One of two or more atoms with the same A number equivalent to the number of protons in the nucleus of an atom, commonly abbreviated as Z. Click on Term to Read More (Z), but different mass (A). For example, Lightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • 1H (99.9885 %) • 2H (0.0115 %), also called deuterium. • 3H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere. Click on Term to Read More has three isotopes: 1H, 2H (Also called heavy hydrogen, deuterium is an isotope of hydrogen (D, or 2H) whose nucleus contains one proton and one neutron. As a trace element formed during the nucleosynthesis epoch of the Big Bang, deuterium is an important indicator of the baryon density in the universe. The larger the density, the Click on Term to Read More), and 3H (tritium). Different isotopes of a given Substance 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 Click on Term to Read More have different numbers of neutrons in the Core of an atom, where nearly the entire mass and all positive charge is concentrated. It consists of protons and neutrons. Click on Term to Read More.