2.4 - Isotopes and Its Uses

Meaning of Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Thus, isotopes of an element have:

the same proton number and number of electrons;
different nucleon numbers and masses because their numbers of neutrons differ.

Examples of isotopes

Hydrogen: $_{1}^{1} H$ , $_{1}^{2} H$ and $_{1}^{3} H$ .
Silicon: $_{14}^{28} Si$ , $_{14}^{29} Si$ and $_{14}^{30} Si$ .
Magnesium: $_{12}^{24} Mg$ , $_{12}^{25} Mg$ and $_{12}^{26} Mg$ .
Phosphorus: $_{15}^{31} P$ and $_{15}^{32} P$ .

Chlorine isotopes

chlorine-35, $_{17}^{35} Cl$ : 17 protons, 18 neutrons and 17 electrons;
chlorine-37, $_{17}^{37} Cl$ : 17 protons, 20 neutrons and 17 electrons.

An isotope may be written as chlorine-35, Cl-35, $_{17}^{35} Cl$ or $^{35} Cl$ .

Relative Atomic Mass of Isotopes

Most elements occur naturally as mixtures of two or more isotopes. Natural abundance is the percentage of an isotope present in a natural sample of an element.

Relative atomic mass = \frac{\sum ( % isotope \times isotope mass )}{100}

Example: chlorine

Chlorine contains 75% chlorine-35 and 25% chlorine-37.

Relative atomic mass of chlorine = \frac{( 75 \times 35 ) + ( 25 \times 37 )}{100} = 35.5

Uses of Isotopes

Field and isotope	Use
Medicine — Cobalt-60	Radiotherapy to kill cancer cells without surgery; sterilising surgical tools
Medicine — Iodine-131	Treating thyroid disorders such as hyperthyroidism and thyroid cancer
Agriculture — Phosphorus-32	Studying plant metabolism
Nuclear energy — Uranium-235	Generating electricity in a nuclear power generator
Archaeology — Carbon-14	Estimating the age of artefacts or fossils
Archaeology — Lead-210	Determining the age of sand and earth layers up to 80 years
Industry — Hydrogen-3	Acting as a detector in studies of sewage and liquid waste
Engineering — Sodium-24	Detecting leaks in underground pipes

The use of isotopes supports sustainability of life and has both positive and negative effects on society and the environment.