explain what a polar bond is and how the electronegativities of the two atoms in the bond compare.

Explanation:

Ionic bonds are polar and some covalent bonds are also polar.

it depend upon the electronegativity of bonded atoms. If electronegativity difference of both  bonded atoms are greater than 0.4 polar covalent bond is formed. While in case of ionic bond there is huge difference between electronegativity of bonded atoms one with higher electronegativity complete gain the electron from atom with lower electronegativity and ionic bond is formed.

Ionic bond:

It is the bond which is formed by the transfer of electron from one atom to the atom of another element.  

Both bonded atoms have very large electronegativity difference. The atom with large electronegativity value accept the electron from other with smaller value of electronegativity.

For example:

Sodium chloride is ionic compound. The electronegativity of chlorine is 3.16 and for sodium is 0.93. There is large difference is present. That's why electron from sodium is transfer to the chlorine. Sodium becomes positive and chlorine becomes negative ion.

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial positive.

c (diamond),

sio2 (quartz)