The mass number is determined by which particles

The total number of protons and neutrons in an atomic nucleus determines mass number. It is also called atomic mass number or nucleon number. The mass number is written either after the element name or as a superscript to the left of an element's symbol. Like in the element carbon, its most common isotope is carbon-12, or can be written as 12C, which has 6 protons and 6 neutrons.