What is Fractional Atomic Mass?
Answer:
All nuclei are composed of integral numbers of protons and neutrons. Electrons' mass is less than 0.03% of nuclear mass. However... a bound system must be lower in energy than its dissociated constutuents. Nuclear binding energies are very large.
m = E/c^2
All nuclei are therefore missing fractional mass. The largest binding energy/nucleon is at iron-56.
http://t2.lanl.gov/data/astro/molnix96/m...
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Your question is a bit vague and incomplete.
Maybe you are asking why some elements have an atomic mass that is not an integer? Normally the atomic mass is the number of protons plus the number of neutrons, so how could it ever be a decimal number and not a whole number?
Well, many elements have so-called isotopes - they come in different varieties. Since the number of protons determines what element it is, the difference in the various isotopes is the number of neutrons.
The atomic weight of Chlorine, for example, is often given as 35.5. This is because it occurs as isotopes with atomic masses 35 and 37. Actually, isotopes with mass 36 and 38 exist as well, but they decay radioactively, so they do not occur naturally on Earth except in very small trace amounts.
So why 35.5? Because the isotope with mass 35 occurs three times as often on Earth as the one with mass 37. (3*35 + 37)/4 = 35.5. In other words, it's an average over the isotopes that exist on Earth.
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