Isotopes are atoms of the same element with different numbers of neutrons. The relative atomic mass is a weighted average over those isotopes, so it is rarely a whole number.
This lesson is part of matter and atomic structure. It uses the counting from calculating protons, neutrons and electrons.
What stays the same and what changes?
Isotopes of one element share the proton number and the electron arrangement. They differ only in neutron count.
| Isotope | Protons | Neutrons | Nucleon number |
|---|---|---|---|
| Copper-63 | 29 | 34 | 63 |
| Copper-65 | 29 | 36 | 65 |
Because the electrons are identical, both isotopes react the same way. Their masses differ, so physical properties such as density differ slightly.
Why is relative atomic mass not a whole number?
A sample of copper contains both isotopes mixed together. The periodic table reports an average, weighted by how common each isotope is.
If copper were half and half, the average would sit exactly halfway, at 64. Copper-63 is more common, so the average is pulled closer to 63.
Worked example: copper
A sample of copper contains 69% copper-63 and 31% copper-65. Calculate the relative atomic mass.
- Multiply each nucleon number by its percentage: 63 × 69 = 4 347 and 65 × 31 = 2 015.
- Add: 4 347 + 2 015 = 6 362.
- Divide by 100: 6 362 ÷ 100 = 63.62, which rounds to 63.6.
Reasonableness check. The answer lies between 63 and 65. It is closer to 63, which matches the larger percentage. If you got 64.4, you swapped the percentages.
The mistakes to avoid
Two slips are common. The first is to average only the two nucleon numbers, (63 + 65) ÷ 2 = 64, which ignores the percentages. The second is to forget to divide by 100 and leave the answer as 6 362.
Write the percentages beside each isotope before you start. That habit stops the swap, and the reasonableness check catches the rest. Use the mole and stoichiometry steps tool later when relative atomic mass feeds a mole calculation.
Check yourself
Boron has two isotopes: 20% boron-10 and 80% boron-11. Calculate its relative atomic mass and state one property the two isotopes share.
Answer
(10 × 20) + (11 × 80) = 200 + 880 = 1 080, and 1 080 ÷ 100 = 10.8.
Check: 10.8 lies between 10 and 11 and is closer to 11, the more abundant isotope. Both isotopes have 5 protons and 5 electrons, so they share the same chemical properties.
What to study next
Move on to explaining heating and cooling curves using particles. When the four lessons are done, use the matter and atomic structure practice set.
If you want a teacher to go through definitions and calculations with you, see online one-to-one Chemistry tuition.