What is the Mass Number of an atom ?

Definition of 'Mass Number':

The Mass Number of an atom is the total number of protons and neutrons it contains (all of which are located in the nucleus of the atom).

More about the Mass Number of an atom ...

• The Mass Number of an atom is represented by the symbol A.
• The Mass Number of an atom is also known as the Nucleon Number.

Use of the Mass Number to calculate how many neutrons are in an atom

Number of Neutrons (in an atom) = Mass Number (A) - Atomic Number (Z)

or, using alternative words whose meaning is the same:

Number of Neutrons (in an atom) = Nucleon Number (A) - Proton Number (Z)

Examples of Use of Mass Number and Atomic Number to calculate the number of neutrons in an atom

Element	Symbol	Atomic Number	Mass Number	Calculation of number of neutrons	Number of neutrons
		(= Number of Protons)	(= Number of Protons + Neutrons)
Nitrogen	N	7	14	14 - 7 =	7
Magnesium	Mg	12	24	24 - 12 =	12
Potassium	K	19	39	39 - 19 =	20
Cobalt	Co	27	59	59 - 27 =	32
Lead	Pb	82	207	207 - 82 =	125

An element can have atoms with different Mass Numbers (but not different Atomic Numbers)

The Atomic Number of an atom defines which element the atom is a particle of.

The Mass Number of an atom of a particular element (so, given a specific atomic number), defines which isotope of that element the atom is an example of. (An isotope of an element is an atom of that element that has a particular Mass Number and is often identified according to it, e.g. 'carbon-14'.)

Understanding about isotopes is necessary for GCSE Chemisty. For more information see What is an isotope ?

'Mass Number' and 'Relative Atomic Mass'

The Mass Number (= Nucleon Number) of an atom is not necessarily the same as the Relative Atomic Mass of that element. The Mass Number (of the atoms) is only the same as the Relative Atomic Mass (of the element) when all the the atoms of the element are the same, that is when the element has only one isotope.

Note: This is one of a series of simple pages introducing key concepts in introductory chemistry. Other pages in this section include elements, mixtures and compounds and individual pages about substances, elements, mixtures and compounds, plus pages about atoms, molecules and isotopes. If you need further information ask your chemistry tutor.