Molecules are the basic units of matter and the defining structure of objects in the physical world. All knownmatter is composed of molecules, including solids, liquids, gases, and plasma. They range in size from extremely small (such as atoms and subatomic particles) to very large (such as DNA). In addition to their size, molecules can be distinguished by their chemical composition and structure.
A molecule consists of two or more atoms that are held together by chemical bonds. These bonds can be either ionic (electrostatic) or covalent (polar or non-polar). Ionic bonds occur when one atom donates an electron to another atom; these electrons then act like a glue to hold the atoms together. Covalent bonds occur when two atoms share electrons; this sharing creates a strong bond between the atoms.
The type of bond that forms between two atoms depends on their relative electronegativity; Electronegativity is a measure of how strongly an atom attracts electrons to itself. When two atoms have equal electronegativities, they form a non-polar covalent bond. When one atom has a higher electronegativity than the other, it will form a polar covalent bond. If the difference in electronegativities is great enough, an ionic bond will form.
The structure of a molecule also plays an important role in its properties. The way that the atoms are arranged within the molecule affects how it interacts with other molecules and its overall shape. For example, water molecules are arranged in a particular way that gives them unique properties, such as being able to dissolve other substances or absorb heat without changing temperature themselves. The arrangement of atoms within a molecule can also give rise to optical activity—the ability to rotate plane-polarized light—as well as magnetic properties.