Molecular Weight: Even or Odd?
This phenomenon is a result of the fact that the most common elements in organic compounds, carbon and oxygen, have even atomic masses (12 and 16, respectively), so any number of carbons and oxygens will have even masses. The most common isotope of hydrogen has an odd molecular weight, but because carbon and oxygen both have even valences (carbon forms four bonds and oxygen forms two), there is always an even number of hydrogen atoms in an organic compound containing those elements, so they also add up to an even numbered molecular mass.
Nitrogen has an even atomic mass (14), so any number of nitrogen atoms will add up to an even molecular mass. Nitrogen, however, has an odd valence (it forms three bonds), and as a result there will be an odd number of hydrogens in a nitrogenous compound, and the molecular mass will be odd because of the presence of an extra hydrogen.
Of course, if there are two nitrogens in a molecule, there will be two extra hydrogens, so the molecular mass will actually be even. That means the rule about molecular mass and nitrogen should really be expressed as:
- odd numbers of nitrogen atoms in a molecule in an odd molecular mass.
What about those other atoms that sometimes show up in organic chemistry, such as the halogens? Halogens all have odd molecular mass (19 amu for fluorine, 35 or 37 for chlorine, 79 or 81 for bromine, and 127 for iodine). However, halogens all have a valence of 1, just like hydrogen. As a result, to add a halogen to methane, we would need to erase one of the hydrogen atoms and replace it with the halogen. Since we are just substituting one odd numbered atomic mass for another, the total molecular masss remains even.
Calculate molecular weights for the following compounds.