Hydrogen is the chemical element with the symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table.Hydrogen is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state.The most common isotope of hydrogen, termed Dictionary.com is the world’s leading online source for English definitions, synonyms, word origins and etymologies, audio pronunciations, example sentences, slang phrases, idioms, word games, legal and medical terms, Word of the Day and more. For over 20 years, Dictionary.com has been helping millions of people improve their use of the English language with its free digital services. Welcome to H&M online. Find fashionable clothing for men, women and kids at affordable prices, plus stylish home décor. Shop online today.