(B) COVALENT BOND:
What about reactions between two non metals? Many non metals do bond together. Hydrogen atoms, for instance, frequently react with other hydrogen atoms. Which will turn out to be positively charged and which negative? Actually none. Neither atom has any stronger pull (or affinity) for electrons than the other, consequently these reactions do not create ions. In fact the two atoms share each others' electrons in what is called a covalent bond. Covalent bond forms in atom that are electro negative, especially those with 4 or more valance electrons. These atoms 'share' their electrons with each other in order to form completely filled valance shells and achieve chemical stability. For example, a chlorine atom which just lacks a single electron to completely fill up its valance shell (it just has 7 electrons out of 8 needed to fill up the shell), can pair up with another chlorine atom to form a chlorine molecule by covalent bonding. Each chlorine atom will then have 6 unshared electrons and 2 'shared' electrons, for a total 8 electron that fill up its valance shell. Atoms can form multiple covalent bonds if they need more than one electron to complete their valance shells. Oxygen, for instance, bonds with itself to create 2 bonds between the atoms (since each atom needs to share 2 electrons). So if there is one atom has a much greater affinity for electrons than another, the two many form an ionic bond. If two atoms have equivalent electron affinities they form covalent bonds. What if two atoms are somewhat unequal? In a molecule of water enough to pull the electrons away completely and form ionic bonds. This is possible because there are two types of covalent bonds. Non-polar covalent bonds are formed when atom shared electrons equally, such as in the examples above. But when an atom has a greater affinity for electrons in a molecule, the shared electrons will spend more time around that atom and the bond formed will be a electrons polar covalent bond. This is the case with the water molecule. Each water molecule consists of 2 atoms of hydrogen bonded to 1 atom of oxygen and thus has the chemical formula bonded H2O, the electrons tends to spend more time around the oxygen atom than the hydrogen atoms. The unequally shared electrons will cause a partial charge (called a dipole).