Friday, May 4, 2012


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Chapter 4 Atomic Structure

I. Atom�basic unit of matter

A. Composed of main subatomic particles

1. Protons

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. Neutrons

. Electrons

B. Atomic Number (Z)�indicates number of protons in nucleus

1. Unique number for each element

. Also shows number of electrons for neutral atoms

C. Mass Number (A)�number of nucleons in nucleus of an atom

1. Nuclide�atom where number of nucleons is known

a. May be designated as follows

b. Also designated by

. Isotopes�atoms of same element that have different numbers of neutrons

a. Have same Z since number of protons identifies each element

b. A differs for each isotope since number of neutrons differ

c. Example carbon-1 and carbon-14

. To find number of neutrons for any atom,

D. Example Complete the following table for neutral atoms.

Name Symbol Atomic # Mass # p+ no e-

calcium 4

8 11


1 77

II. Average Atomic Mass

A. Weighted average of the masses of all known isotopes for an element

B. Reported for each element on the Periodic Table to the number of known significant digits

C. Uses scale for atomic mass units (amu’s or u’s) since actual values for atomic masses are so small

1. Based on mass of a carbon-1 atom

. 1 amu = ½ mass of one carbon-1 atom

. This relationship enables chemists to calculate the amounts of elements that will react to yield a certain amount of product

D. Masses of main subatomic particles

1. 1 e- = .105 x 10-8 g = 0.00054 amu

. 1 p+ = 1.6765 x 10-4 g = 1.007 amu

. 1 no = 1.6745 x 10-4 g = 1.0087 amu

E. Calculating average atomic mass

1. Convert the percent abundance of each isotope to a decimal

. Multiply the mass of each isotope by its corresponding decimal percentage

. Add the results to obtain the average atomic mass for that element

F. Example Problem

Chlorine has two known isotopes. Chlorine-5 has a mass of 4.68 amu, and chlorine-7 has a mass of 6.65 amu. In a given sample of chlorine atoms, 75.771% will be chlorine-5 and 4. % will be chlorine-7. Calculate the average atomic mass of chlorine.

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