Ion
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Description
An ion is an electrically charged part of a molecule, consisting of an atom or a radical with one or more electrons missing or in excess. A positively charged ion is called a cation, because it is attracted to a negative electrode, called a cathode. A negatively charged ion is called an anion and is attracted to an anode.
Detailed description
A neutral water molecule consists of two atoms of hydrogen H and one of oxygen O, H2O. If ionised, one of the hydrogen atoms loses an electron and the remaining hydroxyl group acquires the electron. The cationic hydrogen ion is thus positively charged and the anionic hydroxyl group is negatively charged: H+ and OH- respectively.
Most salts are easily split into ions. Common salt or sodium chloride, NaCl, is ionised when dissolved in water, constantly splitting into Na+ and Cl- ions and reverting back to salt. The bond between the atoms in such a salt is termed ionic. Electroplating works because of the polarity of the ions. The object to be plated is made the cathode of a cell. If it contains, for example, a solution of copper sulfate and the anode is copper, applying a DC voltage will cause copper ions Cu++ to migrate to the cathode, where they will gain their "lost" electron and revert to metallic copper. At the same time, the sulfate anions SO4-- ions will migrate to the copper anode, where they will combine with the metal, giving up their electrons to the electric current, and dissolving it into copper sulfate.
Except for amphoteric metals, metal salts usually form the cations. The polarity of the cations (i.e., the number of lost electrons in the ion) is the same as the valency of the metal in an ionic solution.
