A chlorine atom shows a tendency to gain an electron, as indicated by its electron affinity (-349 kJ/mol). From which atom, sodium or hydrogen, can the electron most readily be extracted? Neither atom gives up an electron freely, but the energy required to extract an electron from Na (I = 496 kJ/mol) is much smaller than that for H (I = 1312 kJ/mol). In Chapter 9 we learned that the lower its ionization energy, the more metallic an element is; sodium is much more metallic than hydrogen (recall Figure 9-12). In fact, hydrogen is considered to be a nonmetal. A hydrogen atom in the gaseous state dose not give up an electron to another nonmetal atom. Bonding between a hydrogen atom and a chlorine atom involves the sharing of electron, which leads to a covalent bond.
To emphasize the sharing of electron, let us think of the Lewis structure of HCL in this manner.