Average Atomic Mass

Every element has at least 2 naturally occurring isotopes.

This begs the question, "If elements can have different masses, why is there only one mass on the Periodic Table?"

The mass on the Periodic Table is actually the Average Atomic Mass of all of that element's isotopes.

Quick notes on Average Atomic Mass:

• Units: amu (Atomic Mass Unit)
• The mass on the PT
• Comprised of the average of all isotopic masses

This is the formula for Average Atomic Mass:



This is what the formula means:

1. Isotopes don't exist in equal amounts
2. Take each isotope's mass
3. Multiply it by its % abundance (in decimal form)
4. Do this for all isotopes of the element
5. Add them together

Ex.

Carbon has 2 main isotopes: Carbon-12 and Carbon-13.

C-12 makes up about 98% of all of the Carbon in the world.
C-13 makes up about 1.1% of all of the Carbon in the world.

To find the Average Atomic Mass of Carbon:

1) Change % to decimal

98% = .98
1.1% = .011

2) Multiply decimal by corresponding mass

.98 x 12 = 11.76 amu
.011 x 13 = .143 amu

3) Add the products together

11.76 amu + .143 amu = 11.903 amu

4) The Average Atomic Mass of Carbon is 11.903 amu

Websites
Here is a video explaining how to calculate Average Atomic Mass.
This website gives a brief explanation and a few examples.
Worksheets
This is a great worksheet on Average Atomic Mass.

Isotopes and Ions

Notes for Page 23.

Atoms are identified by their # of protons.

• The # of protons DOES NOT change
• The # of neutrons CAN change
• The # of electrons CAN change

When the # of neutrons and electrons change the atomic structure/description also changes.

Isotopes

• Atoms of the SAME element with DIFFERENT atomic masses
• Occur when a change in neutrons occurs
• All elements have at least two naturally occurring elements

Two ways to write isotopes

1)

Ex.

2)

Ex.

Ions

• Atoms with a charge
• Occur when there is a change in # of electrons

Two types:

1) Cation - positively charged ion

2) Anion - negatively charged ion

Charge is written in the top right corner of the chemical symbol

Quizzes

Quiz #1 is a great quiz!

Quiz #2 is another great quiz!

Quiz #3 is a very basic matching quiz.

Quiz #4 is a really great quiz.

Worksheets

This is a great worksheet on the Subatomic Chart that includes Isotopes and Ions.

This is a great worksheet on atomic structure with isotopes and ions.