The atoms of a particular element will all have the same number of protons. Their atomic number will be the same. However, the atoms of an element can have different numbers of neutrons - so their mass numbers will be different.
Atoms of the same element with different numbers of neutrons are calledisotopes. The different isotopes of an element have identical chemical properties. However, some isotopes are radioactive.
Chemical symbols
The full chemical symbol for an element shows its mass number at the top, and atomic number at the bottom. Here is the full symbol for carbon:
It tells us that a carbon atom has six protons. It will also have six electrons, because the number of protons and electrons in an atom is the same.
The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 – 6 = 6 neutrons.
Isotopes of hydrogen
Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons. The table summarises these isotopes.
| Isotope | Protons | Electrons | Neutrons |
|---|---|---|---|
| 1 | 1 | 1 - 1 = 0 | |
| 1 | 1 | 2 - 1 = 1 | |
| 1 | 1 | 3 - 1 = 2 |
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