Answer
Ions are reactive atoms that are formed when stable atoms lose electrons from or add electrons to their valence shell. The number of positive protons and the number of negative electrons are equal in a stable atom. If this stable atom loses an electron from its valence shell, the number of its protons(+ve) will become greater than the number of its negative electrons(+ve) and a cation ( +ve ion) will have been formed.
On the other hand, if an atom gains an electron and that results in the total number of its electrons exceeding the number of protons in its nucleus, it will have an excess negative charge. An atomic species with an excess negative charge is called an anion.
Work Step by Step
The element sodium has 11 protons in its nucleus and 11 electrons in its electron energy shells. However, there is only one electron in the sodium 3S valence shell. If the Sodium atom loses the 3S electron, it will be left with only 10 electrons. But it will still have 11 protons in its nucleus giving it a positive charge and making it a cation.
Anions are formed when electro-neutral chemical species acquire a negative charge. A chlorine atom has 17 protons in its nucleus, and 17 orbital electrons-- it has 7 electrons in the valence shell. If the valence shell accepts one electron, this will result in a chemical species with 18 electrons (18-ve). But the nucleus still has only 17 protons (18+ve), so the chlorine atom becomes a chloride anion with a single negative charge(1-ve).
