What is the first element on the periodic table?

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The first element on the periodic table is hydrogen, symbol H, with atomic number 1: every hydrogen atom has one proton in its nucleus. It sits alone at the top of group 1 in period 1 and is the lightest element, with an atomic mass of about 1.008 u, and it is a colorless, odorless gas at room temperature. Hydrogen is also the most abundant element in the universe, making up roughly three quarters of ordinary matter by mass, and it powers the Sun through nuclear fusion. Henry Cavendish identified it as a distinct substance in 1766, and Antoine Lavoisier named it. Helium (atomic number 2) is the second element and ends period 1, which holds only two elements. Hydrogen's position at the very top reflects its structure: a single proton and a single electron, the simplest atom of any element. Its most common isotope, protium, has no neutrons, while deuterium has one and tritium has two. Hydrogen forms water with oxygen, makes up nearly all organic compounds together with carbon, and is used in ammonia production, oil refining and as a fuel in rockets and fuel cells. It was the first element to form after the Big Bang, and stars like the Sun fuse hydrogen into helium. On the periodic table it heads group 1 and period 1, but it is not an alkali metal, and some tables place it above fluorine in group 17 or alone at the top. Counting by atomic number, the first elements are hydrogen, helium (2), lithium (3), beryllium (4) and boron (5), then carbon (6), nitrogen (7), oxygen (8), fluorine (9) and neon (10), which complete period 2. Hydrogen is also the element most people meet daily, as part of water, sugar and most fuels. Hydrogen is often placed above the alkali metals but behaves as a reactive nonmetal, so some periodic tables put it above the halogens or draw it separately.