The simplest element, containing a single proton in its nucleus. Normal hydrogen is , but varieties with and two neutrons also exist: (deuterium) and ) tritium). The hydrogen atom also has a single electron. A hydrogen spin-flip transition can occur when the electron changes the sign of its spin () relative to the proton spin. The hydrogen molecule consists of two bound hydrogen atoms (), and exhibits different thermodynamic properties depending on the relative spins of the two protons.

Balmer Formula, Balmer Lines, Deuterium, Fusion, Hydrogen Atom, Hydrogen Bomb, Hydrogen Maser Frequency Standard, Hydrogen Molecule, Hydrogen Molecule Ion, Hydrogen Lines, Hydrogen Ortho-Para States, Hydrogen Spin-Flip Transition, Liquid Metallic Hydrogen, Tritium