Relative Formula Mass.
Relative Atomic Mass - Ar.
1) This is just a way of saying how heavy different atoms are compared with the mass of an atom of carbon-12 has Ar of exactly 12.
2) It turns out that the relative formula mass Ar, is usually just the same as the mass number of the element (to the nearest whole number.)
3) In the periodic table, the elements all have two numbers. The smaller number is the atomic number (how many protons it has) and the bigger number is the mass number or relative atomic mass.
Helium has an atomic symbol of He it has an …