What are ionic bonds?
Ionic bonds are the bonds that are formed during the formation of electrovalent or ionic compounds.
This happens when a metallic atom reacts with a non-metallic atom.Electrovalent compounds are formed due to the high difference in electronegativities between the metallic atoms and the non-metallic atoms.
#Na#atom consists of 1 valence electron in its valence shell.
#Na#needs to lose this electron to attain octet configuration.
#Cl#atom consists of 7 valence electrons in its outermost shell. Thus, #Cl#needs to gain 1 valence electron to attain an octet configuration.
You can refer to the image for better understanding:
Hope it was of some help!!