So then, why isn't the atomic mass of Hydrogen exactly 1?

If you check a periodic table, you'll see that Hydrogen actually has a mass of 1.00794. If hydrogen is the lightest of all substances, then why not give it a mass of exactly 1 on our relative mass scale? 

There are three reasons: 

  • First, atoms have isotopes, and these isotopes do not all have the same mass. The mass of the atoms in nature - what we use as the atomic mass - is a weighted average of all these different isotopes. 

Here are the exact atomic masses and abundances of an atom with two imaginary stable isotopes.

Isotope Natural abundance (%) Isotope atomic mass
X-10 18.50 10.0129
X-11 81.50 11.0093

To 4 significant digits, what would be the calculated atomic mass of naturally occurring X?

10.51  10.82    11.00    none of these is right

  • The second reason is historical. Once upon a time, way back before 1961, there actually were two sets of atomic masses (though everybody called them atomic weights then). One scale was used by physicists; the other by chemists.  Both were based on weights compared to Oxygen, rather than Hydrogen.  Oxygen was used because it combines with a lot of things to form oxides.  This made it a better choice as a standard because of the ease of chemical analysis.  Oxygen was set to have an atomic mass of 16, which was just about 16 times as heavy as Hydrogen being 1.   Unfortunately, Chemists picked naturally occurring Oxygen, which is a mixture of isotopes of Oxygen-16, Oxygen-17, and Oxygen-18.  After all when you made an oxide of an element you would do so in naturally occurring oxygen.  Physicists picked the pure isotope Oxygen-16, because they tended to make their measurements on the basis of mass spectrometry.

    Though the ratio of any two atom's masses was the same on either scale, it was horribly confusing, so in 1961, a compromise was reached. Instead of using either Hydrogen, or Oxygen as the standard, the isotope of Carbon with 6 protons and 6 neutrons in its nucleus (Carbon-12) was given a mass of exactly 12. It was a good choice, since it was in between the two previously used standards, and meant that nothing had to change too much.
Which of the following statements is correct?
a) Chemists and physicists have always used the same atomic weights.
b) Chemists chose oxygen as their standard reference weight, because of its high reactivity.
 c)  The modern atomic weights are based on naturally occurring oxygen having a mass of 16.
 d)  Carbon-12 was originally chosen as an atomic weight standard  by physicists.
  • The third reason is the most important of all.  If a hydrogen atom has only one proton, and carbon-12 has 6 protons and 6 neutrons to make up its mass of twelve, why isn't the mass of hydrogen 1/12 of that of carbon-12?
    Mass of 1 hydrogen atom Mass of sub-atomic particles Mass of  1 carbon-12 atom
    1.00794
    6 protons = 6 x 1.007277 6.043662
    6 neutrons = 6 x 1.008665 6.051990
    6 electrons = 6 x  0.000548 0.003288
    Total 12.098940
    12.0 exactly

    If you think about it, Hydrogen at 1.00794 is more than 1/12 of the weight of carbon-12 (as you can see from the above table, if you multiply 12 times the mass of a single hydrogen atom it comes to more than 12).  The reason for this effect is nuclear binding energy.  After all, the protons in the nucleus are all positive, and so the nucleus should just repel itself apart.  It doesn't of course, so something must be "binding" it together.  This nuclear binding energy makes the mass of all atoms (except hydrogen-1, which only has 1 proton) slightly lighter that what you'd get by adding up the mass of the sub-atomic particles.   Einstein's famous equation E = mc2 shows us that we can get the necessary binding energy from the mass of the sub-atomic particles.  So the mass of any multi-nucleon atom is less than the sum of the weights of its separated parts.   Its this change in mass when the nucleus changes size that is the source of the enormous amount of energy in nuclear reactions.

So we could have set hydrogen to be exactly 1, but then we'd have had to really revise the atomic weight table back in 1961.  If hydrogen was assigned a mass of 1 exactly, then oxygen would have become 15.87, quite a difference from the mass chemists were using.  Choosing carbon-12 as the reference standard meant the least change was necessary.   Still, if you do really accurate calculations based on the old and the new scale you can see some differences. For example, on the pre-1961 atomic weight scale the molecular weight of table salt, Sodium chloride NaCl would have been 58.45. On today's scale it is 58.44. The difference is just 0.02%, so for most purposes it wouldn't matter. 

Hold it! You just used the term molecular weight. Isn't that wrong? Yes, of course it is, but for Sodium chloride, we shouldn't even use the term molecular mass. Instead we should use the term "formula mass", because Sodium Chloride really isn't a molecule of NaCl. 

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Copyright 1998 - 2008 David Dice