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

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

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Question What element has 8 neutrons in its nucleus? Don’t know because atoms of the same element can have different numbers of neutrons.

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Isotopes Isotopes: atoms of the same element with different numbers of neutrons Almost all elements have many different isotopes that are found in nature.

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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?

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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

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

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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

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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

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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?

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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!!!

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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

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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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