# Mass vs relative atomic mass on periodic table

Hi, I’m sure this has probably been asked before but I can’t find any answers.

I am doing GCSE AQA science and in chemistry the teacher handed out periodic tables, the ones I and a few others got were A-level ones rather than GCSE.

These ones had the relative atomic mass under each symbol. This has really confused me, as I thought that the mass number was protons+neutrons and the atomic number was both protons and electrons. While the relative atomic mass is sort of the average mass of all isotopes of that element compared to carbon 12.

The teacher has been treating the relative atomic mass as the same as the mass number and using it to work out how many neutrons. I can’t understand this and Google just repeats the same useless things telling me what each different name means.

So how are they working the same if they are totally different numbers from totally different things?

Thanks for any help!
Original post by ratterpat
Hi, I’m sure this has probably been asked before but I can’t find any answers.

I am doing GCSE AQA science and in chemistry the teacher handed out periodic tables, the ones I and a few others got were A-level ones rather than GCSE.

These ones had the relative atomic mass under each symbol. This has really confused me, as I thought that the mass number was protons+neutrons and the atomic number was both protons and electrons. While the relative atomic mass is sort of the average mass of all isotopes of that element compared to carbon 12.

The teacher has been treating the relative atomic mass as the same as the mass number and using it to work out how many neutrons. I can’t understand this and Google just repeats the same useless things telling me what each different name means.

So how are they working the same if they are totally different numbers from totally different things?

Thanks for any help!

The periodic table shows relative atomic mass - which is calculated from % abundance of each isotope x relative isotopic mass
For example, chlorine relative atomic mass is 35.5. That doesnt mean chlorine has 17 protons and 18.5 neutrons, it is merely the average of the isotopes.

Chlorine could have 17 protons and 18 neutrons, 17 protons and 19 neutrons, 17 protons and 20 neutrons, other isotopes are unstable.

Mass number is protons + neutrons, you work it out for a specific atom

this is your gcse periodic table
https://secondaryscience4all.files.wordpress.com/2014/06/filestore_aqa_org_uk_subjects_aqa-chemistry-data-sheet_pdf3.png
Original post by BankaiGintoki
The periodic table shows relative atomic mass - which is calculated from % abundance of each isotope x relative isotopic mass
For example, chlorine relative atomic mass is 35.5. That doesnt mean chlorine has 17 protons and 18.5 neutrons, it is merely the average of the isotopes.

Chlorine could have 17 protons and 18 neutrons, 17 protons and 19 neutrons, 17 protons and 20 neutrons, other isotopes are unstable.

Mass number is protons + neutrons, you work it out for a specific atom

this is your gcse periodic table
https://secondaryscience4all.files.wordpress.com/2014/06/filestore_aqa_org_uk_subjects_aqa-chemistry-data-sheet_pdf3.png

The teacher was using the relative atomic mass to work out the neutron numbers and it was working??? I am extremely confused.
But looking at it, many of the relative atomic masses are odd numbers (not to mention decimals). Also, having done a very low level of science before, I was using periodic tables with the atomic number and the mass number. I’m guessing that at this level you can’t use the mass number to work out neutron numbers anyway because you are aware of isotopes so each one would have a different mass number.
I’m still quite confused but typing this out has helped a little bit, it probably doesn’t make much sense to anyone else though.
The mass number is as you say the mass of the protons and the neutrons for ANY particular atom.
However since there are isotopes of elements this leaves a bit of a problem, since isotopes have extra neutrons in the nucleus then they will have different masses, so what mass would you record in the periodic table for an element with lots of isotopes. The average mass of an element taking into account the relative abundance of each isotope is the relative atomic mass , its like an average of the masses of all the isotopes taking into account the % abundance of each.

Since all elements have isotopes then all the relative atomic masses in the periodic table will be decimals, but in GCSE these are usually just rounded up to the nearest whole number.

https://www.science-revision.co.uk/atoms%20and%20elements.html