Helium

Helium (He) Second lightest and second most abundant elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of (after HydrogenLightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • H (99.9885 %)
• H (0.0115 %), also called deuterium.
• H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere.
) in the universeThat which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all that exists physically, including matter, energy, physical laws, space, and time. Also, a cosmological model of the universe.. The most abundant isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: H, H (deuterium), and H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. is ⁴He (99.9998%), ³He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burningProcesses by which hydrogen (H) is fused into helium (He) with in a star. The five possible fusion paths can be divided into two sets of processes: the Proton-Proton (PP) process, which depends only on the amount of H and He in the star, and the CNO cycle (carbon-nitrogen-oxygen), which inside stars, but this process cannot, by itself, account for the amount of helium actually observed. Production of He in the Big BangBeginning point of time and space for the universe. A state of extremely high (classically, infinite) density and temperature from which the universe began expanding. (when temperatures were ~109 K) can account for the deficit, an observation strongly favoring the Big Bang theory.