Covalent Bonding

A covalent bond is a shared pair of electrons between two atoms. Covalent bonds usually form between non-metal atoms, which each need to gain electrons to fill their outer shell. By sharing, both atoms count the shared electrons as part of their outer shell.Each shared pair is shown as a line in a displayed formula. A single bond is one shared pair (C–H), a double bond is two shared pairs (C=O). The number of covalent bonds an atom usually forms depends on how many electrons it needs:H: 1    O: 2    N: 3    C: 4Almost all biological molecules, such as glucose, amino acids and fatty acids, are built from carbon, hydrogen, oxygen and nitrogen held together by covalent bonds. Covalent bonds are strong, so these molecules are stable in cells and are only broken by reactions such as hydrolysis, usually catalysed by enzymes.Polar covalent bonds. If the two atoms attract the shared electrons unequally, the electrons are pulled towards the more electronegative atom. That atom gains a slight negative charge (δ−) and the other a slight positive charge (δ+). Oxygen and nitrogen are more electronegative than hydrogen and carbon, so O–H and N–H bonds are polar, while C–H bonds are almost non-polar.Worked example: in methane, CH₄, carbon shares one pair with each of four hydrogen atoms, so there are 4 single bonds and 8 shared electrons. In carbon dioxide, O=C=O, carbon forms two double bonds, which still adds up to four bonds.Whether a molecule has polar bonds decides how it behaves with water, which is why polarity comes up again and again in biology.