Ionic Bond or Electrovalent Bond
Electronic configurations of the noble gases show that the outermost shells of all the noble gases (except helium) contain eight electrons. This is known as an octet configuration and is considered a very stable configuration. During the formation of a chemical bond, atoms gain, lose or share electrons until they acquire stable configuration in their valence shell.
When atoms combine, one atom may transfer one or more electrons to the other atom. In this process each atom gets the stable octet structure in its outermost shell. This type of bonding is called electrovalent bond or ionic bond. The atom losing the electrons becomes a positive ion. The atom gaining the electron becomes a negative ion. The electrostatic attractive force existing between two oppositely charged ions in a molecule is known as ionic bond.
The sodium atom has electronic configuration 2, 8, 1. It has one electron in its outermost shell. Sodium atom has one electron more than stable inert gas configuration.
So the sodium atom transfers its excess electron to chlorine.
Now the electronic configuration becomes 2, 8 for sodium and 2, 8, 8 for chlorine. So both the atoms have attained the stable octet configuration.
By the Lewis structure (valence electrons are shown by dots around the symbol) this can be written as:
Thus sodium atoms become the cation and chlorine atoms become the anion. Thus ionic or electrovalent bond is formed due to the electrostatic force of attraction between the Na+ and Cl- ions.