Isotopes

Atoms of the same element always have the same number of protons. They can, however, have different numbers of neutrons. These different forms of the element are called isotopes.

The proton number determines which element an atom belongs to. Changing the number of neutrons does not change the element, but it does change the total number of protons and neutrons in the atom. This total is called the mass number.

For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. Their mass numbers are therefore 12 and 14, respectively.

Atomic and ionic symbols

An atomic symbol gives the element and the numbers needed to work out its subatomic particles. In $$\ce{^{12}_{6}C}$$, C is the symbol for carbon.

• The proton number, at the lower left, is 6. The atom has 6 protons.
• The mass number, at the upper left, is 12. This is the total number of protons and neutrons.

The neutron number is the mass number minus the proton number. This carbon atom therefore has 6 neutrons. It is neutral, so it has the same number of electrons as protons: 6 electrons.

Charge in ionic symbols

An ion has a charge shown at the upper right of its symbol. A negative charge means the atom has gained electrons; a positive charge means it has lost electrons. The size of the charge shows how many electrons have been gained or lost.

For example, $$\ce{^{35}_{17}Cl^-}$$ has 17 protons and 18 neutrons. Its single negative charge means it has gained one electron, so it has 18 electrons.

Forming an ion changes the number of electrons, not the numbers of protons or neutrons shown by the symbol.