<Li> Molecule (1794)--"extremely minute particle", from French molécule (1678), from New Latin molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the vogue for the word (used until the late 18th century only in Latin form) can be traced to the philosophy of Descartes . </Li> <P> The definition of the molecule has evolved as knowledge of the structure of molecules has increased . Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties . This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules . </P> <P> Molecules are held together by either covalent bonding or ionic bonding . Several types of non-metal elements exist only as molecules in the environment . For example, hydrogen only exists as hydrogen molecule . A molecule of a compound is made out of two or more elements . </P> <P> A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms . These electron pairs are termed shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding . </P>

How large a change to the structure of an organic molecule