# How do you determine the atomic mass of an element?

How do you determine the atomic mass of an element?

I can think of only two good ways to determine the atomic mass of an element.

#### Explanation:

Most naturally-occurring elements consist of mixtures of isotopes, so you cant just add the masses of the protons and neutrons in a single atom.1. The easy wayLook up the atomic mass in the Periodic Table.

(Adapted from http://www.ptable.com/)For example, the atomic mass of Cl is 35.45 u.2. The harder wayTake the weighted average of the atomic masses of the elements isotopes.In a weighted average, we multiply each value by a number representing its relative importance (its percentage as a decimal fraction).For example, chlorine consists of 75.78 chlorine-35 (atomic mass 34.97 u) and 24.22 chlorine-37 (atomic mass 36.97 u)bbAtomic Mass/u color(white)(m)bbFractional abundance color(white)(m)bbContribution/u

color(white)(mml)34.97color(white)(mmmmmmmmm) 0.7578color(white)(mmmmmmmmm) 26.50

color(white)(mml)36.97color(white)(mmmmmmmmm)0.2422color(white)(mmmmmmmmmll)8.954

color(white)(mmmmmmmmmmmmmmmm)Atomic mass = color(white)(mll) stackrel(———)(35.45)The atomic mass of chlorine is 35.45 u.