Chemical reactions (chemical changes) are always accompanied by the formation of new chemical bonds or breaking of existing chemical bonds or both. For example when sodium chemically reacts with chlorine, a chemical bond is formed between the two atoms i.e. the sodium atom and the chlorine atom. This type of chemical bonds which is formed due to complete transfer of an electron from an atom like sodium into another atom like chlorine is called ionic bond. Another kind of chemical bonds, called covalent bond, is formed when identical atoms chemically combine to make molecules. For example two hydrogen atoms chemically combine to give us a covalently bonded molecule of hydrogen gas. Similarly atoms of different elements having close electronegativity values may also form covalent bonds with slight to moderate ionic characters. However, atoms of different elements having very different electronegativity values do not form covalent bonds rather they form ionic bonds.