Editing Chemistry (section)

====Molecule====
{{Main|Molecule}}
[[File:Caffeine (1) 3D ball.png|upright=1.05|thumb|right|A ball-and-stick representation of the [[caffeine]] molecule (C<sub>8</sub>H<sub>10</sub>N<sub>4</sub>O<sub>2</sub>)]]

A ''molecule'' is the smallest indivisible portion of a pure [[chemical substance]] that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. However, this definition only works well for substances that are composed of molecules, which is not true of many substances (see below). Molecules are typically a set of atoms bound together by [[covalent bond]]s, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in [[lone pair]]s.

Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named a [[molecular ion]] or a polyatomic ion. However, the discrete and separate nature of the molecular concept usually requires that molecular ions be present only in well-separated form, such as a directed beam in a vacuum in a [[mass spectrometer]]. Charged polyatomic collections residing in solids (for example, common [[sulfate]] or [[nitrate]] ions) are generally not considered "molecules" in chemistry. Some molecules contain one or more unpaired electrons, creating [[Radical (chemistry)|radicals]]. Most radicals are comparatively reactive, but some, such as [[nitric oxide]] (NO) can be stable.

[[File:Benzene-2D-full.svg|thumb|upright=0.7|left|A 2-D [[structural formula]] of a [[benzene]] molecule (C<sub>6</sub>H<sub>6</sub>)]]
The "inert" or [[Noble gas|noble gas elements]] ([[helium]], [[neon]], [[argon]], [[krypton]], [[xenon]] and [[radon]]) are composed of lone atoms as their smallest discrete unit, but the other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various [[pharmaceutical]]s.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as [[ionic compounds]] and [[network solids]], are organized in such a way as to lack the existence of identifiable molecules ''per se''. Instead, these substances are discussed in terms of [[formula unit]]s or [[unit cell]]s as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as [[table salt]]), solids like carbon and diamond, metals, and familiar [[silica]] and [[silicate minerals]] such as quartz and granite.

One of the main characteristics of a molecule is its geometry often called its [[molecular structure|structure]]. While the structure of [[diatomic]], [[triatomic]] or tetra-atomic molecules may be trivial, ([[Linear molecular geometry|linear]], angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.