On 10/15/2020 3:45 PM, Ivan Tubert-Brohman wrote:
Hi Steven,
MolWt uses naturally occurring average atomic weights, the ones you find in
a typical periodic table. For example, Cl = 35.453.
ExactMolWt uses the weight of a specific isotope (the most naturally
abundant isotope unless the structure specifies a different one for an
atom). These are the atomic weights you find in an isotope chart, not a
regular periodic table. For example, 35Cl = 34.96885268.
To add to this, if you want to really Do It Right, take a look at
https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses
and weep.
We defined ours as "monoisotopic natural abundance mol. weight" but
quite a few of them have 60/40 or 75/25% natural abundance split, and
e.g. Br is 50/50 and Xe is 20/20/20/10/10. I.e. "natural abundance" is
not really monoisotopic.
Even the "standard weight" in the NIST table is not always one value.
Dima
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