BACKGROUND:
A compound consists of two or more different types of
atoms that are chemically bonded. Halite, composed of sodium and chlorine,
is an example. Electrons move around the nucleus of an element in
specific and set orbitals. There are a finite number of electrons in each of
these orbitals.
If an atom does not have the full number of electrons
in each orbital, it seeks a partner that can "loan" one or more
electrons to "fill" its molecular orbital. This is the essential cause of
chemical bonding. For example, a sodium ion, which has a positive charge
wants to give up an electron whereas a chlorine ion, which has a negative
charge wants to accept an electron. The two elements combine to form an
ionic bond (bond formed by the attraction of unlike charges) and thus form
the compound, halite. There are several additional types of bonding of
molecular orbitals which
students will learn in high school.
The type of bonding between atoms and the
characteristics of those atoms determines to a large degree how a compound
will "appear" when the atoms combine. In halite, the chlorine atom
is twice the size of the sodium atom. When the chlorine atoms
"nestle" into a "packed" position, the sodium atoms fill
in the gaps. This packed position has a cubic structure, which is reflected
in the cubic nature of halite. You can demonstrate this by placing small and
large plastic beads in a small, cubic, clear, plastic box. Shake the box. If
one bead is twice the size of the other, they will pack in a cubic pattern.
The large beads represent chlorine (Cl) atoms and the smaller beads
represent sodium (Na) atoms.
PROCEDURE:
- In this lab, the students will look at different specimens of
commercial salt from Cargill Salt Company in Newark, California. These
can include: mill feed - kiln dried, used for animal feed; blending - vacuum
dried, used in food processing; granulated - vacuum dried, used in food
processing and table salt; pellets - kiln dried, used as water
conditioner; medium - kiln dried, used as water conditioner; bakers -
vacuum dried, used in making butter. Kiln and vacuum dried refer to the
type of process used to make the salt.
 Summarize the
composition and bonding behavior of halite. You might tell your students
that the atoms are "holding hands" and are brought together by
an "attractive" force. Draw the following diagram on the board
for the students to see this "bonding" , or use the electronic
presentation. Explain that since chlorine is twice the size of sodium,
when they combine sodium fills in the spaces between the chlorine,
forming cubes.
- The students will try to determine if all types of salt are
"cubic." They should use a magnifying glass or a microscope to
see the specimens in detail. Have them examine the specimens without
taking them out of the plastic bags.
- Ask students to think about why each type is different. Do not give
them too many hints, but have them "guess" the use of
each specimen. Have them record their guesses on the lab sheet.
- As they look at the samples, ask the students to draw the salt
crystals. Monitor their progress as they work their way through the
samples. Remind them to draw accurate pictures (pencils work best for
this exercise).
- Review their answers, then answer the conclusion together. While
all of these samples are composed of halite, only the blending,
granulated, and baking (all vacuum dried) specimens are cubic. The mill
feed and pellets are not cubic, and the medium is only roughly cubic.
This is because the cubic structure can be broken if the process of
making the salt crushes the crystals (mill feed) or combines the
crystals (pellets).
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