The seven diatomic molecules are hydrogen, nitrogen, oxygen, fluorine, chlorine, iodine and bromine. This means that under normal pressures and temperatures most of them appear as two covalently bonded atoms. All of them are gases.
The only ones that are not diatomic at room temperature are iodine and bromine. Iodine is a solid at standard room temperature and pressure, and bromine is a liquid.
Four of the elements, fluorine, chlorine, iodine and bromine, are halogens. These are quite reactive elements that lack electrons in their outer shells and grab them from other elements. Fluorine, which is at the top of the halogen list, is so reactive that it causes steel wool to catch fire. Halogens get their name because they are derived from salts or minerals. Though dangerous as pure elements, halogens are useful when they form compounds. Fluorine is used to prevent tooth decay, and chlorine is used as a disinfectant. Bromine and iodine are used in halogen light bulbs.
Though uncommon in Earth's atmosphere, hydrogen is the universe's most plentiful element. Stars that are on the main sequence fuse hydrogen to create helium, and hydrogen joins with oxygen on Earth and other planets to make water.
Oxygen and nitrogen make up most of the chemicals of air. Nitrogen makes up about 78 percent of air, while oxygen is about 21 percent of air.