Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 6: 31 January


The Nuclear Atom, cont.


Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also differ substantially in terms of their nuclear stability. Let's look at some examples of isotopes:

 Symbol Z A p n e-
14 6 14 6 8 6
238U6+ 92 238 92 146 86
35Cl- 17 35 17 18 18
 18O2-  8 18 8  10 10 

You should be able to fill in the blanks in a table like this with, the aid of a periodic table, on a quiz.


Stoichiometry is the quantitative study of the composition of compounds (e.g. determining the ratios of atoms in a molecule) and/or the ratios of substances in chemical reactions.

The Mole

This is the SI unit of amount of substance. 1 mole = the number of carbon atoms in 12 g of 12C. This number, called Avogadro's Number, has been measured as 6.022 x 1023 mol-1 (current value: 6.022 141 99 x 1023mol-1). Notice that this number can refer to anything (a mole of eagles, a mole of pennies, etc.). In each case we are talking about 6.022 x 1023 items or entities.

For chemists a mole has two common uses:

Note that Avogadro's number, 6.022 x 1023 is thus the conversion factor from amu's to grams!

Mole Samples Demo

Atomic Mass

Notice that atomic masses have two meanings:


Determination of Atomic Mass

We want to be able to figure out the atomic mass of a sample with a particular isotopic composition.

Example: Cu occurs as an isotopic mixture of 69.09% 63Cu (mass = 62.93 amu) and 30.91% 65Cu (64.93 amu). What is the atomic mass of copper in this sample?

Assume the sample consists of 1 atom for convenience, then

(0.6909 atoms)(62.93 amu/atom) + (0.3091 atoms)(64.93 amu/atoms) =

43.478 amu + 20.070 amu = 63.558 amu for 1 atom

= 63. 558 amu/atom

How about sig figs? 1 is a count, therefore exact. The two multiplications each have 4 sig figs so the calculations each have 4 sig figs (note I keep one extra, that is 5 sig figs, in the calculations to avoid rounding errors.) . For the addition we use the add/subt. rule and look at decimal place, for our four sig figs the hundredth's place is then the sig fig (again, during calculation its best to keep one extra sig fig to avoid rounding errors). The final answer then has 4 sig figs:

= 63. 56 amu/atom

An example of the reverse problem can be found on the posted Final, number II.3.




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© R A Paselk

Last modified 31 January 2011