What must always be true if a covalent bond is to be polar?

If a covalent bond is polar the electronegativity difference between the elements of the compound is between 0.4 and 1.7

Explanation:

The electronegativity measures the power of an atom to attract electrons; if the electronegativity difference between the elements of the compound is big (more than 1.7) the compound will be ionic, that means, an atom or a group of atoms will give electrons and another atom of group of atoms will receive them; if the electronegativity difference is small (less than 0.4), the electrons will be shared by the atoms in a equitative way, i. e. the electrons will spend similar amount of time in the orbitals of each atom; but if the electronegativity difference is between 0.4 and 1.7  a covalent polar bond will be formed, that means valence electrons will spend more time in the orbitals of the most electronegative element.

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons