Hydrogen

Having only one electron in each of its atoms, hydrogen has two options for combining chemically with another atom. For one thing it can pair up its single electron with an electron from a non-metal atom to make a shared-pair covalent bond. Examples of such compounds are H₂O, H₂S and NH₃ (water, hydrogen sulfide and ammonia) and virtually all of the millions of organic compounds. Or, it can take on an extra electron to become the negative ion H^-, called a hydride ion, and combine with a metallic positive ion. Examples are lithium hydride LiH and calcium hydride CaH₂, but these compounds are unstable in water and decompose to form hydrogen gas.

Hydrogen reacts with all the halogens to form hydrogen halides, such as hydrogen chloride HCl and hydrogen fluoride HF. These compounds are acids when dissolved in water, and are used among other things to dissolve metals and, in the case of HF, to etch glass.

With sulfur, hydrogen forms hydrogen sulfide, H₂S, a highly poisonous gas. Fortunately, hydrogen sulfide has such a strong and disagreeable odor that people can smell very tiny amounts of it in the air and take steps to put some distance between it and them.