Isotopes
atomThe smallest part of an element that can exist. of an elementA substance made of one type of atom only. that have the same number of protonSubatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1., but different numbers of neutronUncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0. in their nucleiNuclei is the plural of nucleus. The nucleus is the central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. are called isotopeAtoms of an element with the same number of protons and electrons but different numbers of neutrons.. Isotopes of an element have:
- the same atomic numberThe number of protons in the nucleus of an atom. Also called the proton number.
- different mass numberThe number of protons and neutrons found in the nucleus of an atom.
Three isotopes of hydrogen
All hydrogen atoms contain one proton (and one electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.), but they can contain different numbers of neutrons. Hydrogen-1 is the most abundanceIf there is a lot of something, it is described as being abundant. isotope of hydrogen.
| Isotope | Symbol | Protons | Neutrons | Electrons |
| Hydrogen-1 | \(_{1}^{1}\textrm{H}\) | 1 | 1 - 1 = 0 | 1 |
| Hydrogen-2 | \(_{1}^{2}\textrm{H}\) | 1 | 2 - 1 = 1 | 1 |
| Hydrogen-3 | \(_{1}^{3}\textrm{H}\) | 1 | 3 - 1 = 2 | 1 |
| Isotope | Hydrogen-1 |
|---|---|
| Symbol | \(_{1}^{1}\textrm{H}\) |
| Protons | 1 |
| Neutrons | 1 - 1 = 0 |
| Electrons | 1 |
| Isotope | Hydrogen-2 |
|---|---|
| Symbol | \(_{1}^{2}\textrm{H}\) |
| Protons | 1 |
| Neutrons | 2 - 1 = 1 |
| Electrons | 1 |
| Isotope | Hydrogen-3 |
|---|---|
| Symbol | \(_{1}^{3}\textrm{H}\) |
| Protons | 1 |
| Neutrons | 3 - 1 = 2 |
| Electrons | 1 |
An isotope is named after the element and the mass number of its atoms. For example, carbon-12 is an isotope of carbon with a mass number of 12.
All three isotopes of hydrogen have identical chemical propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point.. This is because the number of electrons determines chemical properties, and all three isotopes have one electron in their atoms.
Learn more on isotopes in this podcast.
Listen to the full series on ±«Óătv Sounds.
Relative atomic mass
relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom. are given in the periodic table. They have the symbol Ar.
Take care not to confuse mass numbers and relative atomic masses:
- mass numbers are always whole numbers (it is not possible to have part of a proton or neutron)
- relative atomic masses are often rounded to the nearest whole number, but are actually not whole numbers
For example, the relative atomic mass of chlorine is 35.5 rather than a whole number. This is because chlorine contains two different isotopes, chlorine-35 and chlorine-37.
More guides on this topic
- Equations and formulae - Edexcel
- Hazards and risks - Edexcel
- The periodic table - Edexcel
- Ionic compounds - Edexcel
- Simple molecular substances - Edexcel
- Giant covalent substances - Edexcel
- Metals and non-metals - Edexcel
- Chemistry calculations - Edexcel
- Mole calculations (higher) - Edexcel
- Sample exam questions - key concepts in chemistry - Edexcel