Chemical substance can be transmuted into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons. As of 2012, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements. Some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O) and ozone (O). The majority of elements are classified as metals.
Defining equation (physical chemistry) and "z" be its valency, the half reaction takes the form of a reduction reaction: Defining equation (physical chemistry) In physical chemistry, there are numerous quantities associated with chemical compounds and reactions; notably in terms of "amounts" of substance, "activity" or "concentration" of a substance, and the "rate" of reaction. This article uses SI units. Theoretical chemistry requires quantities from core physics, such as time, volume, temperature, and pressure. But the highly quantitative nature of physical chemistry, in a more specialized way than core physics, uses molar amounts of substance rather than simply counting numbers; this leads to the specialized