The building blocks of all things are the tiny atoms that make up the many elements which we can see on the periodic table. The minerals we will be studying are made of these elements or combinations of elements called
compounds
. The basic atom is composed of positively-charged
protons
, negatively-charged
electrons
, and
neutrons
, which have no electrical charge at all. Protons an neutrons are located in the nucleus of a cell and have almost the same mass which is measured by a special unit called the
atomic mass unit
, or
amu
. A single proton or a single neutron has a mass of about one amu. The electron has a much smaller mass, only 1/1836 amu. There are always the same number of protons as electrons, and therefore the same number of positive charges as negative charges are found in the neutral atom.
A. The Bohr Model
Although our best understanding of the atom tells us that the electrons are in constant and rapid motion around the nucleus and the exact location of any electron cannot be known, we can by experimental means determine the most probable locations of electrons and establish energy levels or shells which we can use to help us understand the relations of atoms. In 1913 a Danish Scientist, Niels Bohr, developed a model that can be used by children to understand the way atoms combine into compounds. This model is called the Bohr model and shows a central nucleus with the protons and neutrons either drawn in or simply enumerated. Around this central core are the shells or energy levels. These levels are sometimes referred to as the K, L, M, N, O shells or alternatively, the first, second, third, fourth, fifth, energy levels. Each of these levels has a specific number of electrons it can maximally hold. The degree to which these shells are filled determines how readily it will combine with other elements and in what proportion.
Figure 1 illustrates a specific labelling technique that is quite easy to use with children and facilitates accuracy. Since the shells fill using definite laws and patterns involving energy, they are predictable. Beyond the element calcium, however, the laws become quite complex and need extensive explanation and understanding of subshells, It is wise, therefore, to limit the Bohr model drawing to that point at this grade level. Some periodic tables give the number of electrons in the energy levels as part of the information in the block for each element. By looking at this data, some of the students might come up with partial explanations for the building of these shells. In the absence of such data it is sufficient to declare the numbers 2 and 8 “magic numbers” and let them build the smaller atoms.
(figure available in print form)
After drawing out a few, or many, of the Bohr models the children will notice that some of the models have an outer shell that only needs one or two more electrons to be filled, and that others have one or two electrons that seem to hang awkwardly by themselves in their outer shell. These conditions can of course be used to develop an understanding of the terms
metal
and non-
metal
. Since the children can “see” the electrons they can judge their availability for donating or receiving. This technique makes easy work of explaining compounds and their chemical formulas. It also aids in the explanation of ionic and covalent bonding.
B. Ionic Bonding
By drawing a Bohr model of sodium we can see that its K and L shells are filled and the one remaining electron is by itself in the outer M shell. It will take very little energy to coax that electron away from its atom, leaving only ten electrons with ten negative charges to balance with the eleven protons with their eleven positive charges. When this happens we will have an ion with a +1 charge. In the same way if we draw a shell that has seven electrons, one short of that “magic number” eight. In this case the chlorine atom would be very happy (anthropomorphically speaking) to grab another electron and fill that shell. In doing so the chlorine atom now would have seventeen protons and eighteen electrons, resulting in a net charge on the atom of -1. This would be a negative ion. When two elements come together in this way we refer to it as an ionic bond. The opposing charges on the two ions cause them to be attracted together.
(figure available in print form)
C. Covalent Bonding
Next we will draw out an oxygen atom. In this case we can see that its outer shell has six electrons, two short of the “magic number” eight. And if we draw a hydrogen atom we can see that it has one electron in its outer shell, half of what it needs to fulfill the “magic” two count. From this we could deduce that by bringing in another hydrogen atom we can combine the three atoms into H2O, and each atom’s outermost shell would be filled. This is the basis for an understanding of
covalent bonds
, in which atoms come together and share electrons in their outer shells. The electron clouds overlap and the electrons circle both atoms.
(figure available in print form)
The bonds of compounds can influence some substances’ physical properties. And bonds exist not just between individual atoms but also throughout a crystal. We can look at two forms of the element carbon for an example. Graphite is a slippery black solid, the bonds form sheets of carbon which slide loosely over one another. Diamond on the other hand is a hard, clear crystal with tight tetrahedral bonding that holds the carbon atoms of diamonds securely in place. The difference can be seen below in the illustration.
(figure available in print form)
Further investigation of the importance of chemical bonds can be accomplished by a study of sugar and salt crystals. By comparing melting points and ease of crushing some simple inferences can be made about their bonds. Directions for this experiment are to be found in Appendix 1, Activity 1.