Average Atomic MassIf there are 2 naturally occurring isotopes ofNeon includingNe-20 and Ne-22.

then the average mass of Neon atoms should be ___ amu.

Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

Why does the PT lists it as 20.18 amu?

A weighted average atomic mass is calculated from both themasses of each isotope and therelative abundance (%) of each isotope.

63Cu is 69% abundant

65Cu is 31% abundant

To which will theaverage mass be closer,63 or 65?

Average Atomic Mass

copper-63has a mass of 62.93 amu and natural abundance of 69.15%

copper-65has a mass of 64.93 amuand has a natural abundance of 30.85%

= (62.93)(0.6915) + (64.93)(0.3085)

= 63.55

Avg. Mass = (Mass1)(%) + (Mass2)(%) …

Calculating Average Atomic Mass

because…the weighted average atomic mass of neon from the PT is 20.18 amu.

neon-20 neon-22

Which isotope is in greater relative abundance in natural samples of neon?

1. If an unusually selected sample of sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, what would be its average atomic mass?

A. 32.2 amu

B. 32.9 amu

C. 33.5 amu

D. 34.2 amu

Quick Quiz!

= (32)(0.900) + (34)(0.100)

= 32.2

Avg. = (M1)(%) + (M2)(%)