Why is the atomic mass not double of the atomic number?

Atomic number is the number of protons in an atom and atomic mass is the mass of protons, neutrons, and electrons in an atom. Many atoms don't have a number of neutrons equal to the number of protons. read more

It usually is because the atomic number describes how many protons there are in an atom's nucleus and the atomic mass is the total mass of all protons, neutrons and electrons. read more

But atomic mass is not at all times double of atomic number sometimes it's more than the double of the atomic number. Eg : argon( 40 = A ; 18= Z ) (A = atomic mass ,Z = atomic number ). read more

The mass quoted on the Periodic Table is the weighted average of the individual isotopes. The element tin, Z=50, has 10 stable isotopes, and the atomic mass quoted on the Periodic Table, 118.710*g*mol^-1, is the weighted average of the individual isotopes. read more