All atoms can be identified by the number of protons and neutrons they contain. The number of protons in the nucleus of each atom of an element is called the atomic number (Z). The chemical identity of an atom can be determined solely by its atomic number. For example, the atomic number of nitrogen is 7; this means that each neutral nitrogen atom has 7 protons and 7 electrons. Or viewed another way, every atom in the universe that contains 7 protons is correctly named “nitrogen.”

The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element. In general, the mass number is given by

Isotopes

Atoms of the same element with the same number of protons but different number of neutrons are called Isotopes. Isotopes have the same atomic number but different mass numbers. For example, there are three isotopes of carbon. The first isotope contains 6 neutrons. The second contains 7 and the third contains 8 neutrons.

The three isotopes of hydrogen have the following common names:

The isotopes of an element are not found in nature in equal distribution. Among the isotopes of a given element, one of them will be found in greater quantity and others in smaller quantity. Among the three isotopes of carbon, for example, ¹²C is the most abundant. Among the three isotopes of hydrogen, protium is the most common.

Note that since it is the electrons which are responsible for chemical properties, isotopes of an element have the same chemical properties. However, they differ in physical properties such as density.