AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
IB® (SL and AHL)
YouTube Channel
YouTube Channel
@chemistry.student
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

S1.4 - Counting particles by mass - The Mole

1.4.1 The Mole and Avagadro's Constant 1.4.2 Relative Atomic Mass (Ar) and Relative Formula Mass (Mr) 1.4.3 Molar Mass, Mass and the Mole 1.4.4 Empirical and Intermolecular Formulae 1.4.5 Molar Concentration and Solution Calculations 1.4.6 Avagadro's Law and Gas Volumes

Relative Atomic Mass (Ar) and Relative Formula Mass (Mr)

Specification Reference S1.4.2

Quick Notes:

  • Relative atomic mass (Ar) compares the mass of an atom to ¹²C, which is defined as exactly 12.
  • Relative formula mass (Mr) is the total of the Ar values of all atoms in a formula.
  • Both Ar and Mr are unitless (no units).
  • To calculate Mr, simply add up the Ar values for each atom in the compound’s formula.

Full Notes:

Relative Atomic Mass (Ar)

The relative atomic mass (Ar) of an element is a weighted average of the atomic masses of its naturally occurring isotopes, compared to carbon-12.

Carbon-12 is the standard: 1 atom of ¹²C = exactly 12 units.

Ar values are not whole numbers because they reflect isotope averages.

Examples:

Relative Formula Mass (Mr)

The Relative Formula mass (Mr) of a compound is the sum of the Ar values of all atoms in its chemical formula.

It is sometimes also called molecular mass (for molecules) or formula mass (for ionic compounds).

Mr = (Number of atoms × Ar) for each element, then add them up.

Example: Mr of Water (H2O)

Hydrogen Ar = 1.0
Oxygen Ar = 16.0
Mr = (2 × 1.0) + (1 × 16.0) = 18.0

IB Chemistry example showing Mr calculation for water H2O using Ar values of hydrogen and oxygen.

Example: Calcium chloride (CaCl2)

Ca = 40.08
Cl = 35.45 × 2 = 70.90
Mr = 40.08 + 70.90 = 110.98

Example: Ammonium sulfate ((NH4)2SO4)

N = 14.01 × 2 = 28.02
H = 1.01 × 8 = 8.08
S = 32.07
O = 16.00 × 4 = 64.00
Mr = 28.02 + 8.08 + 32.07 + 64.00 = 132.17

Photo of Matt
Matt’s exam tip

Remember Ar and Mr are relative, not actual masses. They are unitless. Arways use Ar values from the IB data booklet, not whole number rounded estimates.

Summary

Linked Question

Structure 3.1 – Linked Course Question

Atoms increase in mass as their position descends in the periodic table. What properties might be related to this trend?

As atomic mass increases down a group, several related properties change systematically. Heavier atoms have more electron shells, so atomic radius and ionic radius increase. This greater distance and shielding reduce the attraction between the nucleus and outer electrons, lowering ionisation energy and electronegativity. The higher mass and larger number of electrons also increase London dispersion forces, leading to higher melting and boiling points for many non-metallic elements.