# Isotope

## Key Stage 4

### Meaning

Isotopes are atoms of the same element (have the same number of protons) but different numbers of neutrons.

Each element has many isotopes but some are stable so they will exist for billions of years and others are unstable so they decay quickly into other isotopes or a different element.
Different isotopes of the same element have the same atomic number but different atomic mass due to the different numbers of neutrons.
Different isotopes of the same element have the same chemical properties but they have different physical properties. This means they cannot be separated by chemical processes but they can be separated by physical ones.
Different isotopes of the same element may have different boiling points, different melting points, may diffuse at different rates and, as has already been stated, have a different mass. This means isotopes can be separated by processes such as Distillation, Chromatography and Centrifuge.

### Examples

 Hydrogen-1 Hydrogen-2 Lithium-7 Lithium-6 Hydrogen always has 1 proton but isotope there are no neutrons. Hydrogen always has 1 proton but in this isotope there is 1 neutron. This isotope of Hydrogen is known as Deuterium. Lithium always has 3 protons but in this isotope there are 4 neutrons. Lithium always has 3 protons but in this isotope there are 3 neutrons.

### Isotopic Abundance

Different isotopes of the same element do not all appear in the same quantities. Some isotopes are more common than others.
The Periodic Table orders the elements due to their chemical properties but since isotopes of the same element have the same chemical properties then different isotopes are not included.
On the Periodic Table an average atomic mass is given based on the mass of each isotope and how common it is.
To find the average atomic mass the mass of each isotope is considered along with the ratio of abundance.

#### Calculating the Average Relative Atomic Mass

 Chlorine has two common stable Isotopes; Chlorine-35 and Chlorine-37. Chlorine-35 is three times more common than Chlorine-37 so the ratio is 3:1. Find the average atomic mass of Chlorine. Lithium has two common stable Isotopes; Lithium-7 and Lithium-6. 92.5% of all Lithium is the isotope Lithium-7 while Lithium-6 makes up only 7.5%. Find the average atomic mass of Lithium. 1. State the known quantities. Ratio: 3:1 3xChlorine-35 : 1xChlorine-37 1. State the known quantities. Ratio: 92.5:7.5 92.5xLithium-7 : 7.5xLithium-6 2. Substitute the numbers into the equation and solve. To find the average$Average=\frac{35+35+35+37}{4}$ $$Average=\frac{35 \times 3 + 37 \times 1}{4}$$ $$Average=35.5$$ So the Relative Atomic Mass of Chlorine is quoted as 35.5 on the Periodic Table. 2. Substitute the numbers into the equation and solve. To find the average$Average=\frac{7 \times 92.5 + 6 \times 7.5}{100}$ $$Average=6.925$$ So the Relative Atomic Mass of Lithium is quoted as 6.9, 6.93 or 6.925 depending on the precision of that particular Periodic Table.

### References

#### AQA

Isotope, pages 109-10, 117, GCSE Physics; Student Book, Collins, AQA
Isotope; definition, page 110, GCSE Physics; Student Book, Collins, AQA
Isotopes, page 112, GCSE Combined Science Trilogy; Physics, CGP, AQA
Isotopes, page 124, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
Isotopes, page 17, GCSE Chemistry; Third Edition, Oxford University Press, AQA
Isotopes, page 44, GCSE Physics; The Revision Guide, CGP, AQA
Isotopes, page 90, GCSE Physics, Hodder, AQA
Isotopes, pages 119-20, 340, GCSE Combined Science Trilogy 1, Hodder, AQA
Isotopes, pages 13, 21, GCSE Chemistry; The Revision Guide, CGP, AQA
Isotopes, pages 25, 26, 51, GCSE Chemistry, CGP, AQA
Isotopes, pages 27, 28, 51, GCSE Combined Science Trilogy; Chemistry, CGP, AQA
Isotopes, pages 4-5, GCSE Chemistry, Hodder, AQA
Isotopes, pages 96-97, 104-107, 109, GCSE Physics; Third Edition, Oxford University Press, AQA
Isotopes, pages 97, 106, 198, GCSE Combined Science; The Revision Guide, CGP, AQA

#### Edexcel

Isomers, page 173, GCSE Chemistry, Pearson, Edexcel
Isotopes, page 155, GCSE Physics, CGP, Edexcel
Isotopes, page 17, GCSE Chemistry; The Revision Guide, CGP, Edexcel
Isotopes, page 93, GCSE Physics, Pearson Edexcel
Isotopes, pages 166-167, GCSE Combined Science, Pearson Edexcel
Isotopes, pages 22-23, GCSE Chemistry, Pearson, Edexcel
Isotopes, pages 357, GCSE Combined Science, Pearson Edexcel
Isotopes, pages 37, 38, GCSE Chemistry, CGP, Edexcel
Isotopes, pages 51, 53, GCSE Physics; The Revision Guide, CGP, Edexcel
Isotopes, pages 80, 174, 176, GCSE Combined Science; The Revision Guide, CGP, Edexcel

#### OCR

Isotopes, pages 15, 31, Gateway GCSE Chemistry; The Revision Guide, CGP, OCR
Isotopes, pages 170-171, 183, Gateway GCSE Physics, Oxford, OCR
Isotopes, pages 28-29, Gateway GCSE Chemistry, Oxford, OCR
Isotopes, pages 74, 77, 79, Gateway GCSE Physics; The Revision Guide, CGP, OCR
Isotopes, pages 86, 195, Gateway GCSE Combined Science; The Revision Guide, CGP, OCR