# Chemistry relative atomic mass

Hi, please could someone tell me the difference between mass number and relative atomic mass? On videos I watched they keep saying that on the periodic table for elements we say mass number but it should relative atomic mass which means total number of protons and neutrons but that doesn’t make sense because it actually means average weighted mass of all the atoms of the atoms relative to 1/12th of the mass of a C-12 atom so idk which one is right?
Also is relative atomic mass basically the average of all the atoms of the different isotopes of an element?
Thank you!!
Original post by Ashirs
Hi, please could someone tell me the difference between mass number and relative atomic mass? On videos I watched they keep saying that on the periodic table for elements we say mass number but it should relative atomic mass which means total number of protons and neutrons but that doesn’t make sense because it actually means average weighted mass of all the atoms of the atoms relative to 1/12th of the mass of a C-12 atom so idk which one is right?
Also is relative atomic mass basically the average of all the atoms of the different isotopes of an element?
Thank you!!

Atomic number - number of protons
Mass number = atomic number + number of neutrons
Relative isotopic mass - mass of isotope relative to 1/12th mass of an atom of C12
Relative atomic mass - weighted mean mass of an atom of an element relative to 1/12th mass of atom of C12
Weighted mean mass = relative isotopic mass x percentage abundance

for example, look at chlorine with atomic number 17, with relative atomic mass of 35.5. The periodic table uses relative atomic mass.

mass number is used with reference to a single atom/ion (to find the number of protons, neutrons, electrons using atomic number)
You are correct that the relative atomic mass is the average mass of the atoms of the different isotopes weighted by their abundance. The term relative isotopic mass is used for the mass of a single isotope, relative to 1/12th of a carbon-12 atom, and is exactly equal to mass number for carbon. However, for other elements, the relative isotopic mass and mass number can differ by up to around 1% due to the mass defect caused by binding energy of different nuclei, different masses of proton and neutron, and masses of the electrons. The issue is quite well summarised on Wikipedia page.