Average atomic Mass

What does the atomic mass tell us on the Periodic

table?

Atomic Mass

Weighted average of element’s natural isotopes

Some isotopes are more abundant than others….

SO atomic mass leans towards more abundant mass

How do we calculate atomic mass?

1) Masses of Isotopes

2) Natural abundance of each isotope usually a percentage

Average atomic mass = mass contributed by all isotopes Fraction of abundance (isotope mass) = mass from a

particular isotope

Example 1Neon has 3 natural isotopes.

Ne-20 (90.51%, 19.99244 u)

Ne-21 (0.27%, 20.99395 u)

Ne-22 (9.22%, 21.99138 u)

What is the weighted average atomic mass for Ne?

Example 2: A certain metallic element has two stable isotopes: M-45 and M-48. Given that the atomic weights are 45.0010 and 48.0014, respectively, what is the percent abundance of the lighter isotope if the average atomic weight of M is 47.2002?

Example 3: Calculate the atomic weight of chlorine if 75.77% of the atoms have a mass of 34.97 amu and 24.23% have a mass of 36.97 amu.