How to calculate the atomic mass of an element with its atomic number?

Atomic number of an element is defined as number of unit positive charges on the nucleus of an atom of the element.Since positive charge on the nucleus is due to protons,therefore atomic number of an element is equal to the number of protons.Also number of protons is equal to the number of electrons as an atom is electrically neutral.Hence atomic number is given as -
Atomic number(Z) = number of protons = nummer of electrons
Atomic mass means mass of an atom which is mainly concentrated in the nucleus.In nucleus there are protons and neutrons thus atomic mass is geiven as-
Atomic mass(A) = number of protons + number of neutrons
or,
Atomic mass = Atomic number + number of neutrons (since atomic number is equal to number of protons)
Generally it is found that atomic mass is double that of atomic number.For eg- N with Z = 7 has A = 14 and O having Z = 8 has A = 16 etc..In case of isotopes average atomic mass is taken.