chemical bonding?

Question

cf4 has polar bonds yet is a non-polar molecule. explain.

Answer 1

F atoms are more electronegative than C atoms. This means that when they bond together, the F atom will attract the electrons in the covalent bond towards itself. This makes the F atoms have a slight negative charge (delta -). The carbon therefore has a slight positive charge (delta +). Therefore the bonds are polar covalent bonds.

As for the shape of the molecule...
you need to work out how many electron pairs are distributed around the C atom. There are 4 electrons in the outer shell (or energy level) of the carbon, plus 1 each from the fluorines. In other words, there are 8 electrons = 4 bonding pairs around the central carbon atom. As electrons repel each other, they will position themselves as far away from each other as possible - tetrahedrally at 109 degrees approx. As this is a symmetrical arrangement, the slightly negatively charged fluorine atoms will be evenly distributed around the molecule. hence, no overall polarity in the molecule.

Answer 2

+ve and _ve bonds neutralise each other leaving a non polar molecule.