Atoms are neutral; they contain the same number of protons as electrons. By definition, an ion is an electrically charged particle produced by either removing electrons from a neutral atom to give a positive ion or adding electrons to a neutral atom to give a negative ion. When an ion is formed, the number of protons does not change.
Neutral atoms can be turned into positively charged ions by removing one or more electrons. A neutral sodium atom, for example, contains 11 protons and 11 electrons. By removing an electron from this atom we get a positively charged Na + ion that has a net charge of +1.
Atoms that gain extra electrons become negatively charged. A neutral chlorine atom, for example, contains 17
protons and 17 electrons. By adding one more electron we get a negatively charged Cl - ion with a net charge of -1.
The gain or loss of electrons by an atom to form negative or positive ions has an enormous impact on the chemical and physical properties of the atom. Sodium metal, for example, which consists of neutral sodium atoms, bursts into flame when it comes in contact with water. Neutral chlorine atoms instantly combine to form Cl2 molecules, which are so reactive that entire communities are evacuated when trains carrying chlorine gas derail. Positively charged Na + and negatively charged Cl - ions are so unreactive that we can safely take them into our bodies whenever we salt our food.