Not every atom of an element is identical. Chlorine atoms all have 17 protons, but some carry 18 neutrons and some 20. That is why the relative atomic mass of chlorine is 35.5 and not a whole number.
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers. (Extended) Isotopes of an element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. The relative atomic mass, Ar, is the average mass of the isotopes weighted by their percentage abundances.
Fig. 1: the two isotopes of chlorine and the weighted average they give
Section 01
Same protons, different neutrons.
Pick an element in the simulator and add or remove neutrons. The element name never changes; only the mass number does. Then compare the isotopes in the table.
Isotope
Protons
Neutrons
Electrons
11H hydrogen-1
1
0
1
21H hydrogen-2
1
1
1
126C carbon-12
6
6
6
146C carbon-14
6
8
6
3517Cl chlorine-35
17
18
17
3717Cl chlorine-37
17
20
17
Stage 1 · Learn
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Four quick checks. Each correct answer earns XP and records this skill on your profile.
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Isotopes are atoms of the same element with:
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Carbon-12 and carbon-14 differ in the number of:
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How many neutrons are in an atom of 3717Cl?
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Which pair of atoms are isotopes of each other?
Section 02
Why isotopes react the same.
Extended. Chemical reactions are about electrons, especially the outer-shell electrons. Neutrons sit in the nucleus and take no part.
Same electrons: isotopes of an element have the same number of protons, so a neutral atom has the same number of electrons.
Same configuration: both chlorine isotopes are 2,8,7, with seven outer electrons, so both gain one electron to form Cl− in exactly the same reactions.
What does differ: the mass of the atom, so physical properties that depend on mass, such as density, are slightly different.
Section 03
Relative atomic mass.
Extended. Multiply each isotope's mass number by its percentage abundance, add the results, and divide by 100.
Sense check: the answer must lie between the isotope masses and closer to the more abundant one.
Examiner trap
Do not just average the mass numbers. For chlorine, (35 + 37) ÷ 2 = 36 is wrong because there are three times as many 35Cl atoms as 37Cl atoms. Each mass must be weighted by its abundance, and the total divided by 100, not by 2.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
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(Extended) Why do 35Cl and 37Cl have the same chemical properties?
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(Extended) Copper is 69% 63Cu and 31% 65Cu. What is the relative atomic mass of copper, to one decimal place?
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(Extended) Lithium is 7.5% 6Li and 92.5% 7Li. What is its relative atomic mass?
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An atom has 12 protons and 14 neutrons. Which is an isotope of this atom?
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Isotopes, mastered.
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