# How would you find atomic mass?

May 5, 2018

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:

$\text{Isotope}$$\textcolor{w h i t e}{m}$$\text{fractional abundance}$$\textcolor{w h i t e}{m}$$\text{Isotopic Mass }$$\textcolor{w h i t e}{m m}$$\text{Wt Avg}$
$\text{O-16} \textcolor{w h i t e}{m m m m m}$$0.997620$$\textcolor{w h i t e}{m m m m m}$$15.9949$$\textcolor{w h i t e}{m m m m}$$\textcolor{w h i t e}{m}$$15.95683$
$\text{O-17} \textcolor{w h i t e}{m m m m m}$$0.000373$$\textcolor{w h i t e}{m m m m m}$$16.9991$$\textcolor{w h i t e}{m m m m}$$\textcolor{w h i t e}{m}$$00.00634$
$\text{O-17} \textcolor{w h i t e}{m m m m m}$$0.002000$$\textcolor{w h i t e}{m m m m m}$$17.9992$$\textcolor{w h i t e}{m m m m}$$\textcolor{w h i t e}{m}$$00.03510$

$\text{Sum""of" "Wt" "Avg." "contributions}$ = Atomic Mass (amu) =$\textcolor{w h i t e}{m}$15.9994

Atomic Mass Oxygen published on Periodic Table = 15.9994 amu