Chapter 3 Atoms and Elements

1. Chapter 3 Atoms and Elements 3.5Isotopes and Atomic Mass

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 atom of an element • gives the mass number in the upper left corner and the atomic number in the lower left corner Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol: mass number atomic number 23 Na 11

4. 16 65 31 O P Zn 15 8 30 Nuclear Symbols For an atom, the nuclear symbol gives the number of • protons (p+) • neutrons (n) • electrons (e–) 8 p+ 15 p+ 30 p+ 8 n 16 n 35 n 8 e– 15e– 30 e–

5. 13 14 12 C C C 6 6 6 Learning Check Naturally occurring carbon consists of three isotopes, C-12, C-13, and C-14. State the number of protons, neutrons, and electrons in each of the following. protons ______ ______ ______ neutrons ______ ______ ______ electrons ______ ______ ______

6. 13 14 12 C C C 6 6 6 Solution protons 6 p+ 6 p+ 6 p+ neutrons 6 n 7 n 8 n electrons 6 e– 6 e– 6 e–

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

8. 16 O 8 37 Cl 17 107 Ag 47 Solution A. 8p+, 8n, 8e– B. 17p+, 20n, 17e– C. 47p+, 60n, 47e–

9. Learning Check 1. Which of the following pairs are isotopes of the same element? 2. In which of the following pairs do both atoms have 8 neutrons? A. B. C. 12 15 X X 6 7 12 14 Z Z 6 6 15 16 Z Z 7 8

10. Solution 1. B Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8. 2. C An atom of nitrogen (atomic number 7) and an atom of oxygen (atomic number 8) each have 8 neutrons.

11. Isotopes of Magnesium In naturally occurring magnesium, there are 3 isotopes.

12. 32 S 16 33 S 16 34 32 33 34 36 S S S S S 16 16 16 16 16 36 S 16 Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances: Isotope% Abundance 95.02 0.75 4.21 0.02

13. Atomic Mass 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 C-12 • is not the same as the mass number Na 22.99

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

15. Atomic Mass for Cl The atomic mass of chlorine is • based on all naturally occurring Cl isotopes • not a whole number • the weighted average of the Cl-35 and Cl-37 isotopes

16. Atomic Mass for Mg The atomic mass of Mg is • based on all naturally occurring Mg isotopes • not a whole number • the weighted average of the Mg-24, Mg-25, and Mg-26 isotopes

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

18. Solution Using the periodic table, give the atomic mass of each element. A. calcium 40.08 amu B. aluminum 26.98 amu C. lead 207.2 amu D. barium 137.3 amu E. iron 55.85 amu