BondPolarity

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Chemical Bonding or sharing of electrons with two atoms do not require that the shared electrons be located equally between the two atoms.
- In fact most often these electrons are located closer to one of two atoms.
- The more electronegative element has more desire for the electron thereby the shared electrons are located closer to this atom.
- Therefore, there is an uneven distribution of electron thereby making one end of the bond have more electrons (partially negative) and the other end have less electrons (partially positive).

A general rule of thumb for predicting the type of bond based upon electronegativity differences:

If the differences in electronegativities (delta EN) is less than 0.80, the bond is non-polar covalent
If the difference in electronegativities (delta EN) is greater than or equal to 0.80, the bond is polar covalent
If the difference in electronegativities (delta EN) greater or equal to 1.7, the bond is ionic

In real life, there are no hard and fast cut-offs (i.e. greater than 1.7, ionic), it is a gradual change. You would need to know additional chemical concepts to be certain. Therefore, this is why we use the above cut-offs in 1st year cheimstry.