What is Relative Atomic Mass ?

Definition of "Relative Atomic Mass":

Background Information

Key terms that it is useful to understand in order to be able to explain relative atomic mass include: atom, atomic number, element, mass number and isotope.

At the center of each atom is a "nucleus" that consists of two types of particles called "protons" and "neutrons" (and known collectively as "nucleons"). The number of protons is called the "atomic number" and determines which element the atom is a particle of (and therefore also its chemical properties). The total number of protons and neutrons in the nucleus of the atom is called the "mass number" and is determined by the type of atom (i.e. which element it is) and also by the number of neutrons present in that particular atom. In many cases, different atoms of the same element can have different numbers of neutrons in their nuclei ("nuclei" is the plural word for "nucleus"; one nucleus - many nuclei). The number of neutrons in an atom of a specific element therefore determines is "mass number" and so the "isotope" of which that particular atom is an example.
For example, most atoms of the element phosphorus have a mass number of 31 and so may be referred to as phosphorus-31, however, some atoms of phosphorus have an extra neutron and therefore a mass number of 32. They are therefore atoms of the isotope phosphorus-32.

Although there are three types of particles that form atoms (protons, neutrons and electrons), electrons have very low mass compared with protons and neutrons so the mass of atoms is mainly due to the numbers of protons and neutrons that make-up the atom, i.e. the "mass number" of the atom.

• The atomic number of an atom is the same as the atomic number of the element of which that atom is a particle because the atomic number of an atom determines which element it is an atom of.
  
  however ...
  
  
• The mass number of an atom is not the same as the mass number of an element because atoms of the same element can have different masses due to having different numbers of neutrons (i.e. exist in the form of different isotopes, which therefore have slightly different physical properties e.g. boiling points).

That is the Atomic Number of an element is meaningful, but the Mass Number "of an element" is not meaningful.

Calculation of Relative Atomic Mass

In order to calculate the relative atomic mass of a sample of an element, it is necessary to know which isotopes of that element are present in the sample, and in what proportions.

The following calculation of relative atomic mass uses the example of chlorine.
Consider a sample of chlorine that contains the isotopes chlorine-35 and chlorine-37 in the ratio 3:1, meaning that for every one atom of chlorine-37 there are 3 atoms of chlorine-35.

The relative atomic mass of this sample of chlorine may be calculated by taking into account the atomic masses and relative proportions of its isotopes as follows:

	Isotope 1		Isotope 2
Name	chlorine-35		chlorine-37
Mass Number	35		37
Relative Proportion	3	:	1
(Mass Number)x(Relative Proportion)	35 x 3 = 105		37 x 1 = 37
(Mass Number)x(Relative Proportion) of Isotope 1 + (Mass Number)x(Relative Proportion) of Isotope 2				= 105 + 37 = 142
Answer from above divided by the sum of the relative proportions of the isotopes				= 142 / (3+1)
Relative Atomic Mass of this sample				= 142 / 4 = 35.5

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.