Isotopes
- Elements are defined by a fixed number of protons in their atoms
- For example, all hydrogen atoms have 1 proton, and all carbon atoms have 6 protons
- However, atoms of an element can have different numbers of neutrons
- These different versions of elements are called isotopes
- An isotope is defined as:
Nuclei that have the same number of protons but different numbers of neutrons
- For example, hydrogen has two isotopes, deuterium and tritium
- All three isotopes contain 1 proton, but different numbers of neutrons
The three atoms shown above are all forms of hydrogen, but they each have different numbers of neutrons
- Since nucleon number A includes the number of protons and neutrons, an isotope of an element will have
- A fixed proton number, Z
- A different nucleon number, A
- Some isotopes have an imbalance of neutrons and protons which makes them unstable
- This means they constantly decay and emit radiation to achieve a more stable form
- This can happen from anywhere between a few nanoseconds to 100,000 years
Differences between isotopes
- The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
- This is because neutrons have no charge but do have mass
- The charge of the nucleus of a particular element is always the same
- In the periodic table, the mass number of Chlorine is often given as 35.5
This section of the periodic table shows chlorine as having a mass number of 35.5, but other elements have an integer mass number
- The mass number of Chlorine is given as 35.5 because it has two isotopes with mass numbers of 35 and 37, and these occur in a ratio of 3:1 respectively so 35.5 is the average nucleon / mass number
- The number of electrons and protons in different isotopes remains the same
- Less common isotopes tend to be more unstable due to the imbalance of protons and neutrons
Isotopic Data
- Isotopic data is defined as:
The relative amounts of different isotopes of an element present within a substance
- The mass of an element is often given as relative atomic mass
- The relative atomic mass of an element can be calculated using the relative abundance values
- The percentage abundance of different isotopes in a sample can be obtained using a mass spectrometer
Table of isotopic data for a sample of oxygen
- For example, a sample of oxygen may contain three isotopes: , and
- The relative abundance of this sample of oxygen can be calculated using:
(16 × 0.9976) + (17 × 0.0004) + (18 × 0.002) = 16.0044
- To two decimal places, the relative abundance of the sample of oxygen is 16.00
- A common use of isotopic data is carbon dating of archaeological artefacts
- This involves using the ratio of the amount of stable isotope carbon-12, to the amount of unstable isotope, carbon-14
- The age of a sample of dead tissue can be determined by comparing the ratio of these isotopes to the ratio in a sample of living tissue
Worked example
Which of the following rows shows a pair of nuclei that are isotopes of one another?
nucleon number | number of neutrons | ||
A. |
nucleus 1 nucleus 2 |
39 35 |
19 22 |
B. |
nucleus 1 nucleus 2 |
37 35 |
20 18 |
C. |
nucleus 1 nucleus 2 |
37 35 |
18 20 |
D. |
nucleus 1 nucleus 2 |
35 35 |
20 18 |
Answer: B
- In nucleus 1:
- Nucleon number: 37
- Neutrons: 20
- Protons = 37 − 20 = 17
- In nucleus 2:
- Nucleon number: 35
- Neutrons: 18
- Protons = 35 − 18 = 17
- They have the same number of protons but different numbers of neutrons hence, they are isotopes of each other