In normal conditions the number of electrons that revolve round the nucleus of an atom is exactly the same as the number of protons. The negative and positive charges are therefore the same and atom is electrically neutral. But when there is no such balance of positive and negative inside the atom, because an electron has been lost or one extra one has been gained from another atom, the protons in the nucleus can no longer keep the negative-positive balance equal. From that point the atom ceases to be neutral and becomes charged with electricity. The charge is negative if there are too many electrons, and positive if the electrons are fewer in number.

Atoms form various elements according to how many protons and electrons they possess. For example, the atom which has only one proton and one electron forms hydrogen; the atom with two protons and two electrons forms helium; the atom with three of each forms lithium.

In nature, about ninety elements exist, many of them very rare. Other elements have been created artificially by nuclear processes, adding protons and electrons to atoms to form new elements such as einsteinium, mendelevium, nobelium and lawrencium. This last element has 103 electrons and a nucleus of 103 protons.