Isotopes

Defining Isotopes

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

The proton number defines which element an atom belongs to. Carbon, for example, always has 6 protons. The neutron number, however, can vary without changing the element’s identity.

A familiar example is carbon. Carbon-12 contains 6 protons and 6 neutrons, while carbon-14 contains 6 protons and 8 neutrons. Both are carbon because both have 6 protons.

Hydrogen has three natural isotopes: protium (¹H, 0 neutrons), deuterium (²H, 1 neutron) and tritium (³H, 2 neutrons). All three are hydrogen because each has exactly 1 proton.

Isotope Notation ˣᵧA

The notation ˣᵧA places the mass number x at the top-left and the proton number y at the bottom-left of the element symbol A. This compact format communicates both nuclear values in a single label.

Reading the notation:

• The top-left number (x) is the mass number, also called the nucleon number. It equals protons + neutrons.
• The bottom-left number (y) is the proton number, also called the atomic number. It equals the number of protons.
• The number of neutrons is found by subtracting: $\text{neutrons}=x-y$.

For a neutral atom, the number of electrons equals the proton number. For example, ²³₁₁Na contains 11 protons, 11 electrons and $23-11=12$ neutrons.

Identical Chemical Properties of Isotopes

Isotopes of the same element show identical chemical properties because chemical behaviour depends only on the number and arrangement of electrons.

All isotopes of an element have the same proton number. In a neutral atom, the number of electrons equals the number of protons. Therefore all isotopes of an element have the same electron configuration.

Chemical reactions involve the loss, gain or sharing of electrons in the outer shell. Because isotopes share identical electron configurations, they form the same bonds and undergo the same reactions. Neutrons sit in the nucleus and play no role in bonding.

MisconceptionStudents often write that isotopes “react slightly differently” because they have different masses. Mass affects rate of reaction marginally but the chemistry itself is identical. The products formed and the reaction types are unchanged. Exam cue: For “explain why” marks, always link to identical electron configuration, not just “same protons”.

Different Physical Properties of Isotopes

Isotopes of the same element show different physical properties such as mass and density because their nuclei contain different numbers of neutrons.

Each additional neutron adds approximately one atomic mass unit to the nucleus. A heavier isotope therefore has a greater atomic mass than a lighter isotope of the same element. For example, ²H has roughly twice the mass of ¹H.

Density is mass per unit volume. Because isotopes have effectively the same atomic volume (the electron cloud is identical), the heavier isotope packs more mass into the same volume. The heavier isotope therefore has a greater density.

The syllabus limits physical property differences to mass and density. These differences arise solely from the differing neutron numbers in the nucleus.