Isotopes · Fathom

By the end of this lesson

By the end you can define an isotope, explain why isotopes of an element behave the same chemically but differently in mass and physical properties, and give the numbers of protons and neutrons in a named isotope.

Definition

Isotopes

Isotopes are nuclei (atoms) of the same element that have the same number of protons but different numbers of neutrons.

Equivalently: same proton number $Z$ , different nucleon number $A$ . Because the neutron count is $A - Z$ , a different $A$ with the same $Z$ means a different number of neutrons — that is the whole definition.

Same element, different mass

The three isotopes of hydrogen are the cleanest example, because the nucleus is a single proton plus whatever neutrons it has:

isotope	notation	protons	neutrons	nucleon number
protium	$^{1}_{1}\text{H}$	1	0	1
deuterium	$^{2}_{1}\text{H}$	1	1	2
tritium	$^{3}_{1}\text{H}$	1	2	3

All three are hydrogen: each nucleus has one proton, so each atom has one electron when neutral. Adding neutrons changes the mass but not the charge or the element.

Carbon gives the example you will meet most often. $^{12}_{6}\text{C}$ has 6 protons and 6 neutrons; $^{14}_{6}\text{C}$ has 6 protons and 8 neutrons. Both are carbon.

Why chemistry survives but mass does not

Chemistry is the behaviour of the electrons, and the electron arrangement of a neutral atom is set by the number of protons it has to balance. Isotopes share $Z$ , so they share an electron arrangement, so they form the same bonds, the same ions and the same reactions. A sample of $^{14}_{6}\text{C}$ burns in oxygen exactly as $^{12}_{6}\text{C}$ does.

Mass is a different story. Neutrons carry about $1\ \text{u}$ each, so the two extra neutrons make a carbon-14 atom about 17% heavier than a carbon-12 atom ( $14\ \text{u}$ against $12\ \text{u}$ ). Anything that depends on mass — density, rate of diffusion, the speed at which the atoms move at a given temperature — is different. That is why isotopes are separated by mass-based methods rather than chemical ones.

Worked example: Comparing two isotopes of carbon

A sample contains $^{12}_{6}\text{C}$ and $^{14}_{6}\text{C}$ . For each, state the number of protons, neutrons and electrons in the neutral atom, and explain which nuclide is heavier.

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