1. Isotopes and Atomic Mass
Atoms and Elements
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

2. Isotopes
Isotopes
Are atoms of the same element that have different mass numbers.
Have the same number of protons, but different numbers of neutrons.

3. Nuclear Symbol A nuclear symbol
Represents a particular isotope of an element.
Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol:
mass number 23 Na
atomic number 11

4. Information from Nuclear Symbols
The nuclear symbol can help you find the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom.
O P Zn
In atoms, the protons = the electrons. If you know one, you know the other.
Mass number – # protons = neutrons
(so neutrons + protons = Mass number)

5. Information from Nuclear Symbols
O P Zn
8 p+ 15 p p+
8 n 16 n 35 n
8 e- 15 e e-

6. Learning Check
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.
12C 13C 14C
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
(notice the ONLY difference in the nuclear symbols is the mass number)

7. Solution
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.
12C 13C 14C
protons 6 p+ 6 p+ 6 p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-

8. Learning Check A. 8 p+, 8 n, 8 e- ___________
Write the nuclear symbols for atoms with the following subatomic particles:
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________

9. Solution
Write the nuclear symbols for atoms with the following subatomic particles:
16O
A. 8 p+, 8 n, 8 e
B. 17p+, 20 n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47

10. Learning Check
Which of the following pairs are isotopes of the same element?
A. 15X 15X
B. 12X 14X
C. 15X 16X
7 8

11. Solution
1. B. 12X 14X
Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.

12. Learning Check 2. In which of the following pairs do both atoms have
8 neutrons?
A. 15X 15X
B. 12X 14X
C. 15X 16X
7 8

13. Solution
2. C. 15X 16X
7 8
The atom of nitrogen (7) and the atom of oxygen (8) each have 8 neutrons. We know they are nitrogen and oxygen because nitrogen is 7 on the PT and oxygen is 8 on the PT.

14. Atomic Mass…a bit more complicatedNa
22.99
Atomic Mass…a bit more complicated
Atomic Mass is the average mass of all isotopes of an element. Its rarely a whole number
ATOMIC MASS vs. MASS NUMBER
Mass Number is the mass of one particular isotope. Its always a whole number
12 x x

15. Calculating Atomic Mass
The calculation for atomic mass requires multiplying the…
Percent(%) abundance of each isotope.
Mass of each isotope of that element.
Then add the results for each isotope to get the atomic mass for that element:
[Mass isotope 1 x (% ) ] + [Mass isotope 2 x (% )]
(100) (100)
IMPORTANT: to turn a percent into a decimal, just move the decimal point two places to the left…avoid the % / 100 calculation.

16. Atomic Mass of Magnesium
The atomic mass of Mg on the periodic table…
The most abundant isotope contributes the most; least abundant contributes the least
Can’t be found by just adding the masses and dividing by 3…is not a “straight average”

17. Correctly Calculating Atomic Mass
Isotope Mass Abundance
24Mg = 24 amu x = amu
25Mg = 25 amu x = amu
26Mg = 26 amu x = amu
Atomic mass (average mass) Mg = amu
Wrong way: amu
3 isotopes
= amu Mg
24.31

18. Learning Check Using the periodic table, specify the atomic mass of
each element:
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________

19. Solution Using the periodic table, specify the atomic mass
of each element:
A. calcium amu
B. aluminum amu
C. lead amu
D. iron amu

20. Atomic Mass for Cl
The atomic mass of chlorine is the weighted average of two isotopes 35Cl and 37Cl.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

21. Calculating Atomic Mass for Cl
35Cl has atomic mass 35 (75.78%) and 37C has
atomic mass 37 (24.22%).
The atomic mass and percent of each isotope are used to calculate the contribution of each isotope to the weighted average.
35 x = amu
37 x = amu
35.48 amu
The sum is the weighted average or atomic mass of Cl amu on the Periodic Table

22. Learning Check
Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.11% of 69Ga (atomic mass 68.93) atoms and 39.89% of 71Ga (atomic mass 70.93) atoms.
What is the atomic mass of gallium?

23. Solution 69Ga 68.93 amu x .6011 = 41.43 amu (from 69Ga) 71Ga
Atomic mass Ga = amu 31 Ga
69.72