Chemical compounds are formed by the joining of two or more atoms. a chemical bond. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom. The "unzipping" of DNA is a breaking of hydrogen bonds which help hold the two strands of the double helix together.

A molecule is two or more atoms linked by a chemical bond. Methane has four covalent bonds between carbon (C) and hydrogen (H). Notice how these different views represent the atoms and their bonds differently. Chemical reactivity of molecules- tendency to break and form chemical bonds.

Non-covalent bonds and other weak forces are important in biological structures. In water, these bonds are very weak. Van Der Waals bonds-are short range attractive forces between chemical groups in contact. When an electrovalent chemical is dissolved in water it will dissociate.

bonds are formed when atoms share electrons. Covalent bonds hold the atoms of oxygen and sulphur together. These are all chemically combined by covalent bonds into a long chain. Hydrogen bonds are caused by attractions between the positively charged nucleus of a hydrogen atom and a pair of electrons in the outer shell of another atom, either oxygen or nitrogen.