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In molecular biology and pharmacology, a small molecule or micromolecule is a low molecular weight (≤ 1000 daltons [1]) organic compound that may regulate a biological process, with a size on the order of 1 nm [citation needed]. Many drugs are small molecules; the terms are equivalent in the literature.
A molecule may be homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O 2); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H 2 O).
Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to attach and detach from each other is responsible for most of the physical changes observed in nature. Chemistry is the science that studies these changes.
However, molecule size is a much stronger factor on viscosity than polarity, where compounds with larger molecules are more viscous than compounds with smaller molecules. [citation needed] Thus, water (small polar molecules) is less viscous than hexadecane (large nonpolar molecules).
Likewise, larger molecules are generally more polarizable than smaller ones. Water is a very polar molecule, but alkanes and other hydrophobic molecules are more polarizable. Water with its permanent dipole is less likely to change shape due to an external electric field. Alkanes are the most polarizable molecules. [9]
Water is an inorganic compound with the chemical formula H 2 O.It is a transparent, tasteless, odorless, [c] and nearly colorless chemical substance.It is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent [20]).
The mole is widely used in chemistry as a convenient way to express amounts of reactants and amounts of products of chemical reactions. For example, the chemical equation 2 H 2 + O 2 → 2 H 2 O can be interpreted to mean that for each 2 mol molecular hydrogen (H 2) and 1 mol molecular oxygen (O 2) that react, 2 mol of water (H 2 O) form.
Note how the size of the carbon appears smaller than the hydrogen. The importance of stereochemistry was not then recognised and the model is essentially topological (it should be a 3-dimensional tetrahedron). Jacobus Henricus van 't Hoff and Joseph Le Bel introduced the concept of chemistry in three dimensions of space, that is, stereochemistry.