II.

Occurrence

Hydrogen is the tenth most common element on Earth. Because it is so light, though, hydrogen accounts for less than 1 percent of Earth's total mass. It is usually found in compounds. Pure hydrogen gas rarely occurs in nature, although volcanoes and some oil wells release small amounts of hydrogen gas. Many minerals and all living organisms contain hydrogen compounds. Hydrogen is in nearly every compound in the human body. For example, it is in keratin, the main protein that forms our hair and skin, and in the enzymes that digest food in our intestines. Hydrogen is in DNA, the molecules that code our genetic information and make each species of plant or animal unique (and every person unique). Hydrogen is in the molecules in food that provide energy: fats, proteins, and carbohydrates.

Hydrogen is in nearly all organic compounds, or compounds that contain carbon (see Organic Chemistry). Fats, proteins, and carbohydrates are all organic compounds. Other organic compounds that contain hydrogen include the hydrocarbon fuels methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). Alcohols, such as methanol (CH₃OH) and ethanol (C₂H₅OH), are organic compounds that contain hydrogen, carbon, and oxygen. Hydrogen also forms inorganic compounds, or compounds that do not include carbon, such as water (H₂O), ammonia (NH₃), hydrochloric acid (HCl), and sodium hydroxide (NaOH) (see Inorganic Chemistry).

Hydrogen accounts for about 73 percent of the observed mass of the universe and is the most common element in the universe. Spectroscopes, instruments that measure properties of light to detect the element producing it, reveal that hydrogen exists in the Sun and in most, if not all, other stars. Most scientists believe that hydrogen atoms were the first atoms to form in the early universe and that the atoms of the other elements formed later from the hydrogen atoms. Scientific experiments show that about 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent.

Heavier elements form from hydrogen through a process called fusion, the joining of two atoms or parts of atoms to produce a new, larger atom. Fusion is not the same as chemical bonding. In chemical bonding, atoms share electrons to join together as a molecule that can be broken apart to yield the atoms again. In fusion, an atom permanently changes into an atom of another element. Fusion only occurs in nature in stars that reach a temperature of about 200 million degrees C (400 million degrees F). At this temperature, atoms collide with each other at great speeds, enabling them to fuse together. Hydrogen atoms in stars fuse together to form helium atoms, and the fusion reaction releases energy. Once this fusion starts, it heats the star such that heavier elements can form: elements through atomic number 22 (titanium) can form at about 1 billion degrees C (roughly 2 billion degrees F). At higher temperatures, all the natural elements can form.

Hydrogen exists in interstellar space (between stars) as atoms of the gas and as hydrogen molecules, spread out at about one atom or molecule per cubic centimeter. However, a surprisingly large amount of ionized hydrogen (H⁺), hydrogen atoms missing their electron, also exists in the galaxy. Scientists do not understand where it comes from, but they are reexamining theories of astronomy and cosmology to explain its presence.