Mass number

The mass number, [math]\left(A\right)[/math] (also called nucleon number), refers to the total number of protons and neutrons in the nucleus of an atom, and is used to organize the chart of nuclides. Each chemical element has a different number of protons, often with different numbers of neutrons. Both protons and neutrons weigh approximately 1 atomic mass unit (the approximately is a complication from the mass deficit that's described by E=mc²), so the mass number is an approximate mass of a given isotope of an element.

Mass number should not be confused with atomic number, [math]\left(Z\right)[/math] the total number of protons in a particular atom. When writing out descriptions of a particular atom, sometimes the mass number is included. When it is included, the mass number is put in a superscript before the element (14 in this case, with 7 being the number of protons, the atomic number): [math]\ce{^{14}_{7}N}[/math].

Different isotopes will have the same atomic number, but different mass numbers. For example: [math]\ce{^{16}_{8}O}[/math], [math]\ce{ ^{17}_{8}O}[/math], [math]\ce{^{18}_{8}O}[/math].

PhET: Build an Atom

The University of Colorado has graciously allowed us to use the following PhET simulation. This simulation builds atoms from protons, neutrons, and electrons and tests knowledge of the periodic table. The simulation shows how the neutrons and protons must balance for the nucleus to be stable. Note how this simulation allows changing the atomic number and mass number by changing the number of protons and neutrons.