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Topic: Why is the atomic weight of oxygen less than 16 when its isotopes are 16,17,18?  (Read 4519 times)

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Offline gracie7

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Why is the atomic weight of oxygen less than 16 when its isotopes are 16,17,18?
« on: December 18, 2009, 11:33:26 AM »
I cannot figure out why the atomic wt. for oxygen is less than 16 (i.e., 15.999), when its three main, stable isotopes have weights of 16, 17, and 18. Can anyone please explain this to me?
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Offline JGK

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Re: Why is the atomic weight of oxygen less than 16 when its isotopes are 16,17,18?
« Reply #1 on: December 18, 2009, 12:09:19 PM »
Naturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance).

I would say it's due to rounding up of the actual weights rather than reporting to a load of decimal places for each isotope.
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Offline Borek

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Re: Why is the atomic weight of oxygen less than 16 when its isotopes are 16,17,18?
« Reply #2 on: December 18, 2009, 12:13:32 PM »
What is EXACT mass of 16O?

Hint: it is not 16.
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Offline AWK

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Re: Why is the atomic weight of oxygen less than 16 when its isotopes are 16,17,18?
« Reply #3 on: December 18, 2009, 01:44:19 PM »
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