How do covalent bonds affect physical properties?
For a covalent bond to form, electrons are shared between the bonding atoms. But not all atoms have an equal attraction for electrons.
Electronegativity is the strength of attraction an atom has for electrons. So if an atom with low electronegativity bonds with an atom of high electronegativity, then the atom with high electronegativity is going to take most of the electrons because it "pulls them harder."
This unbalance in the sharing of electrons creates a partial negative charge on the atom hogging more electrons, this is what we call the negative dipole on the molecule.
The less electronegative atom will have less electrons creating a partial positive charge or positive dipole on that side of the molecule.
These dipoles will attract dipoles of other similar molecules which is known as dipole-dipole attraction. Dipole-dipole attraction is one type of the several Intermolecular Forces of Attraction.