Period 1 element

A chemical element in the first row of the periodic table. The only Period 1 elements are hydrogen and helium.

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Hydrogen: The chemical element with atomic number 1, one proton in each atom’s nucleus, and symbol H. Hydrogen starts Period 1 and occupies the table’s first position.
Electron configuration: A description of how electrons occupy an atom’s orbitals. Hydrogen and helium differ by whether their 1s orbital is half-filled or full.
Alkali metal: A Group 1 metal with one valence electron and a strong tendency to form a +1 ion. Hydrogen shares Group 1 placement but is a nonmetal with very different chemistry.
Hydrogen fuel: Hydrogen used as an energy carrier, either by combustion or in electrochemical fuel cells. Hydrogen’s ability to release energy makes it a candidate fuel, though production and storage pose challenges.
Dmitri Mendeleev: A Russian chemist who formulated an influential periodic table in 1869 and predicted properties of undiscovered elements. His table established the periodic framework in which the first row is understood.
Helium: The chemical element with atomic number 2, symbol He, and a filled first electron shell. Helium completes Period 1 with the first filled electron shell.
1s orbital: The lowest-energy atomic orbital, which can hold up to two electrons. Both Period 1 elements place their electrons in this orbital.
Halogen: A Group 17 element with seven valence electrons, often forming a −1 ion or a covalent bond. Hydrogen can resemble halogens by needing one electron for a filled shell.
Fuel cell: An electrochemical device that converts the chemical energy of a fuel and oxidant directly into electricity. Hydrogen fuel cells use hydrogen and oxygen to generate electricity and water.
Henry Cavendish: An English natural philosopher who identified hydrogen as a distinct substance in the eighteenth century. His experiments characterized hydrogen before it received its modern name.