1.1.3Chemical Substances, Reactions and Essential Resources
Relative Atomic Mass and Relative Molecular Mass
5 min read 6 parts Lesson 4 of 46 1 quiz
Section 1 of 6Comparing the masses of atoms
Why a comparison is needed
- Why a comparison is needed Atoms are far too small to weigh directly, so chemists compare their masses with a standard atom.
- The standard is carbon-12, which is given a mass of exactly 12 units.
Relative atomic mass, Ar
- Relative atomic mass, Ar The relative atomic mass of an element shows how heavy its atoms are compared with carbon-12. It has no units.
- Magnesium has Ar = 24, so its atoms are twice as heavy as carbon atoms.
- Hydrogen has Ar = 1, so its atoms are one twelfth of the mass of a carbon-12 atom.
Finding it on the Periodic Table
- Finding it on the Periodic Table On the Periodic Table the larger number of the pair beside each symbol is the relative atomic mass and the smaller one is the atomic number.
- Hydrogen is the only element where the two numbers are the same, since Ar = 1 and Z = 1.
Section 2 of 6Relative formula mass, Mr
1
Write down the formula and list each type of atom in it.
2
Count how many atoms of each element there are. A subscript number applies to the symbol before it.
3
If there are brackets, multiply everything inside the brackets by the subscript outside them.
4
Look up the Ar of each element and multiply it by the number of atoms.
5
Add all the results together. The total is the Mr, which has no units.
Section 3 of 6Worked calculations
| Substance | Working | Mr |
|---|---|---|
| Hydrogen, H2 | 2 × 1 | 2 |
| Water, H2O | (2 × 1) + 16 | 18 |
| Sodium chloride, NaCl | 23 + 35.5 | 58.5 |
| Copper oxide, CuO | 63.5 + 16 | 79.5 |
| Potassium carbonate, K2CO3 | (2 × 39) + 12 + (3 × 16) | 138 |
| Calcium hydroxide, Ca(OH)2 | 40 + (2 × 16) + (2 × 1) | 74 |
| Magnesium nitrate, Mg(NO3)2 | 24 + (2 × 14) + (6 × 16) | 148 |
| Ammonium sulfate, (NH4)2SO4 | (2 × 14) + (8 × 1) + 32 + (4 × 16) | 132 |
Section 4 of 6Atomic mass, mass number and Ar (HT only)
Mass of an atom
Mass number (HT only)
- Mass number (HT only) The mass number is the total number of protons and neutrons in the nucleus of one atom, so it is always a whole number.
Relative atomic mass (HT only)
- Relative atomic mass (HT only) Ar is the average mass of the atoms of an element, taking its isotopes and their abundances into account, compared with one twelfth of a carbon-12 atom.
Why Ar can be a decimal (HT only)
- Why Ar can be a decimal (HT only) Chlorine is a mixture of isotopes with mass numbers 35 and 37, mainly 35,
- so its average is 35.5 rather than a whole number.
Formula mass versus molecular mass
- Formula mass versus molecular mass Relative molecular mass is used for simple molecules; relative formula mass is the term used for ionic compounds,
- but both are found by adding Ar values.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Relative atomic mass (Ar) | The mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom. |
| Relative formula mass (Mr) | The sum of the relative atomic masses of all the atoms shown in the formula. |
| Carbon-12 | The standard atom, with a mass of exactly 12 units, used for comparing atomic masses. |
| Mass number | The number of protons plus neutrons in the nucleus of an atom. |
| Atomic number | The number of protons in the nucleus of an atom. |
Section 6 of 6Exam technique
1
Confusing the atomic number with the relative atomic mass. The relative atomic mass is the bigger number on the Periodic Table.
2
Ignoring the number outside a bracket, for example counting only one nitrate group in Mg(NO3)2.
3
Writing units such as grams after Ar or Mr. They are ratios and have no units.
4
Rounding chlorine to 35 or 36. Use 35.5 unless the question gives something different.