Why is the relative atomic mass of chlorine not a whole number?

Chlorine is a mixture of isotopes with different masses

Why did Mendeleev place tellurium before iodine even though tellurium has the higher relative atomic mass?

So that iodine was in the same group as elements with similar properties

Isotopes, relative atomic mass and Mendeleev's order

Most elements are a mixture of isotopes. That is why many relative atomic masses are not whole numbers, and why Mendeleev's order sometimes seemed wrong.

GCSE · 1CH0 · 1.9, 1.10, 1.11, 1.12, 1.15
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Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so different mass numbers.
Chlorine has two main isotopes: chlorine-35 (17 protons, 18 neutrons) and chlorine-37 (17 protons, 20 neutrons).
Isotopes have the same chemical properties because they have the same electron arrangement.

Calculating relative atomic mass

The relative atomic mass (A_r) is the mean mass of the atoms, taking the abundance of each isotope into account:
A_r = (mass₁ × % abundance₁ + mass₂ × % abundance₂) ÷ 100
Chlorine: 75% chlorine-35 and 25% chlorine-37.
A_r = (35 × 75 + 37 × 25) ÷ 100 = 35.5

Mendeleev's puzzle

Mendeleev ordered elements by relative atomic mass, but he swapped some pairs so that elements with similar properties were in the same group.
For example, tellurium (A_r 127.6) comes before iodine (A_r 126.9), so that iodine sits with the other halogens.
The reason: the order really depends on atomic number. Tellurium has fewer protons than iodine, but a higher A_r because of the abundance of its heavier isotopes.

Worked example

Copper is 69% copper-63 and 31% copper-65. Calculate its relative atomic mass to 1 decimal place.

A_r = (63 × 69 + 65 × 31) ÷ 100
= (4347 + 2015) ÷ 100
= 63.62 = 63.6

Answer: 63.6

Key idea

Isotopes have the same number of protons but different numbers of neutrons. A_r = (mass × abundance + mass × abundance) ÷ 100. Some pairs, such as tellurium and iodine, are out of A_r order because of their isotopes; the real order is by atomic number.

Key words

Isotopes: Atoms of the same element (same number of protons) with different numbers of neutrons.
Mixture: Two or more substances not chemically combined. They can be separated by physical methods.
Atomic number: The number of protons in the nucleus of an atom. Elements are arranged in order of atomic number in the periodic table.
Element: A substance made of only one type of atom (all atoms with the same atomic number).
Electron: A tiny negative particle (relative charge -1) found in shells around the nucleus.
Relative atomic mass (A_{r}): The weighted mean mass of the isotopes of an element, compared with 1/12 of the mass of a carbon-12 atom.

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Extra practice

2 more questions

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How many neutrons are in an atom of chlorine-35? (Atomic number of chlorine = 17)
Show the answer
Answer: 18
neutrons = 35 − 17 = 18
Carbon-12 and carbon-14 are isotopes. What do they have the same number of?
1. Mass number
2. Protons
3. Nucleons
4. Neutrons
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Answer: Protons
Isotopes have the same atomic number.

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Sources and references

Checked by the GCSE-ready editorial team, 16 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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