1. Periodic Table of the Elements – Lesson 4
Isotopes
Periodic Table of the Elements – Lesson 4

2. Alchemy Unit – Investigation III
Lesson 3:
Subatomic Heavyweights
EQ: How do isotopes of an atom account for the atomic weight of an element?

3. Warm-Up:
A chemist investigating a sample of lithium found that some atoms have a lower mass than other atoms. The chemist determined that the structures of the two types of atoms would be similar to the two drawings below.
What is different about the 2 atoms?
What is the atomic number for each?
What is the atomic mass?
Are they both lithium?
Unit 1 • Investigation III

4. Notes
Atoms of the same element that have different numbers of neutrons are called isotopes.
Atomic mass units (amu) are “invented” measurement units of the atomic mass.
Unit 1 • Investigation III

5. Activity
1. Count and record the numbers of protons, neutrons, and electrons for each atom pictured below:
# electrons
# neutrons
# protons 10 9 8 7 6 5 4 3 2 1
boron atom
Unit 1 • Investigation III

6. 3. How many of each isotope is present in the sample of 10?
2. How many different isotopes of boron can be found in the pictures? Explain what they are.
3. How many of each isotope is present in the sample of 10?
4. What is the atomic mass of each isotope? How do you know?
Calculate the average atomic mass of the 10 atoms.
If you were to look at 100 atoms of boron, how many of each type would you expect to find?
Unit 1 • Investigation III

7. 7. Complete the chart using your periodic table:
18, 20, or 22
39.9 Ar
Argon
Magnesium 7
Nitrogen 23 V
Vanadium
6.94
Lithium 17
Chlorine
5 or 6 B
Boron
# of neutrons
# of electrons
# of protons
Atomic Weight
Atomic Number
Chemical Symbol
Element
Unit 1 • Investigation III

8. 8. How many isotopes does lithium have?
9. Which isotope of lithium is more commonly found? How do you know?
10. Explain why the atomic weights listed in the periodic table are not usually whole numbers.
Unit 1 • Investigation III

9. Notes
While the element iron is defined as being made up of neutral atoms with 26 protons and 26 electrons, not every iron atom has the same number of neutrons.
Atoms that have the same number of protons but different numbers of neutrons are called isotopes.
(cont.)‏
Unit 1 • Investigation III

10. Notes (cont.)‏
What we call the atomic weight on the periodic table is actually the average atomic mass of that element’s naturally occurring isotopes.
Isotopes have similar chemical properties in that they combine with other elements to form similar compounds.
(cont.)‏
Unit 1 • Investigation III

11. (cont.)‏
Atomic Weight is the weighted average of the atomic masses of different isotopes taking into account their abundance.
(cont.)‏
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12. (cont.)‏
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13. Wrap-Up
Elements may have anywhere from 2 to 10 naturally occurring isotopes.
The atomic weight of an element listed on the periodic table is actually the average mass of the naturally occurring isotopes of that element.
Isotopes have the same number of protons and electrons, but different numbers of neutrons.
(cont.)‏
Unit 1 • Investigation III

14. Isotopes are referred to by their mass numbers.
(cont.)‏
Isotopes of a single element exhibit similar properties in that they form similar compounds.
Isotopes are referred to by their mass numbers.
Unit 1 • Investigation III

15. Ticket –Out:
Predict the isotopes of carbon, C. Which isotope is more abundant? How do you know?
Unit 1 • Investigation III