Knowing that Carbon has 4 valence electrons, Nitrogen 5, Oxygen 6, etc. we know how many bonds each needs to form to gain an octet. This may be the consequence of equal (covalent) sharing of electrons with atoms of similar electronegativities, or transfer of electrons (ionic) between atoms of quite different electronegativities. There is no exact cutoff between ionic and polar covalent bonds, however we can usually get close. In these four examples, the following bonding patterns emerge from consideration of the octet rule and E.N. values. Note that there are two different formalisms used for charge; the bonds between metals (Na and Li) and their highly electronegative partners (O and N here) are ionic, while the charges in the first and fourth examples are formal charges showing that these atoms are bonded to more, or fewer, atoms than when they are neutral. These ideas are expanded upon in Organic 1, however it is essential that students know the fundamental differences between ionic and covalent bonding before starting the Organic sequence.