8.3 - Isotope

Definition and Examples of Isotopes

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Isotopes of an element occupy the same position in the Modern Periodic Table of Elements.
• The word isotope comes from the Greek words isos (same) and topos (place).

Isotopes	Number of protons	Number of neutrons
Hydrogen-1, hydrogen-2, hydrogen-3	1	0, 1, 2
Carbon-12, carbon-13, carbon-14	6	6, 7, 8
Oxygen-16, oxygen-18	8	8, 10

• Neutral isotopes of the same element also have the same number of electrons.
• An isotope is normally written using its nucleon number, for example hydrogen-3.

Nucleon number

$\text{nucleon number}=\text{number of protons}+\text{number of neutrons}$

$\text{number of neutrons}=\text{nucleon number}-\text{number of protons}$

Worked particle counts

Isotope	Protons	Neutrons
Chlorine-35	17	$35-17=18$
Chlorine-37	17	$37-17=20$
Sodium-23	11	$23-11=12$
Sodium-24	11	$24-11=13$
Bromine-79	35	$79-35=44$
Bromine-81	35	$81-35=46$

Uses of Isotopes in Various Fields

• A radioisotope is an unstable isotope that emits radioactive radiation.
• Radioisotopes emit dangerous radiation but have important uses when handled correctly.
• Radioisotopes are used in medicine, agriculture, industry, food technology, archaeology and geology, electricity generation and research.

Uses of radioisotopes

Field and isotope	Use
Medical — gamma rays from cobalt-60	Kill cancer cells.
Food technology — gamma rays from cobalt-60	Kill microorganisms on vegetables without changing the quality of the food.
Agriculture — phosphorus-32	Injected into plant roots to study the rate of absorption of phosphorus fertiliser in plants.

Isotopes identified for information gathering

Field	Isotopes
Medical	Sodium-24, cobalt-60, iodine-131
Agriculture	Phosphorus-32, carbon-14
Archaeology and geology	Carbon-14
Food technology	Cobalt-60
Paper industry	Americium-241
Electricity generation	Uranium-235