molecular Polarity

[drgshahbaz1193]

I have a question about molecular polarity. I have never been able to deceide if a molecule has a dipole moment . Like with BCl3 I don't understand how Dr. Ferdinand says that the center of negative charge is in the middle. Any help or an easy way to do this would be greatly appreciated. Thanks.

[mcat_premed3832]

Sorry for the delay! Your question was missed.

An example of this can be found for molecular polarity for carbon dioxide (see Organic Chemistry Chapter 1):

O = C = O

The molecule is symmetric, the distance between O and C is equal on both sides and the center of positive and negative charges is the same. Why? The center of positive charge is the carbon which is precisely at the center of the whole molecule. There is a partially negative charge on the right (oxygen) and the same on the left. So if you had to put a dot at the center of negative charge, you would put that dot on the carbon (an equal distance to both oxygens). Since the centers of charge are the same, no dipole moment.

{keep in mind: oxygen is to the right of carbon in the periodic table meaning it would be partially negative when sharing a bond with carbon}

So, looking at BCl3, I assume that you would agree that the center of positive charge is at the center of an equilateral triangle (with B at the center; it is partially positive because of its placement on the periodic table relative to Cl).

So the question is: where is the center of negative charge. If you draw the molecule yourself: ask where you can put a dot that is equal distance to the 3 identical atoms (Cl) which will all be equally partially negative. Your dot would have to be placed at the very center of the triangle: on the boron atom. Again, therefore, because of symmetry and identical electronegative atoms, the center of positive and negative charges are in the same spot, no dipole. The bonds are all polar and yet the molecule in non-polar.

[admin]

Molecular polarity is based mainly on electronegativity and charge separation between bonding atoms within a molecular structure. A dipole moment occurs when there is a separation of positive and negative charge. The degree of charge differential is dependant on the electronegativity difference between bonds. If two atoms are identical electronegatively, they share their electrons equally and the bond is covalent and non-polar with no dipole moment.

For molecular entities, if bonding occurs between different atoms such as nonmetals (HCl), the electronegativity differences will cause dipole moments. Thus, the polarity of a bond is quantified by the size of the dipole moment which occurs by charge separation as mentioned. To answer if a molecule has a dipole moment you need to also look at the overall structure and geometry of the molecule and not only the individual bonds within the molecule.
Moreover, the geometry of the molecule is very important. For example, if you were to look at the dipole moment of CO2, it would be equal to 0. Why, well the bonding that occurs between the carbon and oxygen atoms does have a dipole moment due to the charge differences and thus the electronegativity differences between carbon and oxygen atoms (which means the tendancy of an atom to withdraw electrons to its own nuclear core), however, the two oxygens bonded to the single carbon have the same electronegativity potential and are linearly shaped with the same distant apart and so as such, the two separate dipole charges between each carbon oxygen bond essentially cancel eachother resulting in a overall dipole charge of 0. As for your Boron trichloride (BCl3) molecule, it has a trigonal planar molecular shape with approximate equivalent bond angles of ~120 degrees between each boron and chlorine bond and as such, the dipole moments cancel eachother analogous to vector additions in physics. So, in dipole moments, not only is the charge separation between bonding atoms important but also the overall direction of charge distribution and direction.

So as for BCl3, look at the structure and geometry as per VSEPR (see below):




You can see that the overall structure allows the cancellation of the dipoles to give an overall charge of 0. Moreover,in a trigonal planar molecule, each B-Cl bond is 120 degrees and this high degree of symmetry means there is no dipole moment in BCl3, so the molecule is non-polar with a dipole moment equal to 0.