Isotopes and Average Atomic MassVocabulary:

1. isotope

2. percent abundance

3. average atomic mass

“Marilyn Monroe”, Andy Warhol, 1962

Question

•What element has 5 protons in its nucleus?• boron!!!

Question

•What element has 8 neutrons in its nucleus?• Don’t know because atoms of the

same element can have different numbers of neutrons.

Isotopes

• Isotopes: atoms of the same element with different numbers of neutrons• Almost all elements have many

different isotopes that are found in nature.

Isotopes

• EX: A lump of carbon is shown on the left. If you analyzed the carbon atoms in it you would find three different isotopes of carbon:1. carbon-12

2. carbon-13

3. carbon-14

Remember what these numbers means?

Isotopes

• How many protons, electrons, and neutrons are found in each of the isotopes?1. carbon-12

2. carbon-13

3. carbon-14

1. 6, 6, 62. 6, 6, 73. 6, 6, 8

• If the three different isotopes of carbon are carbon-12, carbon-13, and carbon-14, what is the average mass of carbon?

• It’s not 13! Any ideas how that could be true?• There are many, many

more naturally occurring carbon-12 atoms than there are carbon-13 or carbon-14 atoms

Percent Abundance

• To calculate the average mass of all the atoms of an element, chemists must determine the percent abundance of each isotope of the element.• Carbon’s percent abundance is:• 98.93% carbon-12• 1.06% carbon-13• 0.01% carbon-14

Percent Abundance

• Carbon’s percent abundance is:• 98.93% carbon-12• 1.06% carbon-13• 0.01% carbon-14

• So which isotope will have a larger effect on the average mass of carbon atoms?• What will the average mass be close to?

Average Atomic Mass

• To calculate the average atomic mass of an element, you need to know two things: the percent abundance of each isotope and the relative mass of each isotope. Then you can use the following formula:

avg atomic mass = (%isotope 1 x massisotope 1) + (%isotope 2 x massisotope 2) + …

must be % as decimal!!!

Example 1

• Let’s try carbon:• What is the average atomic mass of carbon if the

percent abundance of each isotope of carbon is the following: 98.93% carbon-12, 1.06% carbon-13, and 0.01% carbon-14? (Round to 2 decimal places.)

equation:avg atomic mass = (%isotope 1 x massisotope 1) + (%isotope 2 x massisotope 2) + …

substitute:avg atomic mass = (0.9893 x 12 amu) + 0.0106 x 13 amu) + (0.0001 x 14 amu)

solve:avg atomic mass = 12.01 amu

Example 2

• Try this one on your own:• Magnesium has three naturally occurring

isotopes. 78.70% of magnesium atoms exist as magnesium-24, 10.03% exist as magnesium-25, and 11.17% exist as magnesium-26. What is the average atomic mass of magnesium?(Round to 2 decimal places.)

Answer: 24.30 amu