Introduction

You might have heard the term 'radioactive isotopes' mentioned on the news or by people worried about the environment. Sometimes we use radioactive isotopes to help treat cancer. But what is an isotope? In this unit you will find out what an isotope is. We will also see how to calculate the relative atomic mass of atoms, which measures how heavy atoms are relative to one another.

We saw in the unit Atomic Shorthand that each chemical

element

An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.element has its own unique

atomic number

The atomic number is the number of protons in an atom (which equals the number of electrons).atomic number. That means that all the atoms of any particular

element

An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.element must have the same number of protons (and therefore electrons). So any

atom

An atom is the smallest particle of an element that can still be defined as that element.atom of hydrogen has one proton, otherwise it wouldn't be hydrogen! If the atom had two protons, it would be helium.


But there is no reason why the number of neutrons can't vary. In fact, there are three types of hydrogen atom, each containing a different number of neutrons. We call these isotopes of hydrogen. So isotopes are atoms of the same element that have different masses.

Look at the diagrams in Fig. 1 below to see the isotopes of hydrogen. The two heavier isotopes of hydrogen are often known as deuterium () and tritium ().



Different isotopes of an element have the same atomic number, but different mass numbers. That's because each isotope of an element contains different numbers of neutrons.

The chemical properties of an element are determined by its

electronic structure

The electronic structure is a description of the arrangement of the electrons in an atom, starting from the lowest energy level (shell). For example, the electronic structure of sodium is 2, 8, 1. (Also known as an electronic configuration.)electronic structure. Because isotopes have the same number, and therefore arrangement, of electrons, isotopes of an element undergo the same chemical reactions. The only difference is their mass.

Isotopes are sometimes shown by the name of the element, followed by the

mass number

The mass number is the number of protons plus neutrons in the nucleus of an atom.mass number. Thus the three isotopes of hydrogen would be:

Hydrogen-1
Hydrogen-2
Hydrogen-3



This is a diagram of two carbon isotopes:

The mass of atoms can be compared on a relative scale. It is more convenient than quoting the tiny masses of atoms in standard units of mass, like grams or kilograms.

On the relative atomic mass scale, a carbon-12 atom has a mass of exactly 12. For our purposes we can take the mass number as the

relative atomic mass

The relative atomic mass, A_r, is the mass of one mole of atoms of an element (taking into account the proportions of each isotope in a naturally occurring sample of the element).relative atomic mass of a particular atom. Therefore, the relative atomic mass of hydrogen-1 is 1, making it the lightest of all atoms.

However, if you look up the mass of elements in a table of data, you will find that chlorine has a relative atomic mass of 35.5. At first sight this seems a bit strange. How can you have an atomic mass that is a fraction? After all, you can't get half a proton or half a neutron in an atom! But the answer lies in the existence of isotopes.

Chlorine exists as two isotopes: chlorine-35 and chlorine-37. When we calculate the relative atomic mass of an element we have to take into account the proportions of each isotope present. In a naturally occurring sample of chlorine, we find that 75 per cent is chlorine-35 atoms and the other 25 per cent is chlorine-37 atoms.




Use these three steps to answer this question:

Summary


Isotopes of an element have the same number of protons and electrons, but different numbers of neutrons.

Different isotopes of an element have different mass numbers, but react chemically in exactly the same way.

The relative atomic mass of an element is the average mass of the isotopes in a naturally occurring sample of the element, taking into account the proportion of each isotope present.