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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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obarron2 obarron2
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12 years ago
80 protons/neutrons atomic mass 200.5
do you round the atomic mass or do you just cut off the decimals?
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#1
12 years ago
There are different isotopes, each with different numbers of neutrons.

If you subtract the atomic no. of Hg (which is 80) from 200.5, you get (near enough) 120.5 for the average. But as you have correctly reasoned, you can't have half a neutron.

Lots of elements have only one stable isotope (e.g. F), or consist almost entirely on one isotope (e.g. O, N), so they have atomic masses very close toa whole number. If the atomic mass is a log way from a whole number, you can be sure that there are different isotopes.
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#2
12 years ago
Atomic mass is an average of all of the isotopes (atoms of the same element with different numbers of neutrons).  So depending on how accurate you want your calculations to be, the more numbers to the right of the decimal point you should leave unless significant figures say otherwise.  I generally keep my Atomic masses in the hundreth place. 
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