Neutron number

The neutron number, written as N, refers to the number of neutrons in the nucleus of an atom.

When writing out descriptions of a particular nuclear species, neutron number is rarely included, and is written in the lower right hand corner. Nuclear species with the same number of protons but a different number of neutrons are called isotopes. They will have the same atomic number but different mass number; an example would be: [math]\ce{^{16}_{8}O_8}[/math], [math]\ce{ ^{17}_{8}O_9}[/math], [math]\ce{^{18}_{8}O_{10}}[/math]. Neutron number [math]\left(N\right)[/math]is implied from the mass number [math]\left(A\right)[/math] and the atomic number [math]\left(Z\right)[/math] since:

[math]N = A-Z[/math]

Writing [math]\ce{^{16}_{8}O}[/math], [math]\ce{ ^{17}_{8}O}[/math], [math]\ce{^{18}_{8}O}[/math] is generally how these nuclear species are written and [math]\ce{^{16}O}[/math], [math]\ce{ ^{17}O}[/math], [math]\ce{^{18}O}[/math] is sufficient since it's 'known' that O means oxygen, which means 8 protons.

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.