Difference Between Atomic Mass and Atomic Number

ATOMIC NUMBER

ATOMIC MASS

It is the total number of protons in an atom

It is the sum of the protons and neutrons in an atom

Isotopes have the same atomic number 

Isotopes have different atomic masses

Isobars have different atomic numbers

Isobars have the same mass number

It has no units

It is given by atomic mass units (amu)

Two elements cannot have the same atomic number

Two elements can have the same atomic mass 

Example - Hydrogen - 1

                 Helium - 2

                 Oxygen - 8

                 Chlorine - 17

                 Argon - 18

                 Calcium - 20

Example - Hydrogen - 1 amu

                 Helium - 4 amu

                 Oxygen - 16 amu

                 Chlorine - 35.5 amu

                 Argon - 40 amu

                 Calcium - 40 amu