Unit: 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

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

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).

Name: ___________________________________ Date: __________________ Class period: _____

Unit: Atomic Structure

Calculating Atomic Mass – Wksh #4

Directions: Show all work for the following calculations. Include units and round answers to the correct number of significant figures. 1. Calculate the atomic mass of Carbon if the two common isotopes of carbon have masses of

12.000 amu (98.89 % abundance) and 13.003 amu (1.11 % abundance). 2. Calculate the atomic mass of nitrogen if the two common isotopes of nitrogen have masses

of 14.003 amu (99.63% abundance) and 15.000 amu (0.37% abundance). 3. Calculate the atomic mass of oxygen if the three common isotopes of oxygen have masses of

15.995 amu (99.759% abundance), 16.995 amu (0.037 % abundance), and 17.999 amu (0.204 % abundance).

HOMEWORK!