Isotopes
atomThe smallest part of an element that can exist. of the same element must 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 they can have different numbers of neutronUncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0.. Atoms of the same element with different numbers of neutrons 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.
Learn more on isotopes in this podcast.
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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 abundant (most common) isotope of hydrogen.
| Isotope | Symbol | Protons | Electrons | Neutrons |
| Hydrogen-1 | \(_{1}^{1}\textrm{H}\) | 1 | 1 | 1 - 1 = 0 |
| Hydrogen-2 | \(_{1}^{2}\textrm{H}\) | 1 | 1 | 2 - 1 = 1 |
| Hydrogen-3 | \(_{1}^{3}\textrm{H}\) | 1 | 1 | 3 - 1 = 2 |
| Isotope | Hydrogen-1 |
|---|---|
| Symbol | \(_{1}^{1}\textrm{H}\) |
| Protons | 1 |
| Electrons | 1 |
| Neutrons | 1 - 1 = 0 |
| Isotope | Hydrogen-2 |
|---|---|
| Symbol | \(_{1}^{2}\textrm{H}\) |
| Protons | 1 |
| Electrons | 1 |
| Neutrons | 2 - 1 = 1 |
| Isotope | Hydrogen-3 |
|---|---|
| Symbol | \(_{1}^{3}\textrm{H}\) |
| Protons | 1 |
| Electrons | 1 |
| Neutrons | 3 - 1 = 2 |
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 propertiesA description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc.. This is because the number of electrons determines chemical properties, and all three isotopes have one electron in their atoms.
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. can be found 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 (protons or neutrons cannot be split into parts)
- 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.