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Q: Why is the value listed for atomic mass for each element on the periodic table not a whole number? A. It is an average. B. It is a measurement error. C. The numbers of protons can vary for
an element. D. The number of electrons can vary for an element.
A: Elements don't have whole numbers for their atomic masses because the atomic mass of an element must reflect the amounts of different isotopes of that element that occur in nature. A given element always has a fixed number of protons in its nucleus. [ That how we identify that element; it's what gives it its elemental identity. But the number of neutrons in a given element's nucleus can vary.
Atoms of a given element that have different numbers of neutrons represent different isotopes of that element. If we look at hydrogen, we see it's the simplest of all the elements. It's basically one proton with an electron hanging around. Problem is, if we look at a whole bunch of hydrogen atoms, we find one every once in a great while that has a neutron hanging on to the proton in the nucleus. That atom of hydrogen has about twice the mass of a "regular" hydrogen atom. It's a distinct isotope of hydrogen. There are even some atoms of hydrogen that have two neutrons hanging on to that proton in the nucleus. The atoms of hydrogen with the neutrons are rare, but the difference in their masses and the fraction of the naturally occurring element that they make up must be accounted for. As we go on up the periodic table to heaver elements, we find that pretty much all the elements have several isotopes that occur in nature, and some have significant portions distributed over two or more isotopes. Take Boron. It's atomic number is 5. How many neutrons are in a boron atom? Well, roughly 80% have 6 neutrons while about 20% have 5. When we weigh a sample of boron in the lab, we need to account for that. If we look at tin, we find it has 10 naturally occurring isotopes! And we have to account for them all when we have a sample of tin because they are each in there in some amount. That's why the atomic masses of the elements aren't whole numbers. It would be simple if they did, but science demands we account for the variables in nuclear structure. Read more: ]
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