Isotopes and Average Atomic Mass Vocabulary 1 isotope

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

• 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. Question

• 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: Isotopes 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 1. 6, 6, 6 2. carbon-13 2. 6, 6, 7 3. carbon-14 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

• 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? Percent Abundance

• 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!!! Average Atomic Mass

• 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 1

• 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 Example 2