posted by tran on .
what is the difference between polar bonds and polar molecules.
i know a polar bond is a bond between two dipoles? what are polar molecules? wouldnt it be the same?
No. a polar bond is not one between two dipoles although dipole-dipole bonds do have polarity. A polar bond is one in which the electrons are completely transferred from one atom to another (an ionic bond) or a covalent bond in which the electronegativities are different resulting in one atom attracting the electrons more than the other. Two examples, one of each.
1. NaCl is an ionic compound and the bond is polar.
2. HF. The H has an electronegativity (EN) of 2.1 and the F has an EN of 4.0; therefore, the electrons of H are attracted more to F than to H so the F atom carries a slight negative charge and the H carries a slight positive charge. This is a polar covalent bond.
A polar molecule is one which satisfies the following two criteria:
a. There must be a difference in electronegativity between the central atom and at least one of the other atoms.
b. The molecule must not be symmetric in three dimensions.
NH3 has three polar N-H bonds and it is not symmetrical in space; thus it is a polar molecule.
CCl4 has four polar C-Cl bonds but the molecule is not polar because the three dimensional space set up is symmetrical which cancels the effect of the 4 polar bonds.
what do you mean its not symmertrical in space>
Here are two examples. The NH3 molecule is a polar molecule because it is NOT symmetrical. You can see that the 3 NH bonds are pointing downward with the two unpaired electrons on top. (If the two electrons on top were replaced with another H atom, the NH4^+ ion is symmetrical.)
The CCl4 molecule has 4 C-Cl polar bonds because the C and Cl have different electronegativities. But see the diagram at this site. It is a symmetrical molecule in space and the molecular polarity is zero.