Molecule
A molecule is the group of two or more atoms held together by attractive forces required as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, as well as biochemistry, a distinction from ions is dropped in addition to molecule is often used when referring to polyatomic ions.
In the kinetic concepts of gases, the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the prerequisite that a molecule contains two or more atoms, since the noble gases are individual atoms.
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.
Atoms and complexes connected by non-covalent interactions, such(a) as hydrogen bonds or ionic bonds, are typically not considered single molecules.
Molecules as components of matter are common. They also hold up almost of the oceans and atmosphere. nearly organic substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids of which they are composed, the nucleic acids DNA and RNA, sugars, carbohydrates, fats, and vitamins. The nutrient minerals are generally ionic compounds, thus they are not molecules, e.g. iron sulfate.
However, the majority of familiar solid substances on Earth are portrayed partly or completely of crystals or ionic compounds, which are not submission of molecules. These include all of the minerals that form up the substance of the Earth, sand, clay, pebbles, rocks, boulders, bedrock, the molten interior, and the core of the Earth. All of these contain numerous chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for salts nor for covalent crystals, although these are often composed of repeating unit cells that conduct either in a plane, e.g. graphene; or three-dimensionally e.g. diamond, quartz, sodium chloride. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with metallic bonding. Thus solid metals are not made of molecules.
In glasses, which are solids that represent in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals.