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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S1.2 - The nuclear atom

1.2.1 Nuclear Atom and Sub-Atomic Particles 1.2.2 Isotopes and Atomic Mass 1.2.3 Mass Spectrometry and Isotopes (AHL)

Isotopes and Relative Atomic Mass

Specification Reference S1.2.2

Quick Notes

  • Isotopes are atoms of the same element with different numbers of neutrons.
  • They have the same number of protons and electrons, so they behave the same chemically.
  • Physical properties like mass, density, and boiling point can differ between isotopes.
  • The relative atomic mass (Ar) of an element is a weighted average of its naturally occurring isotopes.
  • Ar is often not a whole number because it reflects this average.
  • Ar is calculated using percentage abundance and isotopic masses.

Full Notes

What Are Isotopes?

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons. This means:

For Example: Carbon contains naturally occurring isotopes including Carbon-12, Carbon-13 and Carbon-14:

IB Chemistry diagram showing isotopes of carbon: C-12 with 6 protons and 6 neutrons, C-13 with 6 protons and 7 neutrons, and C-14 with 6 protons and 8 neutrons.

All have 6 protons.

C-12 has 6 neutrons, C-13 has 7 neutrons and C-14 has 8 neutrons.

Physical vs Chemical Properties of Isotopes

Chemical properties are the same because isotopes have the same number of electrons.

However physical properties can differ, such as:

Relative Atomic Mass (Ar)

Most elements exist as a mixture of isotopes. The relative atomic mass (Ar) of an element is the weighted average of the masses of its isotopes.

Ar is calculated from:

Formula:

Relative atomic mass = (mass of isotope 1 × % abundance) + (mass of isotope 2 × % abundance) + … ÷ 100

Example: Calculating Ar for Chlorine

Chlorine has two main isotopes:

The average relative mass of a chlorine atom is based on the amounts of each isotope in a natural sample of chlorine.

IB Chemistry calculation showing weighted average of chlorine isotopes: 35×0.75 + 37×0.25 = 35.5.

Summary

Linked Questions

Nature of Science, Reactivity 3.4 – Linked Course Question

How can isotope tracers provide evidence for a reaction mechanism?

Isotope tracers allow chemists to follow the path of specific atoms through a reaction by replacing one or more atoms in a reactant with isotopes of the same element. Because isotopes behave chemically the same way but can be detected by their mass or radioactivity, their movement reveals which bonds break and form during the reaction. By analysing where the isotope appears in the products, scientists can confirm or reject proposed mechanisms and identify intermediate steps in complex reactions.