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
For some elements atomic mass is roughly double the atomic number but mostly atomic mass is more than 2 times of atomic number. Atomic mass is the addition of number of electrons and number of neutrons while atomic number is only the number of electrons or protons. read more
While the mass number is the sum of the protons and neutrons in an atom, the atomic number is only the number of protons. The atomic number is the value found associated with an element on the periodic table because it is the key to the element's identity. read more