Here's why:
* Electronegativity: Fluorine is much more electronegative than hydrogen. This means fluorine has a stronger pull on the shared electrons in the bond.
* Partial Charges: The shared electrons spend more time closer to the fluorine atom. This gives fluorine a partial negative charge (δ-) and hydrogen a partial positive charge (δ+).
* Polar Covalent Bond: The unequal sharing of electrons creates a polar covalent bond. This means the bond has a positive end and a negative end.
While the overall molecule (HF) is neutral, the individual atoms within the bond have partial charges.