- The definition of an atomic number is the number of protons in the nucleus of an atom of an element.
Three for lithium and 20 for calcium are each an example of an atomic number.
Chem. a number representing the relative position of an element in the periodic table, in which the elements are arranged in the order of their nuclear charges; a number representing the positive charge or the number of protons in the nucleus of the atom of an element: isotopes have the same atomic number but different mass numbers
nounSymbol Z Abbr. at. no.
The number of protons in an atomic nucleus.
(plural atomic numbers)
- attributive form of atomic number
- atomic-number data