Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H2, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, no In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer. Under the Brønsted–Lowry acid–base theory, acids are proton donors, while bases are proton acceptors. A bare proton (H+) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as methane can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen cations attached to other chemical species; it is denoted "H+" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "hydronium ion" ([H3O]+), or still more accurately, [H9O4]+. Other oxonium ions are found when water is in acidic solution with other solvents. The concentration of these solvated protons determines the pH of a solution, a logarithmic scale that reflects its acidity or basicity. Lower pH values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions. Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass. and >90% by number of atoms. In the early universe, protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000 years later during the recombination epoch as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. In astrophysics, neutral hydrogen in the interstellar medium is called H I and ionized hydrogen is called H II. Radiation from stars ionizes H I to H II, creating spheres of ionized H II around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years. These are the source of the 21-centimeter hydrogen line, at 1420 MHz, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha…

The pH of a solution indicates its acidity or basicity. Litmus or pH paper is filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator to test how much acid (acidity) or base (basicity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution. Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H30+). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water. The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water.