1.3.2Principles of Chemistry
Calculate Relative Atomic Mass — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 16 of 50
Part 1 of 5Why relative atomic mass is not a whole number
Isotopes
- Isotopes Most elements exist as a mixture of isotopes,
- which are atoms with the same number of protons but different numbers of neutrons and therefore different mass numbers.
Weighted average
- Weighted average The relative atomic mass, Ar, is an average of the isotope masses that takes into account how much of each isotope is present in a typical sample.
Examples
- Examples Chlorine is about three-quarters Cl-35 and one-quarter Cl-37, so its Ar is 35.5, which lies between the two mass numbers and closer to 35.
Part 2 of 5The calculation
1
List each isotope with its mass number and its percentage abundance.
2
Multiply the mass number of each isotope by its percentage abundance.
3
Add all the products together.
4
Divide the total by 100 (or by the sum of the abundances) to get Ar.
5
Check the answer lies between the lightest and heaviest isotope and is nearer the more abundant one, and give it to the number of decimal places asked for.
Part 3 of 5Formula
| Quantity | Equation |
|---|---|
| Two isotopes | Ar = [(% of A × mass of A) + (% of B × mass of B)] ÷ 100 |
| Three isotopes | Ar = [(% of A × mass of A) + (% of B × mass of B) + (% of C × mass of C)] ÷ 100 |
| Abundances given as ratio or fraction | Use the same pattern, dividing by the total of all the abundance values |
Part 4 of 5Worked example: rubidium
The data
- The data A sample of rubidium contains 72% of the isotope with mass number 85 and 28% of the isotope with mass number 87.
The working
- The working Ar = [(72 × 85) + (28 × 87)] ÷ 100 = (6120 + 2436) ÷ 100 = 8556 ÷ 100.
The answer
- The answer Ar = 85.6 to one decimal place, which is between 85 and 87 and nearer 85, as expected.
Part 5 of 5Spotting isotopes in questions
Same symbol, different mass number
Notation
- Notation Isotopes of one element have the same chemical symbol and atomic number but different mass numbers, such as copper-63 and copper-65.
Which has more neutrons
- Which has more neutrons The isotope with the larger mass number has more neutrons, because the proton number is the same.
Same chemistry
- Same chemistry The isotopes react in the same way because they have the same number of electrons.
Key terms
| Term | Meaning |
|---|---|
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons. |
| Relative atomic mass | The weighted average mass of the isotopes of an element compared with 1/12 of a carbon-12 atom. |
| Percentage abundance | The proportion of atoms in a sample that are a particular isotope, given as a percentage. |
| Mass number | The total number of protons and neutrons in an atom. |
| Weighted average | An average in which each value counts according to how common it is. |
Avoid these mistakes
1
Simply averaging the two mass numbers. The abundances must be used, since the isotopes are not present in equal amounts.
2
Forgetting to divide by 100 at the end, which gives an answer far too big.
3
Giving an answer that is outside the range of the isotope masses. Always check it lies between them.
4
Rounding too early. Keep the full figures until the final line and then round as instructed.
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