When atoms in a covalent bond have a rather high difference in their electronegativities, the bond is said to be : ?

There are two types of bonds in general chemistry: ionic bond and covalent bond. Ionic bond is a combination of a cation and anion, while covalent is a combination of either two cations or two anions. The main rule here is that, the greater the difference between the electronegativities of the atoms, the more ionic the bonds is. Electronegativity is the ability of an atom to attract electron to themselves. In that case, there would be give and take of electrons. 

However, with covalent bonds, there is unequal sharing. The 'high electronegativity difference' referred to in this problem has a degree that is lesser than ionic bonds. So, between covalent bonds, there are polar covalent bonds and nonpolar covalent bonds. It is these polar covalent bonds that have a relatively high difference. In this case, there is unequal sharing of electrons because the electrons are more attracted to one atom than the other.