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Essential Elements

16 17 hydrogEn hElIum the first two elements Hydrogen makes up three quarters of all known matter in the universe and is a large part of most stars. Element one, the simplest atom, consists of one proton orbited by one electron. Hydrogen gas is diatomic, which means two atoms form one molecule. Highly explosive in air, it burns rapidly with oxygen to create water. Under immense pressures and temperatures, everyday conditions in the cores of giant planets like Jupiter and Saturn, hydrogen becomes metallic. Beyond its common form, hydrogen has two isotopes, deuterium, which has one neutron, and tritium, with two. Tritium is rare, and also unstable and therefore radioactive. The second element in the periodic table has two protons, two electrons and 99.99 of the time two neutrons. Called helium, it is the second most abundant element in the material universe, almost a quarter of it. With two electrons filling the 1sorbital, helium is happy to stay on its own and rarely reacts with other elements. It is the first of the noble or inert gases, each of which have completely full outer electron orbitals. Surprisingly, helium was unknown on earth until 1870 when it was discovered through spectrographic analysis of sunlight, a fingerprinting technique for elements opposite below. Lighter than air, though twice as heavy as hydrogen, the specks of helium formed here quickly float off into outer space. It is a much safer gas than hydrogen in balloons, and when inhaled will produce a squeaky voice.
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