The size of atoms is so tiny that we can't really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used

One relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom.

All other elements are measured relative to the mass of a carbon-12 atom, so relative atomic mass has no units 

Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

The relative mass and charge of the sub-atomic particles are shown below: