Name ___________________________________ Date ____________
Period___
Guided Atomic Mass
Directions: Please consult your book (pp. 78-80) to answer the questions.
Earlier in the section you have read about mass numbers. Now you are going to read about atomic mass.
There is a difference between mass numbers and atomic mass.
You know that the mass number of an element is the mass (aka total number of protons and
neutrons) for a SPECIFIC isotope of that element.
The average atomic mass is the _____________________ _______________________ (1) of the
atomic masses of the naturally occurring isotopes of an element.
This means that the average atomic mass accounts for ALL of the isotopes of a particular
element.
(2) What is the difference between the mass number and the average atomic mass of an
element?
The average atomic mass is what is reported underneath the element symbol on the periodic table. The
unit is called the atomic mass unit.
3
Atomic number
Li
Average atomic mass
6.941
The average atomic mass of an element depends on both the ____________________ (3) and the
relative _____________________________ (4) of each of the element’s isotopes.
This dependence is because the average atomic mass accounts for ALL of the isotopes of
an element.
You will be calculating a weighted average in order to calculate the average atomic mass. Which involves
the process below.
You will be given a percentage for the abundance and an atomic mass of an isotope. For
example:
Isotope
Magnesium – 24
Magnesium – 25
Magnesium – 26
Natural Abundance (%)
78.99
10.00
11.01
Atomic Mass (amu)
23.9850
24.9858
25.9826
(5) Convert the natural abundance percentages to decimals. Fill out the blank parts of the
table.
Isotope
Natural
Decimal (Percentage Atomic Mass (amu)
Abundance (%)
conversion)
(6) Multiply
mass. Fill out the blank23.9850
parts of the table.
Magnesium
– 24 the decimal
78.99 by the atomic
0.7899
Magnesium – 25
10.00
24.9858
Magnesium – 26
11.01
25.9826
(6) Multiply the decimal by the atomic mass for each isotope. Fill out the blank parts of
the table.
Isotope
Magnesium – 24
Magnesium – 25
Magnesium – 26
Natural
Abundance (%)
78.99
10.00
11.01
Decimal (Percentage Atomic Mass
conversion)
(amu)
0.7899
23.9850
24.9858
25.9826
Decimal x atomic
mass (amu)
18.94
(7) Add all of the decimal x atomic mass. Fill out the blank part of the table.
Isotope
Magnesium – 24
Magnesium – 25
Magnesium – 26
Natural
Abundance (%)
78.99
10.00
11.01
Decimal (Percentage Atomic Mass
conversion)
(amu)
0.7899
23.9850
24.9858
25.9826
Total
Congrats, you have calculated an average atomic mass!
Decimal x atomic
mass (amu)
18.94