Unit 3: Atomic Structure

Atomic Mass Units (amu) and Calculating Atomic Mass

Remember???• Mass number = protons + neutrons• Isotopes= atoms with different # of

neutrons• Atomic masses occur with decimal

points because they are an average of different isotopes

Atomic Mass Units• Defined as 1/12 the mass of a

Carbon-12 atom.• The way that atomic mass is

measured in the Periodic Table• Abbreviated “amu”

To find the atomic mass, you need to

know:• Number of stable

isotopes• Mass of each

isotope • Natural %

abundance of each isotope

Real-World Application: Weighted Average

Let’s just say that the weights that make up your grade are as follows:

• Tests – 70% of grade• Quizzes – 20 % of grade• Homework –10% of grade

You earn the following grades: test – 80, quiz – 100, homework – 100

To Find Weighted Average…

Final Grade = (.7)(80)+(.2)(100)+(.1)(100) 56 + 20 + 10 = 86%

Don’t forget!All percentages must = 100% All decimals = 1

To Find Atomic Mass :Atomic mass of element =

A.M. isotope1*(%1) + A.M. of isotope2 *(%2)

100 100

Example - Chlorine

Calculate the atomic mass of chlorine if the two common isotopes of chlorine have masses of 35.45 amu (75.00% abundance) and 37.29 amu (25.00% abundance).

Atomic Mass of Chlorine

(35.45amu)(.7500) + (37.29amu)(.2500)

26.59 amu + 9.323 amuAtomic mass of Chlorine= 35.91 amu

a.m.=37.29amu%2= 25.00%

a.m.= 35.45amu%1= 75.00%

What if you don’t have the percentage

or the atomic mass?Calculate the atomic mass of one of indium’s isotopes (there are two) if the atomic mass of indium is 114.8200 amu and one isotope of indium has a mass of 114.9041 amu and an abundance of 95.795%.

114.8200amu = (114.9041amu)*(0.95795)+(x)*(0.04205)

114.8200amu = 110.07 + (x)*(0.04205) -110.07 -110.07

4.75 = 0.04205x 0.04205 0.04205

a.m.= ?%2= ?

a.m.= 114.9041amu

%1= 95.795%

a.m. of indium=114.8200amu

100-95.795= 4.205%

Atomic mass of Indium= 113 amu