Key Concept: Families of the Periodic Table

Key Concept: Average Atomic Mass (436 only)

Sample Questions:

1.(JUNE 1996 436): Copper, whose atomic number is 29. Has two stable isotopes. The first isotope has 34 neutrons and the second has 36 neutrons.

If the first isotope accounted for 20% of all copper atoms, what would be the atomic mass of copper?

35.0 u
35.6 u
64.0 u
64.6 u

2.(JUNE 1995 436): An element has two isotopes that occur naturally. The following table gives some information about these two isotopes.

ISOTOPE	MASS NUMBER	RELATIVE ABUNDANCE
1	63	69.1
2	65	30.9

According to this information, what is the atomic mass of this element?

65.540 u
63.618 u
64.000 u
64.382 u

3.(JANUARY 2000 436): Mark is looking for the atomic mass and the name of the element. He has recorded the following data :

Mass number of each of the natural isotopes of this element.	Relative abundance in nature (%) of each of these isotopes.
20	90.92
21	0.26
22	8.82

Calculate the atomic mass and the name of the element Mark is looking for. Show all work.

Notes:

To find the average, take each mass number and multiply it by the decimal form of its natural abundance. Then add all the products. (see sample calculations in answers to above problems). The answer is in u = atomic mass units. One atom of H has an average mass of 1.00797 u. One mole of atoms would have an average mass of 1.00797 grams.

Note that in finding average atomic mass, we do NOT divide by the number of isotopes. That would only be done if there was an equal abundance for each isotope, which never happens in nature.

Another common mistake is to forget to convert decimal percents into a decimal.

Example 10% = 0.1, but 0.1% = 0.001

If they ask you to look up the element as they do in the January 2000 question, then do so. Otherwise do not always expect calculated answers to agree with those in the periodic table, because the real method of calculating atomic masses is more complicated than what we teach you.