Isotopes and Atomic Mass

3.6

• 3.6

• Isotopes and Atomic Mass

Isotopes

• Isotopes

• are atoms of the same element that have different mass numbers.

• have the same number of protons, but different numbers of neutrons.

An atomic symbol

• An atomic 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 23

• Na

• atomic number 11

The atomic symbol for a specific atom of an element

• The atomic symbol for a specific atom of an element

• gives the

• number of protons (p+),

• number of neutrons (n),

• and number of electrons (e-).

Examples of number of subatomic particles for atoms

• Examples of number of subatomic particles for atoms

• Atomic symbol

• 16 31 65

• O P Zn

• 8 15 30

• 8 p+ 15 p+ 30 p+

• 8 n 16 n 35 n

• 8 e- 15 e- 30 e-

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

• 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

• 6 6 6

• protons ______ ______ ______

• neutrons ______ ______ ______

• electrons ______ ______ ______

12C 13C 14C

• 12C 13C 14C

• 6 6 6

• protons 6 p+ 6 p+ 6 p+

• neutrons 6 n 7 n 8 n

• electrons 6 e- 6 e- 6 e-

Write the atomic symbols for atoms with the following subatomic particles:

• 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- ___________

• A. 8 p+, 8 n, 8 e- 16O

• 8

• B. 17p+, 20 n, 17e- 37Cl

• 17

• C. 47p+, 60 n, 47 e- 107Ag

• 47

1. Which of the pairs are isotopes of the same element?

• 1. Which of the pairs are isotopes of the same element?

• 2. In which of the pairs do both atoms have 8 neutrons?

• A. 15X 15X

• 8 7

• B. 12X 14X

• 6 6

• C. 15X 16X

• 7 8

B. 12X 14X

• B. 12X 14X

• 6 6

• The atomic symbols in “B.” represent isotopes of carbon with 6 protons each, but one has 6 neutrons and the other has 8.

• C. 15X 16X

• 7 8

• These isotopes of nitrogen and oxygen have 8 neutrons.

A sample of naturally

• A sample of naturally

• occurring sulfur contains

• several isotopes with the

• following abundances

• Isotope % abundance

• 32S 95.02

• 33S 0.75

• 34S 4.21

• 36S 0.02

The atomic mass of an element

• 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.

The atomic mass of chlorine is

• The atomic mass of chlorine is

• due to all the Cl isotopes.

• not a whole number.

• the average of two isotopes: 35Cl and 37Cl.

Using the periodic table, specify the atomic mass of

• Using the periodic table, specify the atomic mass of

• each element.

• A. calcium __________

• B. aluminum __________

• C. lead __________

• D. barium __________

• E. iron __________

Using the periodic table, specify the atomic mass of

• Using the periodic table, specify 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

The calculation for atomic mass requires the

• 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.

• mass of isotope(1)x (%) + mass of isotope(2) x (%) +

• 100 100

35Cl has atomic mass 34.97 amu (75.76%) and 37C

• 35Cl has atomic mass 34.97 amu (75.76%) and 37C

• has atomic mass 36.97 amu (24.24%).

• Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average.

• 34.97 x 75.76 = 26.49 amu

• 100

• 36.97 x 24.24 = 8.961 amu

• 100

• Sum is atomic mass of Cl 35.45 amu

Isotope Mass Abundance

• Isotope Mass Abundance

• 24Mg = 23.99 amu x 78.70/100 = 18.88 amu

• 25Mg = 24.99 amu x 10.13/100 = 2.531 amu

• 26Mg = 25.98 amu x 11.17/100 = 2.902 amu

• Atomic mass (average mass) Mg = 24.31 amu

The atomic mass of Mg

• The atomic mass of Mg

• is due to all the Mg isotopes.

• is a weighted average.

• is not a whole number.

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 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.

• 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 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.

• What is the atomic mass of gallium?

69Ga

• 69Ga

• 68.926 amu x 60.10 = 41.42 amu (from 69Ga)

• 100

• 71Ga

• 70.925 amu x 39.90 = 28.30 amu (from 71Ga)

• 100

• Atomic mass Ga = 69.72 amu