An ionic bond forms between a metal and non-metal. Elements aim to become stable by achieving a full octet (a full 8 electrons in their outer valence shell). Metals, almost always from the left side of the periodic table, are less electronegative, meaning they have a weaker hold on their electrons. When combined with a non-metal, from the right side of the periodic table and with a higher electronegativity, metals give up their electrons, fulfilling the outer valence shell of the non metal. Metals, after giving up all of their outermost electrons, reduce by one valence level, and thus also achieve a full octet. Both elements are satisfied in an ionic bond after a transfer of electrons. A sharing of electrons does not occur in ionic bonds (molecular bonds only!), because that entails both elements (usually both non-metals when sharing) using one more electrons simultaneously.
The total amount of energy remains constant in an isolated system. It implies that energy can neither be created nor destroyed, but can be change from one form to another.
