Wikipedia: Hydrogen - Properties of the element, including its history, applications, and characteristics. ChemicalElements.com: Hydrogen - Basic information, atomic structure, and table of isotopes. Chemical of the Week: Hydrogen - Equations for reactions between hydrogen and other substances. It's Elemental: Hydrogen - Basic physical and historical information. Hydrogen Energies and Spectrum - The Bohr model for an electron transition in hydrogen between quantized energy levels with different quantum numbers n yields a photon by emission with quantum energy. Especiál Gas Inc.: Hydrogen - Numeric data, uses, and general information. EnvironmentalChemistry.com: Hydrogen - Atomic structure, chemical and physical properties, and table of nuclides. Lenntech: Hydrogen - Physical data, chemical properties, health and environmental effects.