Noble gases in the context of "Krypton"

A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions.

A molecule may be homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O₂); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H₂O). In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the noble gases are individual atoms. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules.

Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.

On planet Earth, the term 'volatiles' often refers to the volatile components of magma. In astrogeology volatiles are investigated in the crust or atmosphere of a planet or moon. Volatiles include hydrogen, nitrogen, oxygen, water, ammonia, methane, carbon dioxide, nitrogen dioxide, sulfur dioxide, hydrogen sulfide, phosphine, halogens, noble gases and others.

Sir William Ramsay KCB FRS FRSE (/ˈræmzi/; 2 October 1852 – 23 July 1916) was a Scottish chemist who discovered the noble gases and received the Nobel Prize in Chemistry in 1904 "in recognition of his services in the discovery of the inert gaseous elements in air" along with his collaborator, John William Strutt, 3rd Baron Rayleigh, who received the Nobel Prize in Physics that same year for their discovery of argon. After the two men identified argon, Ramsay investigated other atmospheric gases. His work in isolating argon, helium, neon, krypton, and xenon led to the development of a new section of the periodic table.

In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (negative standard Gibbs free energy of formation) yet decomposes at a slow, or negligible rate.

Most of the noble gases, which appear in the last column of the periodic table, are classified as inert (or unreactive). These elements are stable in their naturally occurring form (gaseous form) and they are called inert gases.