Using the Electronegativities we can predict whether a given bond will be non-polar, polar covalent, or ionic. The greater the difference in electronegativity the more polar the bond.
Order the following bonds according to polarity: H-H, O-H, Cl-H, S-H, and F-H.
In practice no bond is totally ionic. There will always be a small amount of electron sharing.
The calculated dipole moment is given by
where $Q$ the magnitude of the charge on each end of the dipole and $r$ is the distance between the positive and negative charges. If we have a proton and electron separated by 1 Å then the dipole moment is calculated to be
In the plot below we see that the % ionic character increases with increasing electronegativity difference.
So what is and ionic compound? There's no clear cut line. Therefore we use the following practical definition for an ionic compound
Ionic Compound is any solid that conducts an electric current when melted or dissolved in water.
Or you can simply say that a "salt" is an ionic compound.
Once again, the picture we're forming is that, in virtually every case, the atom in a stable compound has a noble gas arrangment of electrons. When two non-metals react to form a covalent bond, they share electrons in a way that completes the valence electron configurations of both atoms (i.e., both non-metals attain a noble gas configuration).
Chemisty, The Central Science, 10th Ed.
8.7, 8.9, 8.11, 8.29, 8.35, 8.37, 8.39