Isotope – IGCSE Chemistry Definition

Isotopes are atoms of the same element that differ in the number of neutrons. This concept appears directly in definition questions on Paper 2 and in calculations of relative atomic mass on Paper 4. Understanding isotopes also helps explain why relative atomic masses on the periodic table are not whole numbers.

The 0620 definition

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.

Key points

• Same element = same number of protons = same atomic number
• Different number of neutrons = different mass number
• Same number of electrons = same chemical properties
• Different mass = different physical properties (density, rate of diffusion)

Examples

Chlorine isotopes

Isotope	Protons	Neutrons	Mass number	Abundance
Cl-35	17	18	35	75%
Cl-37	17	20	37	25%

Both isotopes have 17 protons and 17 electrons. They react identically in chemical reactions.

Carbon isotopes

• C-12: 6 protons, 6 neutrons (most common, used as mass standard)
• C-13: 6 protons, 7 neutrons
• C-14: 6 protons, 8 neutrons (radioactive, used in carbon dating)

Calculating relative atomic mass from isotope data

The relative atomic mass (Ar) is the weighted average of the masses of all isotopes of an element, taking into account their relative abundances.

For chlorine: Ar = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5

This is why chlorine’s Ar on the periodic table is 35.5, not a whole number.

Worked exam question

Boron has two isotopes: B-10 (20% abundance) and B-11 (80% abundance). (a) Define the term isotope. (2) (b) Explain why both isotopes have the same chemical properties. (1) (c) Calculate the relative atomic mass of boron. (2)

Mark scheme

(a) Atoms of the same element / with the same number of protons [1]; with different numbers of neutrons / different mass numbers [1]

(b) They have the same number of electrons / same electron arrangement [1]

(c) Ar = (10 x 20 + 11 x 80) / 100 [1]; = (200 + 880) / 100 = 10.8 [1]

Common exam mistakes

• Defining isotopes as “atoms with different numbers of neutrons” without saying “of the same element” or “with the same number of protons”. Both parts are needed for the mark.
• Saying isotopes have different chemical properties. They do not — chemical properties depend on electrons, which are the same.
• In Ar calculations, dividing by the number of isotopes instead of by 100 (or the total number of atoms). It is a weighted average, not a simple mean.