This attraction forms a weak bond called a hydrogen bond . In the correct orientation, a water molecule can form up to 4 hydrogen bonds with other water molecules. But many hydrogen bonds may cooperate to make a stable association between two large molecules, such as base pairing between strands of a DNA double helix. Electron orbitals are shown simplified, compared to the previous diagram of water. In pure water, this happens for one in every 550 million molecules, on average.
Some atoms fill their outermost energy level by gaining or losing electrons.
Cations lose electrons and have an excess positive charge, e.g . For every anion there must be a cation. In a polar solvent like water, however, the attraction between ions is relatively weak and the ions move about separately surrounded by water molecules. Oxidation and Reduction A chemical reaction that involves the loss of one or more electrons from one molecule is called oxidation . Reduction is the complementary gain of electrons by another molecule.