The atomic mass of an element is the average mass of the atoms of an element measured in the atomic mass unit (amu, also known as Dalton, D). The average atomic mass (sometimes called atomic weight) of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. The atomic number (represented by the letter Z) of an element is the number of protons in the nucleus of each atom of that element. The number of electrons in an atom is equal to the number of protons.
