Nitrogen Mass Number
The nitrogen rule states that organic compounds containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens either have 1) an odd nominal mass that indicates an odd number of nitrogen atoms are present or 2) an even nominal mass that indicates an even number of nitrogen atoms in the molecular formula of the molecular ion.[1][2] The nitrogen rule is not a rule as much as a general principle which may prove useful when attempting to solve organicmass spectrometry structures.
Mass Of Nitrogen
The nitrogen rule states that any molecule (with all paired electrons) that contains an odd number of nitrogen atoms will have an odd nominal mass. The nominal mass is the integer mass of an atom, ion, or molecule comprised of only the most stable isotope (s).
Formulation of the rule[edit]
This rule is derived from the fact that, perhaps coincidentally, for the most common chemical elements in neutral organic compounds (hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens), elements with even numbered nominal masses form even numbers of covalent bonds, while elements with odd numbered nominal masses form odd numbers of covalent bonds, with the exception of nitrogen, which has a nominal (or integer) mass of 14, but has a valency of 3.
The nitrogen rule is only true for neutral structures in which all of the atoms in the molecule have a number of covalent bonds equal to their standard valency (counting each sigma bond and pi bond as a separate covalent bond for the purposes of the calculation). Therefore, the rule is typically only applied to the molecular ion signal in the mass spectrum.
The nitrogen rule states that organic compounds containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens either have 1) an odd nominal mass that indicates an odd number of nitrogen atoms are present or 2) an even nominal mass that indicates an even number of nitrogen atoms in the molecular formula of the molecular ion. Nitrogen has an even atomic weight (14), so any number of nitrogen atoms will add up to an even molecular weight. 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 weight will be odd because of the presence of an extra hydrogen. Since the relative atomic mass = 14.01 that means some atoms of N will have more than 7 neutrons to increase the weighted average above 14.003 now let's consider this example. Let's say we have 60% N-15 atoms and enough N-7 to N-12 atoms to bring the weighted average down to from 15.003 to 14.01.
Mass spectrometry generally operates by measuring the mass of ions. If the measured ion is generated by creating or breaking a single covalent bond (such as protonating an amine to form an ammonium center or removing a hydride from a molecule to leave a positively charged ion) then the nitrogen rule becomes reversed (odd numbered masses indicate even numbers of nitrogens and vice versa). However, for each consecutive covalent bond that is broken or formed, the nitrogen rule again reverses.
Therefore, a more rigorous definition of the nitrogen rule for organic compounds containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens would be as follows:
An even nominal mass indicates that a net even number of covalent bonds have been broken or formed and an even number of nitrogen atoms are present, or that a net odd number of covalent bonds have been broken or formed and an odd number of nitrogen atoms are present. An odd nominal mass indicates that a net even number of covalent bonds have been broken or formed and an odd number of nitrogen atoms are present, or that a net odd number of covalent bonds have been broken or formed and an even number of nitrogen atoms are present.
Inorganic molecules do not necessarily follow the rule. For example, the nitrogen oxides NO and NO2 have an odd number of nitrogens but even masses of 30 and 46, respectively.
See also[edit]
References[edit]
- ^Tureček, František; McLafferty, Fred W. (1993). Interpretation of mass spectra. Sausalito, Calif: University Science Books. pp. 37–38. ISBN0-935702-25-3.
- ^David O. Sparkman (2007). Mass Spectrometry Desk Reference. Pittsburgh: Global View Pub. p. 64. ISBN0-9660813-9-0.
Atomic Number Nitrogen
For light elements (the bottom left of the graph), the stable isotopes are the ones with the same numbers of protons and neutrons and hence the ‘line of stability’ follows the straight line of N = Z. For example, the stable isotope carbon-12 has 6 protons and 6 neutrons.Nitrogen Mass Number And Atomic Number
If we got a straight line all the way, then this would tell us that the stable isotopes have the same number of neutrons as protons. However, this is not the case. The line curves upwards. Stable isotopes of the heavier elements (top right of the graph) have more neutrons than protons. For example, Gold-197 is stable. It contains 79 protons and 118 neutrons.
The neutrons in a nucleus can be thought of as acting as a kind of glue to hold the nucleus together. The positively charged protons are in a very confined space but would rather not be, due to the fact that they repel each other.
However, protons and neutrons are all attracted to each other as a result of another force - the strong nuclear force (see below). The neutrons don't contribute any repulsive effects because they are neutral. So having more neutrons around can help to hold the nucleus together. Notice that no amount of neutrons can hold a nucleus together once it has more that 82 protons – the line stops at Z = 82! All of the elements with an atomic number greater than 82 have only unstable isotopes.