Isotopes
- Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary
- An isotope is defined as:
An atom (of the same element) that has an equal number of protons but a different number of neutrons
- The element hydrogen has two other isotopes: deuterium and tritium
The three atoms shown above are all forms of hydrogen, but they each have different numbers of neutrons
- The neutron number of an atom is found by subtracting the proton number from the nucleon number
- Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon / mass number
- Some isotopes have an imbalance of neutrons and protons, they are 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 found within a substance
- It is used to identify an isotopic signature within organic and inorganic materials
- Isotopic data is often used for determining the age of archaeological findings and is used in radioactive dating
- Carbon–14 is a naturally occurring isotope most often used for this since it is present in all living beings and undergoes radioactive decay
- When a plant or animal dies, the natural radioactive decay of this isotope means the concentration of the carbon–14 in its tissue gradually reduces
- Since carbon–14 has a long half-life of around 6000 years, the half-life can be used to determine the age of the plant or animal when it died
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