Taking into account the octet rule and the definition of ionic bond, sodium has one electron in its outer shell and chlorine has 7 electrons in its outer shell. The atoms will form an ionic bond by transfer their electrons.
The octet rule defines the property that atoms have of completing their last energy level with eight electrons to achieve stability. Whether it is an ionic, covalent or metallic bond, the atoms will tend to give up or share to complete 8 electrons in the valence shell.
The basis of this rule are the noble gases, which have 8 electrons in their last shell and are the least reactive elements, this is the most stable, of the entire periodic table.
On the other side, an ionic bond is produced between metallic and non-metallic atoms, where electrons are completely transferred from one atom to another. During this process, one atom loses electrons and another one gains them, forming ions. Usually, the metal gives up its electrons forming a cation to the nonmetal element, which forms an anion.
Considering the previous, sodium has one electron in its outer shell and chlorine has 7 electrons in its outer shell. The atoms will form an ionic bond by transfer their electrons. In this way, the octet rule is fulfilled and the elements acquire stability.
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