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atomic mass vs. atomic number

atomic mass

[uh-tom-ik mas] / əˈtɒm ɪk ˈmæs /

noun

Chemistry.

the mass of an isotope of an element measured in units formerly based on the mass of one hydrogen atom taken as a unit or on 1/16 (0.0625) the mass of one oxygen atom, but after 1961 based on 1/12 (0.0833) the mass of the carbon-12 atom.

atomic number

[uh-tom-ik nuhm-ber] / əˈtɒm ɪk ˈnʌm bər /

noun

the number of positive charges or protons in the nucleus of an atom of a given element, and therefore also the number of electrons normally surrounding the nucleus. at. no.; Z