# Isotopes and Average Atomic Mass Vocabulary: 1.isotope 2.percent abundance 3.average atomic mass “Marilyn Monroe”, Andy Warhol, 1962.

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Isotopes and Average Atomic Mass Vocabulary: 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, 6 2.6, 6, 7 3.6, 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 mass isotope 1 ) + (% isotope 2 x mass isotope 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 mass isotope 1 ) + (% isotope 2 x mass isotope 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

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