Question

How would you find atomic mass?

Answer 1

Assuming you are referring to atomic mass associated with elements on the periodic table, then the listed atomic mass is the weighted average of the isotopes of a given element.

Explanation:

Isotopes of elements are a set of elements all having the same atomic number (number of protons) but different atomic mass (number of neutrons). Typically a mixture of isotopes is separated via mass spectroscopy and characterized as to %Abundance and Isotopic mass. The atomic mass is then determined by multiplying fractional abundance times isotopic mass (atomic mass units-amu) giving the weighted average of the isotopes of the mix.

For example, for this example isotopes of oxygen unclude Oxygen -16, Oxygen - 17 & Oxygen -18. Arranging these in a tabular format one gets the following:

`"Isotope"` `color(white)(m)` `"fractional abundance"` `color(white)(m)` `"Isotopic Mass "` `color(white)(mm)` `"Wt Avg"`
`"O-16"color(white)(mmmmm)` `0.997620` `color(white)(mmmmm)` `15.9949` `color(white)(mmmm)` `color(white)(m)` `15.95683`
`"O-17"color(white)(mmmmm)` `0.000373` `color(white)(mmmmm)` `16.9991` `color(white)(mmmm)` `color(white)(m)` `00.00634`
`"O-17"color(white)(mmmmm)` `0.002000` `color(white)(mmmmm)` `17.9992` `color(white)(mmmm)` `color(white)(m)` `00.03510`

`"Sum""of" "Wt" "Avg." "contributions"` = Atomic Mass (amu) =`color(white)(m)`15.9994

Atomic Mass Oxygen published on Periodic Table = 15.9994 amu