Take a look at their electronegativity values for this one. Electronegativity is the relative attraction that a atom in a molecule has for the shared pair of electrons in a covalent bond. Salt is Sodium Chloride which is NaCl. Na has an electronegativity value of 0.93. Cl has an electronegativity value of 3.16. The difference between the two is 2.23. This is much higher than 1.7 (a pure covalent bond e.g. Oxygen-Oxygen bond O2) therefore it is a highly ionic compound. If this was less than 1.7 it would be a polar covalent molecule.