Water has a higher boiling point than HF, even though HF is more polar thanH₂O. This is because in HF, each molecule can be hydrogen bonded to two othsrs wherease in water, each molecule can be hydrogen bonded to four others. Therefore, even though HF forms some what stronger bond than water. The total strength of two hudrogen bonds to a water molecule exceeds the total strength of two hydrogen bondsin an HF molecule.

Wolfgang Pauli in 1925, put forward a principle that restrict some of the values of quantum numbers of electrons in an atom. Pauli's exclusion principle states," no two electrons in an atom can all the four quantum numbers same." In other word if three of the quantum numbers for two electrons in an atom are identical, the fourth quantum number for the two electrons should differ.

Heitler and London in 1927 developed valence bond theory based on quantum mechanical model of an atom. According to...

NaCl is ionic compounds so it is soluble in polar solvent. Water is a polar solvents, therefore, it can rupture the ionic bond between positive (Na⁺)and negative ( Cl^-) ions of the NaCl as a result of ion- dipole interaction. In other words, the ions of the NaCl gets hydrated.

The attractive force which binds oppositely charged ions acts equally in all directions in case of the ionic compounds. Therefore, ionic compounds are non- directional in nature.