Mass of Individual Atoms

1
Mass of Individual Atoms

• Review
• Mass of proton 1.673 x 10-24 g
• Mass of neutron ?
• Mass of electron 9.11 x 10-28 g
• An atom of K-39, has ____ protons, _____
  neutrons, and ______ electrons.
• Who wants to add up that actual mass?!?!

2
Mass of Individual Atoms (cont.)

• We ALL want to work with whole numbers, so
  scientists came up with the atomic mass unit
  (amu).
• Def. 1/12 the mass of a carbon-12 atom (ALMOST
  equal to the mass of 1 proton or 1 neutron)

3
Mass of Individual Atoms (cont.)

• Review What does the mass number of an element
  tell you?
• The mass recorded on the periodic table for an
  element is called atomic mass.
• Atomic mass is determined by the weighted average
  mass of the isotopes of that element.

4
Mass of Individual Atoms (cont.)

• What is the atomic mass for chlorine?
• Chlorine exists naturally as a mixture of 75.770
  Cl-35 and 24.230 Cl-37.
• What does Cl-35 and Cl-37 mean?
• The ACTUAL masses of these isotopes would be
  34.969 amu and 36.966 amu.
• The math
• (75.770 x 34.969) (24.230 x 36.966) 35.453
  amu
• What was the atomic mass of chlorine? ?

5
Practice Assignment

• To calculate atomic mass
• Calculate mass contribution of each isotope by
  multiplying its mass x its abundance
• Add together all the mass contributions.
• P.104 15-16
• Study Guide Workbook pp. 22-23 (24-33)