# Electronegativity and HCl and HF molecules

In a book I am reading, it is said

In the $\ce{HCl}$ molecule, the shared pair of electrons spend more time nearer the chlorine atom. In the $\ce{HF}$ molecule, the shared electrons spend more time nearer the F atom.

What does "spend more time" mean here? Has this been experimentally validated? If yes, how?