Since the neutrons and protons make up the nucleus, they are also called the nucleons.

In chemistry, an element refers to atoms with the same number of protons. In nuclear physics, a nuclide refers to atoms with the same number of protons and neutrons. In other words, all atoms of a particular nuclide have the same nucleus composition.

A nuclide is identified using the A-Z notation:

A, the mass number (aka the nucleon number) states the number of nucleons in the atom.

Z, the atomic number, states the number of protons in the nucleus. It is also known as the proton number or (even more meaningfully) the charge number.

X, the chemical symbol, states the element of the atom.

For example, refers to carbon atoms with 14 nucleons and 6 protons (and thus neutrons). Since the name of the element (carbon) already determines the atomic number, Z is sometimes omitted, as in , or simply carbon-14.

Illustrated below are the nuclei of the , and nuclides.

Using the A-Z notation, the proton, neutron and electron are denoted by , and respectively.

Isotopes refer to atoms with the same atomic number Z but different mass number A, i.e. atoms with the same number of protons but different number of neutrons. For example, carbon has 15 known isotopes. The four main ones are tabulated below.

	abundance	atomic mass	half-life	decay mode	decay product
	trace	11.011433u	20 min	β+	boron-11
	98.9%	12u	stable	–	–
	1.1%	13.003355u	stable	–	–
	trace	14.003241u	5730 y	β-	nitrogen-14

Note that isotopes have the same chemical properties, but different physical properties. For example, both and undergo the exact same chemical combustion with oxygen to form carbon dioxide. However, is a stable nucleus whereas radioactively decays to form nitrogen-14. and also differ in mass and can be separated using mass spectroscopy.

Illustrated below are the nuclei of three hydrogen isotopes: , and .

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