Hydrogen is an element, usually in the form of a gas, that consists of one proton and one electron. Hydrogen is the most abundant element in the universe, accounting for about 75 percent of its normal matter, and was created in the Big Bang. Helium is an element, usually in the form of a gas, that consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium is the second-most abundant element in the universe, after hydrogen, and accounts for about 25 percent of the atoms in the universe. Most of the helium in the universe was created in the Big Bang, but it also is the product of hydrogen fusion in stars.
