Hydrogen-1.01
Helium-4.00
Oxygen-16.00
Carbon-12.01
The mass of each element comes from the total number of protons and neutrons.
Elements are organized on the periodic table by their atomic number which is their number of protons
This means that further you go down a group (column ) or to the right across a period (row) the larger the atomic number and subsequently mass number will be.
Hope that helped!