1. Learning Check
Naturally occurring carbon consists of three isotopes: Carbon-13, Carbon-14, and Carbon-15. State the number of protons, neutrons, and electrons in each of the following:
C-13 C-14 C-15
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______

2. Solution C-13 C-14 C-15 protons 6 p+ 6 p+ 6 p+ neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-

3. Learning Check
Write the atomic 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- ___________

4. Solution A. 8 p+, 8 n, 8 e- O B. 17p+, 20 n, 17e- Cl
C. 47p+, 60 n, 47 e- Ag

5. X Atomic Symbol Notation
When we look at isotopes, we see a different notation used than what is found on the periodic table. Be sure you understand how to read this different notation:
Where m=mass of isotope
a=atomic number of element
x=atomic symbol X m a

6. Isotopes of Magnesium

7. Isotopes of Magnesium

8. Isotopes of Sulfur A sample of naturally occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
33S
34S
36S
32S, 33S, 34S, 36S

9. Atomic Mass Na 22.99 The atomic mass of an element
is listed below the symbol of each element on the periodic table.
gives the mass of an “average” atom of each element compared to 12C.
is not the same as the mass number. Na
22.99

10. Isotopes of Some Elements and Their Atomic Mass
Most elements have two or more isotopes that contribute to the atomic mass of that element.

11. Atomic Mass for Cl The atomic mass of chlorine is
due to all the Cl isotopes.
not a whole number.
the average of two isotopes: 35Cl and 37Cl.

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

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

14. Calculating Atomic Mass
The calculation for atomic mass requires the
percent(%) abundance of each isotope.
atomic mass of each isotope of that element.
sum of the weighted averages.

15. Calculating Atomic Mass for Cl
35Cl has atomic mass amu (75.76%) and 37C
has atomic mass amu (24.24%).
Find the atomic mass:

16. Calculating Atomic Mass Mg
Isotope Mass Abundance
24Mg = amu
25Mg = amu
26Mg = amu
Atomic mass (average mass) Mg =
24.31 amu

17. Atomic Mass of Magnesium
The atomic mass of Mg
is due to all the Mg isotopes.
is a weighted average.
is not a whole number.

18. Learning Check
Gallium is an element found in lasers used in compact disc players.
In a sample of gallium, there is
60.10% of 69Ga (atomic mass ) atoms and 39.90% of 71Ga (atomic mass ) atoms.
What is the atomic mass of gallium?

19. Solution 31 Ga
69.72