All atoms of the same element have the same number of protons in the nucleus
Atoms of the same element may have different numbers of neutrons in the nucleus
Isotopes
Atoms of the same element that have different numbers of neutrons
Symbol for a specific isotope
Mass number as a superscript to the left of the element symbol
The atomic number is sometimes written as a subscript preceding the symbol, but since this number defines the element's identity, as does its symbol, it is often omitted
