Hydrogen ions, collectively called hydrons, are a type of cation, a positively-charged ion. Hydrogen comes in three forms, or isotopes: proteon, which has a single proton and electron; deuterium, which has a proton, neutron and electron; and tritium, which has a proton, two neutrons and electron. The ions of each, in order, are called proton, deuteron, and triton. Proteon makes up more than 99.9 percent of all hydrogen. There is little difference between the behavior of the three hydrogen ions, but both deuterium and tritium are heavier than proteon, and tritium is also radioactive.
When scientists measure the pH, or acid level, of any substance, they are actually measuring the concentration of hydrogen ions in that substance. A pH of 7 indicates a chemically neutral substance, with no effective free hydrogen ions. Lower pH levels indicate an acid, with more free hydrogen ions as the number decreases. A substance with a pH of zero would have no H2O water; instead, all water would be converted to H3O, with each water molecule also containing a hydrogen ion.
The hydrogen ions in acid cause corrosion to metals by oxidizing the surface. Metals tend to have a weak negative charge. Hydrogen ions, with their strong positive charge, bond easily with metals, changing the chemical composition of, for example, iron to rust. It is not water that causes rust, but rather the hydrogen ions commonly found in water that turn ordinary water into a weak acid; in rain water, for instance, dissolved carbon dioxide in the water forms carbonic acid.
Acids are solutions in which the hydrogen ion from a substance bonds more closely to a water molecule; this results in the hydrogen ionizing, leaving behind a negatively-charged molecule. The high reactivity of the ions left behind creates most of the acid's properties. An acid is called a proton donor because it donates a proton, or hydrogen ion, to water molecules. A base, on the other hand, is called a proton acceptor. In an acid-base reaction, the hydroxide ion in the base pulls the hydrogen ion away from the water molecule it has bonded with in the acid. The remaining ions create a salt. The best-known reaction is probably hydrogen chloride (HCl), the base, blending with sodium hydroxide (NaOH) to create water (H2O) and table salt (NaCl). Without the free hydrogen ion, this important reaction could not occur.