1. 4.5 Isotopes and Atomic Mass
The atomic symbol for an isotope of magnesium with 12 neutrons.
Learning Goal Determine the number of protons, electrons, and neutrons in one or more of the isotopes of an element; calculate the atomic mass of an element using the percent abundance and mass of its naturally occurring isotopes.

2. Isotopes Isotopes are atoms of the same element.
have different mass numbers.
have the same number of protons but different numbers of neutrons.
can be distinguished by their atomic symbols.
Core Chemistry Skill Writing Atomic Symbols for Isotopes

3. Atomic Symbols: Subatomic Particles
Given the atomic symbols, determine the number of protons, neutrons, and electrons.
ANALYZE Atomic Mass Number of
THE PROBLEM Number Number Protons
number in number in equal to
lower left upper left atomic
corner corner number

4. Atomic Symbols: Subatomic Particles
Determine the number of protons, neutrons, and electrons.
Isotope
Atomic
Number
Mass
Number of
Protons
Neutrons 8 16
8 (16–8) 15 31
16 (31–16) 30 65
35 (65–30)

5. Isotopes of Magnesium
Magnesium, with three naturally occurring isotopes, has an atomic mass of amu.

6. Isotopes of Magnesium

7. Study Check
Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the three isotopes.

8. Solution
Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the three isotopes.

9. Study Check
Write the atomic symbols for atoms with the following subatomic particles: A. 8 protons 8 neutrons 8 electrons B. 17 protons 20 neutrons 17 electrons C. 47 protons 60 neutrons 47 electrons

10. Solution
Write the atomic symbols for atoms with the following subatomic particles: A. 8 protons 8 neutrons 8 electrons B. 17 protons 20 neutrons 17 electrons C. 47 protons 60 neutrons 47 electrons

11. Study Check
1. Which of the pairs below are isotopes of the same element?
In which of the pairs below do both atoms have eight neutrons? A. B. C.

12. Solution
1. Which of the pairs are isotopes of the same element? B. 2. In which of the pairs do both atoms have eight neutrons? C.

13. Calculating Atomic Mass
Atomic mass is the
weighted average of all naturally occurring isotopes of that element.
number on the periodic table below the chemical symbol.
Chlorine, with two naturally occurring isotopes, has an atomic mass of amu.

14. Calculating Atomic Mass
To calculate atomic mass,
use the experimental percent abundance of each isotope of the element.
multiply the percent abundance by the atomic mass of that isotope.
sum the total mass of all isotopes.

15. Calculating Atomic Mass
To calculate atomic mass of chlorine, use experimental data for both isotopes. Atomic mass of Mg = amu amu amu = amu (weighted average mass)

16. Atomic Mass of Some Elements

17. Study Check
Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the more prevalent one. (According to the periodic table, the atomic mass of lithium is amu.)

18. Solution
Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the more prevalent one. (According to the periodic table, the atomic mass of lithium is amu.) Since the total atomic mass of 7Li is closer to 7, the mass number of 7Li, the more prevalent isotope is 7Li.

19. Study 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 39.90% of 71Ga (atomic mass ) atoms What is the atomic mass of gallium?

20. Solution
STEP 1 Multiply the mass of each isotope by its percent abundance divided by 100.

21. Solution
STEP 2 Add the contribution of each isotope to obtain the atomic mass. 31 Ga
69.72