What is its relative atomic mass?

10.8. (20 × 10 + 80 × 11) ÷ 100.

Relative atomic masses are measured against which atom?

carbon-12. Carbon-12 is given a mass of exactly 12.

Isotopes and relative atomic mass · Cambridge IGCSE Chemistry (9

In plain words

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These heavier and lighter versions are isotopes. They behave the same in reactions, because reactions are about electrons, and those haven't changed.

Three things to know

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes of an element have the same chemical properties, because they have the same number of electrons.
Relative atomic mass (Ar) is the average mass of an element's atoms, allowing for how common each isotope is, on a scale where carbon-12 is exactly 12.

Worked example

An element is 60% an isotope of mass 69 and 40% an isotope of mass 71. Find its relative atomic mass.

Multiply each mass by its percentage: (60 × 69) + (40 × 71) = 4140 + 2840 = 6980.
Divide by 100.
= 69.8.

Tips and tricks

The definition has three parts: same element, same number of protons, different numbers of neutrons.
Sense-check a relative atomic mass: it must lie between the masses of the isotopes, nearer the commoner one.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Isotopes and relative atomic mass: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

Isotopes of an element have different numbers of what?
- protons
- electrons
- neutrons (the answer)
- shells
The proton number is fixed for an element; the neutron number can vary.
What do carbon-12 and carbon-14 both have?
- 6 protons (the answer)
- 6 neutrons
- 12 electrons
- the same mass
They are both carbon, so both have proton number 6.
How many neutrons are there in an atom of ³⁷₁₇Cl?
- 17
- 20 (the answer)
- 37
- 54
37 − 17.
An element is 20% an isotope of mass 10 and 80% an isotope of mass 11. What is its relative atomic mass?
- 10.2
- 10.5
- 10.8 (the answer)
- 11.2
(20 × 10 + 80 × 11) ÷ 100.
Relative atomic masses are measured against which atom?
- hydrogen-1
- carbon-12 (the answer)
- oxygen-16
- helium-4
Carbon-12 is given a mass of exactly 12.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 7 marks. Write your answers on paper, then check them.

Isotopes and relative atomic mass

Cambridge IGCSE Chemistry (9–1) 0971 · 7 marks · papermunch.org

Name ______________________________ Date ______________

State what is meant by isotopes.[2]
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Atoms of the same element (same number of protons) with different numbers of neutrons.
I got this one
Chlorine is 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.[3]
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35.5. (75 × 35 + 25 × 37) ÷ 100.
I got this one
Explain why the isotopes of an element have the same chemical properties.[2]
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They have the same number of electrons, arranged in the same way.
I got this one

Answers: Isotopes and relative atomic mass

1. Atoms of the same element (same number of protons) with different numbers of neutrons.
2. 35.5. (75 × 35 + 25 × 37) ÷ 100.
3. They have the same number of electrons, arranged in the same way.