Ask A Scientist Chemistry Archive

name         Stephanie S.
status       student
age          18

Question -  I was wondering how do scientists figure out what the bond angles are for each of 
the different kinds of molecular geometry shapes. They obviously could not use a protractor, 
so how do they know that a tetrahedral molecule will have an angle of 109.5 degrees?
----------------------------------------------------------------------
You have two questions here. One is geometry problem.
If (somehow) a molecule is known to have a certain geometry, say a symmetric tetrahedron, such
as methane (CH4) then the geometry itself determines the bond angles H--C--H = 109.5 degrees. 
The classic case of detective work using this approach was the determination of the geometry of
benzene by Kekule. The relative composition was known to be 1 carbon / 1 hydrogen. The 
molecular weight was known to be about 78. He knew that benzene did not have a dipole moment, 
that is, the electronic charge was symmetrically distributed. So, he is said to have had a 
dream of a snake swallowing its tail that gave him the "Eureka" that benzene was a planar six 
membered ring.
In modern chemistry, there are more direct means of determining molecular structure by the 
interaction of molecules with electromagnetic radiation of varying wavelengths (frequencies) 
called molecular spectroscopy. The most direct and accurate means is to measure directly the 
moments of inertia of the molecule for several isotopes, which changes the molecular weight 
but not the geometry. This type of spectroscopy is microwave spectroscopy. But the molecule 
must be able to be in the vapor phase and it must have a dipole moment, so the geometry of 
only certain molecules -- small, polar and volatile -- can be determined by this technique. 
The geometry of molecules that can be prepared as a single crystal, even a very small one, 
can be determined by the way that the diffract electromagnetic radiation, usually x-rays. 
Although there are some "technical" restrictions, x-ray diffraction is used to determine the 
molecular geometry of a large class of molecules. The structure of DNA was determined by 
Watson and Crick using x-ray diffraction. Both of these methods provide both bond angles and 
bond lengths. Other types of spectroscopy, such as infra-red and/or visible, can be used to 
determine the symmetry of a molecule, that is linear vs. bent, but cannot give the actual
bond lengths. The way that molecules diffract neutrons, or electrons, can be used to provide 
information about the structure of other molecules.
Each of these methods has its own area of applicability and utility, as well as its 
limitations. This whole area of the determination of the structure of molecules forms a 
large branch of modern chemistry.

Vince Calder
=====================================================