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A molecule is the smallest identifiable unit of a substance, made up of one or more atoms bound by covalent and ionic electron bonds. The simplest molecules consist of just one atom, such as the noble gases, or two atoms of the same element in a diatomic formation, or two "complementary" atoms of different elements in a biatomic salt. However, molecules, especially in living creatures, can consist of hundreds or even thousands of atoms in incredibly complex structures, such as proteins, the largest known of which being around 100,000 atoms. Carbon nanotubes, a type of fullerene, theoretically have an effectively unlimited size and the longest recorded had about a billion atoms.
The structure of molecules is a direct result of the quantum mechanical constraints on electron energy levels around the nucleus of atoms. The atoms in molecules arrange themselves into consistent geometrical formations and bond angles which maximize the strength of the interatomic bonds.
One of the more popular molecules is water, our good friend H2O, without which "life as we know it" would not be possible. The covalent bonds holding together the two hydrogen atoms and oxygen atom in the water molecule result in the water molecule's "bent" shape. The electric dipole moment caused by this asymmetry in the molecule's structure gives rise to many of water's properties, such as adhesion and its versatility as a solvent.