Hydrogen is an odourless, colourless, neutral gas, and is the lightest of all the elements. Less dense than air, hydrogen-filled balloons quickly rise when released. The atom of its most abundant isotope consists of one proton and one electron. Its second isotope, deuterium, has one neutron, and its third isotope, the radioactive tritium, has two neutrons.
Hydrogen is very reactive, and highly flammable. It burns in air to form water.
Hydrogen is also a very good reducing agent. Passing the gas over heated iron (III) oxide, for example, will reduce the oxide to the metal.
