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Is it possible for an element to have a calculated relative atomic mass that is higher than the relative isotopic mass of its most massive component isotope?
The equation is:
$\mathrm{Relative~Atomic~Mass}=\sum[{\mathrm{(Isotopic~Mass) \cdot (Relative~Abundance)]}} \times100\%$
No. Mathematically, that would not make sense.
Consider an imaginary element with only one known isotope: imaginarium-100.
The percent abundance of imaginarium-100 is $100\%$, so its relative atomic mass is: $$100 \cdot 1=100~\mathrm{amu}$$
Now imagine a group of scientists were to discover that in fact imaginarium-99 consists of $0.0001\%$ of all imaginarium samples.
The new relative atomic mass would be:
$$100 \cdot 0.999999+99 \cdot 0.000001=99.999999~\mathrm{amu}$$
The relative atomic mass goes down with the discovery of a less massive isotope. So the closest that the relative atomic mass of an element can be to the mass of its most massive isotope is if the most massive isotope is also the only known isotope, but the relative atomic mass can never go over this.
