Relative Masses of Atoms and MoleculesCambridge IGCSE Chemistry: Revision notes
Section 1
What Is Relative Atomic Mass?
Atoms are far too small to weigh individually in grams, so chemists compare their masses using a relative scale.
- Relative atomic mass (Ar) is the average mass of the isotopes of an element, compared to 1/12th of the mass of an atom of carbon-12
- It is a weighted average, taking into account the different isotopes of an element and their natural abundances
- Ar has no units, because it is a ratio (a comparison), not an actual mass
Carbon-12 is used as the standard, so it is defined as having a mass of exactly 12. Other elements are compared against this — e.g. magnesium has Ar = 24, meaning its atoms are, on average, twice as heavy as carbon-12 atoms.
Section 2
What Is Relative Molecular (Formula) Mass?
When atoms combine into molecules or ionic compounds, their masses add together.
- Relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms in a molecule
- For ionic compounds (which do not exist as individual molecules), the same idea is called relative formula mass, but it is still given the symbol Mr
- To calculate Mr, add up the Ar values of every atom shown in the formula, counting each atom the correct number of times
For water, H2O: Mr = (2 x 1) + 16 = 18. For calcium carbonate, CaCO3: Mr = 40 + 12 + (3 x 16) = 100.
Section 3
What Is a Molecular Formula and Why Does It Matter for Mass Calculations?
The molecular formula of a compound shows the exact number and type of different atoms present in one molecule of that compound — this is exactly what you need to calculate Mr correctly.
- Each element symbol in the formula tells you which atoms are present
- Any subscript number tells you how many of that atom are in one molecule
- Getting the molecular formula wrong (e.g. missing a subscript) will give an incorrect Mr, so always double-check the formula before adding masses
A common error is forgetting to multiply the mass of an atom by its subscript — e.g. treating H2O as H + O instead of (2 x H) + O.
Section 4
How Are Relative Masses Used to Find Moles?
Relative atomic and molecular mass are the bridge between the mass of a substance (in grams) and the amount of substance (in moles).
The key relationship is:
amount of substance (mol) = mass (g) ÷ molar mass (g/mol)
- The molar mass of a substance (in g/mol) has the same numerical value as its Ar or Mr
- This relationship also allows you to calculate the mass of a given number of moles, or the molar mass if the mass and number of moles are known
- It can also be used to find the number of particles present, using the Avogadro constant
Always check whether a question gives you mass and asks for moles, or gives you moles and asks for mass — rearrange the same equation rather than memorising two versions.
Must Know
- Relative atomic mass (Ar) compares the average mass of an element's isotopes to 1/12th the mass of carbon-12
- Relative molecular mass (Mr) is the sum of the Ar values of all atoms in a molecule; for ionic compounds this is called relative formula mass
- Molecular formula shows exactly which atoms, and how many of each, are present — essential for calculating Mr correctly
- Molar mass (g/mol) has the same numerical value as Ar or Mr
- amount (mol) = mass (g) ÷ molar mass (g/mol) links mass, moles and relative mass together
- Ar and Mr have no units, as they are ratios
That's the notes covered.
Carry on to the next subtopic.