Why are atomic masses not whole numbers?

Answer 1

Because there are different isotopes of the elements. The atomic weight is the average based on the proportions that they occur in naturally. For example, hydrogen comes with a single proton, a proton + a neutron, and a proton plus two neutrons. The isotopes with the neutrons occur in very small amounts so the average weight is slightly greater than 1.

Answer 2

The easy answer is binding energies.

According to Wikipedia, atomic masses were arbitrarily based on the most prevalent isotope of oxygen being defined as the integer 16 by physicists, and the mean mass of all isotopes of oxygen was assigned the integer 16 by chemists. Again, according to the same article, this arbitrary definition has now been changed to the integer 12 for carbon-12 isotope.

In any case, only one atomic mass can be an integer because of the difference in mass between a proton and neutron, and the fact that the sum of the particle masses for any isotope is not equal to the mass of the isotope.

Answer 3

Because they're measured in proton masses, but every atom also has some electrons and they're less massive than protons (about 2000 times less massive, in fact). Also because the generally given atomic mass is the average for all isotopes of that element, and of course this average doesn't always come out whole.

Answer 4

The atomic variety of an factor is desperate with the help of the variety of protons interior the nucleus - e.g. oxygen has 8, nitrogen has 7. although, each and each factor could have a various variety of neutrons, which, through fact they are uncharged, do not regulate the chemical residences of the factor. E.g., carbon, factor 6, has 3 common isotopes, carbon-12 (6 neutrons), carbon-13 (7 neutrons) and carbon-14 (8 neutrons). through fact protons and neutrons weigh with regard to the comparable, the mass of a carbon atom could be the two 12, 13, or 14 reckoning on which isotope it rather is. although, maximum needless to say-occuring components are a mixture of diverse isotopes. So, to apply a trivial occasion, in case you had equivalent factors carbon-12 and carbon-13 in a pattern of carbon, the traditional atomic mass could be 12.5. the actually atomic mass is desperate with the help of the "isotope ratio" of maximum ordinarily happening samples of the factor (what fraction of each and every isotope is modern-day in a common pattern), it is was once certain the "atomic mass" of the factor.

Answer 5

actually, there are several kinds of "atomic mass"

"average atomic mass", "relative atomic mass"

in all of these mass numbers the weight of various isotopes is included

in other words, not all the atoms of carbon are carbon-12, if they were, then carbon would have an atomic mass of exactly 12

in fact, there are other isotopes that are heavier, like carbon-14 and probably lighter isotopes

these occur in predicted or measured ratios and effect the mass of an actual group of carbon atoms

this wikipedia article is fairly informative on the subject:

http://en.wikipedia.org/wiki/Atomic_mass

Answer 6

Compare the weights of neutrons and protons.