• San Diego Mesa College

• CHEMISTRY 152

Chemistry 152 Homework 3 key

1. What are the numbers of protons, neutrons, and electrons for each of the following:

P n e

a. 35Cl 17 18 17

b. 55Fe 26 29 26

c. 139La3+ 57 82 54

d. 79Br- 35 44 36

2. How do the properties of an atom compare to the properties of :

a. an isotope of that atom

nearly identical

b. an ion from that atom

very different

3. If zinc has two isotopes with 35 and 37 neutrons respectively,

a. What are the mass numbers of the two isotopes?

Zn has 30 protons

Mass numbers are 30 + 35 = 65

30 + 37 = 67

b. What are the percentages of each isotope in naturally occurring zinc?

ave = %67Zn(mass67Zn) + %65Zn(mass65Zn)

65.38 = x(67) + (1-x) 65

= 67x + 65 – 65x

0.38 = 2x

0.19 = x = 19% 67Zn

0.81 = 1-x = 81% 65Zn

4. Why are the actual atomic masses of the elements not whole numbers?

They are averages of the masses of the isotopes.