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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

Example:

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:

    • carbon-12
    • carbon-13
    • 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?

    • carbon-12

6p, 6e, 6n

    • carbon-13

6p, 6e, 7n

    • carbon-14

6p, 6e, 8n

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ISOTOPE USES

Some isotopes are radioactive (the nucleus is unstable and breaks apart releasing radiation).

We can use radioactive isotopes to help:

    • determine the age of ancient objects
    • identify blockages in the digestional tract
    • trace the flow of blood through arteries and veins
    • identify leaks in industrial systems

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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:

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

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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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