This is because the electronegativity difference between sodium and chlorine is significant. Electronegativity measures the attraction of an atom for electrons. In the case of NaCl, chlorine has a higher electronegativity than sodium. This means that chlorine atoms attract electrons more strongly than sodium atoms, leading to the transfer of electrons from sodium to chlorine.
As a result, sodium loses an electron, becoming a positively charged ion (Na+), while chlorine gains an electron, becoming a negatively charged ion (Cl-). The electrostatic attraction between these oppositely charged ions forms the ionic bond in NaCl.
In contrast, covalent bonds involve the sharing of electrons between atoms with similar electronegativities. Therefore, NaCl cannot form covalent bonds because of the significant electronegativity difference between sodium and chlorine.
