ELEMENTS AND ATOMIC MASSES
INTRODUCTION
An element is a fundamental substance that cannot be broken down by chemical means into
simpler substances. Elements are the building blocks of all matter. Currently 106 different elements
are known. Of these only 88 are found naturally. Some of the other elements may have existed in
earlier times; all have been produced, usually in extremely small quantities, in the laboratory.
If one could take a small piece of an element and divide it and subdivide it into smaller and
smaller particles, one would finally come to a single unit that can no longer be subdivided and still
show the characteristics of the element. This ultimate particle, the smallest particle of an element
that can exist, is called an atom. An atom is also the smallest unit of an element that can enter into a
chemical reaction. Atoms can be decomposed into smaller parts; however, these subunits no longer
have the properties of elements.
Chemists have assigned specific abbreviations to each element. These are called the
chemical symbols of the elements. Fourteen of the elements have a single letter as their symbol and
all the rest have two letters. The symbol can stand for the element itself, for one atom of that
element or for a particular quantity of that element. When using elemental symbols it is important
that certain rules be followed in the writing of these rules:
1.Symbols consist of one or two letters.
2. If one letter is used, it is capitalized.
3. If two letters are used, the first is capitalized and the second is a lower-case letter.
4. Always print the capital letters.
Pure substances composed of two or more elements are called compounds. Because they
consist of two or more elements, compounds unlike elements are capable of being decomposed into
simpler substances by chemical changes. The atoms of the elements in a compounds are combined
in whole number ratios, not in fractional parts of atoms. For a specific compound these atom ratios
are always the same. Another way of saying the same thing is that a given compound always
contains the same elements in a fixed proportion by weight. This is the Law of Constant
Composition, which basically states that the composition of a substance will always be the same
regardless of origin. It represents one of the cornerstones of chemistry.
Experimentally we find that 1 g of hydrogen gas combines with 8 g of oxygen gas to form 9
g of water. The mass ratio of hydrogen to oxygen is 1/8. In terms of percent we can say that 1 g out
of 9 g or (1/9)(100)% = 11% represents hydrogen. The rest, or 89%, is oxygen. If we also know
that two atoms of hydrogen combine with one atom of oxygen then the mass ratio of one hydrogen
12
atom to one atom of oxygen is 0.5/8 or 1/16. Hence if we assign a relative mass of 1 to a hydrogen
atom then the relative mass of an oxygen atom is 16. These relative masses are the commonly used
atomic weights or more properly the atomic masses.
PROCEDURE
In this experiment you determine the atomic mass of magnesium. The directions given are
quite explicit both to insure your safety and to encourage careful measurement. Small quantities of
magnesium and oxygen are used in this reaction and therefore a small error in measurement will
produce a large error in the results. Enter all data directly in your laboratory notebook. CAUTION:
BE SURE TO WEAR YOUR SAFETY GLASSES.
Step 1. Clean a nickel crucible and a porcelain cover. Support the crucible fitted with the
cover on a triangle on a ring stand. Place the cover slightly ajar so that the moisture can escape and
heat the crucible with a Bunsen burner for 10 minutes. Heat the crucible slowly at first and for the
final 3 minutes of heating lower the crucible into the hotter part of the flame (just above the tip of the
blue cone of a well-adjusted Bunsen burner). To prevent moisture contamination, always handle the
dried crucible and cover with tongs.
Step 2. After the crucible and contents have cooled to room temperature, measure and record
the mass of the covered crucible to the nearest 0.001 g. Place about 0.1 g of magnesium metal into
the dried crucible and then weigh and record the mass (to the nearest 0.001 g) of the covered
crucible and magnesium.
Step 3. Heat the magnesium in the covered crucible for about 30 minutes with a mediumsized flame. Do not lift the cover of the crucible during this time as a violent reaction might occur.
Step 4. At the end of 30 minutes remove the burner and with tongs, momentarily tilt the
cover of the crucible slightly to allow more air to enter. Replace the burner and heat the crucible for
another 5 minutes. Repeat this process at least three more times. If the slightest trace of white
"smoke" is seen rising from the crucible, quickly replace the cover, heat the crucible for an
additional 5 minutes and then proceed as described in the first part of this paragraph.
Step 5. For a final heating period, remove the Bunsen burner and place the cover slightly
ajar to allow for a continual exchange of air. Replace the burner and heat the crucible vigorously for
10 minutes. Allow the crucible to cool, weigh the crucible with its lid. The white powder in your
13
crucible is a compound of magnesium and oxygen.
CALCULATIONS
1. From the data you have collected, determine the mass of magnesium, oxygen, and magnesium
oxide involved in your experiment.
2.
If each atom of magnesium combines with 1 atom of oxygen to form magnesium oxide, what
is the relative mass of 1 atom of magnesium to 1 atom of oxygen?
3. Assigning the atomic mass of 16.000 to oxygen, what is the atomic mass of magnesium?
4. Predict the molecular formula of the compound, magnesium oxide.
5. Calculate the percentage composition of the compound that you predicted in 4.
6. Write a balanced equation for the reaction between magnesium and oxygen gas.
QUESTIONS
1. If 1 atom of sulfur combines with 2 atoms of oxygen to form sulfur dioxide, what is the ratio of
the mass of 1 sulfur atom to 1 oxygen atom?
2. Assigning the atomic mass of 16.000 to oxygen, what is the atomic mass of sulfur?
3. A 0.500 g sample of nitrogen gas combines with 1.140 g of oxygen gas to form NO2. If the
atomic mass of oxygen is 16.000, calculate the atomic mass of nitrogen from this data.
4. Atomic masses (or weights) are based on a scale on which carbon has an atomic mass of 12.00.
Carbon and calcium combine in the ratio of 2:1 to form calcium carbide. If 22.75 g of calcium
combine with 13.62 g of carbon, what is the atomic weight of calcium?
14