What each board prints
- AQA GCSE and Edexcel GCSE round the masses to whole numbers. Chlorine (35.5) and copper (63.5) are the only two they leave unrounded.
- Edexcel's table ends at radon, so period 7 is missing from it.
- OCR Gateway and OCR 21st Century use one sheet between them. It gives each mass to one decimal place, so sodium is 23.0 where AQA has 23.
- OCR leaves the mass box empty for technetium, polonium, astatine, radon and all of period 7.
- The AQA A level booklet also uses one decimal place. It is the only one of the five that shows all 118 elements.
Why the numbers differ
A relative atomic mass is an average. Most elements are a mix of isotopes, which are atoms of the same element with different masses. The average takes each isotope's mass and how common it is.
The boards round that average in different ways. Magnesium is 24 on the AQA sheet and 24.3 on the OCR one.
A number in square brackets works differently. It is the mass number of the element's most stable isotope. AQA prints technetium as [98] at GCSE and [97] at A level.
Using the masses in a calculation
Relative formula mass, written Mr, is the total of the relative atomic masses of all the atoms in a formula. In an exam, take the masses from your own board's sheet.
For water, H2O, AQA numbers give 2 × 1 + 16 = 18. With OCR numbers the sum is 2 × 1.0 + 16.0 = 18.0.
Every board page has a calculator under its table. Type a formula and it lists each step with that board's masses. For more practice, our science games are free and need no login.
Questions students ask
Do I get a periodic table in my GCSE chemistry exam?
Yes. AQA, Edexcel and OCR each give you their own periodic table with the exam paper. Find out which board your school uses, then open that board's page.
Why is chlorine 35.5 on every sheet?
Chlorine is a mix of two isotopes, and their average is not close to a whole number. All five sheets print it as 35.5.
Are the OCR Gateway and 21st Century tables the same?
Yes. The two courses have separate data sheets, and the periodic table on each one is identical.
Does the A level table use different numbers from GCSE?
AQA's A level booklet gives one decimal place, where its GCSE sheet gives whole numbers. Some heavy elements also have different bracketed mass numbers on the two sheets.