Ideally, the sum of the covalent radii is the covalent bond length between atoms. For heteroatomic molecules (molecules with different types of atoms) the ionic terms may enter, as the polarity of the atoms is different. The larger the difference in polarity of the atoms, the larger the role of ionic character of the bond. HF is polar molecule. Typically, the polar covalent bonds, like H–F, are shorter than expected from the sum of covalent radii.