CHAPTER 3. CHEMICAL BUILDING BLOCKS: INORGANIC CHEMISTRY
For the purpose of understanding the liberal arts, the primary reason for studying chemistry is that it is the basis for all life. Moreover, chemistry applies not only to plants and animals, but to human beings themselves for chemistry is the basis for the functioning of the human brain, including its thought processes. In order to understand human behavior, we have to study chemistry. Therefore, this chapter is the basis for succeeding chapters on biology, which then opens to the discipline of ecology. Not only is chemistry the basis of all life; all forms of life have very similar arrangements of chemicals. Given the chemical basis of all nature, it is important to know at least a little chemistry.
Considering the seeming instability of the basic model of the atom, with the electron whirling about its nucleus, there is actually a great deal of stability in the universe. This is due to the power of chemical bonding. A brief review of chemical compounds will show that chemical bonding between elements makes the compounds very stable, because they share or trade electrons amongst themselves. This bonding allows the development of more complicated chemical compounds. In fact, these compounds became so complicated that some of them developed the capability of replication, thereby forming the basis of life itself.
Some Basic Chemistry for the Beginner
Atoms with different numbers of protons, neutrons, and electrons make up the 109 elements of the natural world. These elements are the basic constituents of all matter. There are several basic formulas that measure the differences in the make-up of elements that are useful in summarizing knowledge of the various elements. The atomic number is the number of electrons in an element. The number of electrons must equal the number of protons, so the atomic number is also the number of protons. The atomic weight of an element is the number of protons plus the number of neutrons.
Groups of electrons maintain definite average distances from the nucleus, thereby forming shells of electrons. These electrons vibrate around the atom in a pattern of waves. Only certain wave patterns have the right "fit" for the shells around the atomic nucleus. An electron can "jump" to a higher energy wave pattern (a higher shell) if it absorbs a unit of energy (called a photon). It can also jump back by emitting or sending out a photon of light, thereby returning to a lower energy level.
Many other characteristics of the elements are summarized in Mendeleev's periodic table. This is a chart that organizes the elements into eight basic families -- each member of the family having the same number of electrons in its outer shell. Each shell is capable of containing a definite number of electrons, the number increasing as the distance from the nucleus increases. The first four shells go by the names k, l, m, and n. The maximum number of electrons each shell can hold is 2, 8, 18, and 32, respectively.
The eighteen simplest elements have the simplest atomic structures, i.e., they have the lowest atomic numbers. As the atomic numbers increase from one to eighteen, we find the electron is always added in the outermost shell until the shell is filled (see Table 3.1). Then a new shell is started. For example, the element argon, with an atomic number of eighteen, has two electrons in shell k and eight in shell l. This leaves eight electrons, and these are all in the next highest shell, shell m.
Those elements with atomic numbers higher than eighteen do not follow such a simple, orderly filling of shells. For these elements, the second outermost shell does not completely fill up before electrons are added to the outermost shell. Notice in Table 3.2 that shell m does not fill up to its maximum capacity of eighteen before shell n begins to fill.
Table 3.1 Distribution of Electrons
1 1 hydrogen
2 2 helium
3 2 1 lithium
4 2 2 beryllium
5 2 3 boron
6 2 4 carbon
7 2 5 nitrogen
8 2 6 oxygen
9 2 7 flourine
10 2 8 neon
11 2 8 1 sodium
12 2 8 2 magnesium
13 2 8 3 aluminum
14 2 8 4 silicon
15 2 8 5 phosphorus
16 2 8 6 sulfur
17 2 8 7 chlorine
18 2 8 8 argon
Shell k Shell l Shell m Shell n
19 2 8 8 1 potassium
20 2 8 8 2 calcium
21 2 8 9 2 scandium
This characteristic of having unfilled shells before the next outermost shell starts to fill up permits the division of elements into natural groupings. The simplest elements are those with only one unfilled shell, the transition elements are those with two unfilled shells, and the inner transition elements are those with three unfilled shells.
The Formation of Compounds
An understanding of elements builds to an understanding of more complicated chemical substances, known as compounds. A compound is made up of elements which are chemically combined. When elements combine to form compounds, there arises a new set of properties that are unique to the compound itself. The simplest non-repetitive unit of a compound is a molecule of that compound. About five million compounds are known. Indeed, life itself is a collection of compounds closely interacting with one another.
There are a number of simple rules that summarize important features of all compound formations. First, only electrons are involved in chemical changes. The nuclei of atoms are not altered in any way during these changes. Second, all elements are electrically neutral. Therefore, the number of positive protons in the nucleus of an atom must be set off electrically by an equal number of electrons. And, lastly, only the electrons in the outermost shell are affected during chemical change. (In a few cases, the second outermost shell is also affected.)
The formula of a compound is the ratio of the number of atoms of each element present in the compound. For instance, table salt (sodium chloride) is composed of one atom of sodium and one of chloride (hence, NaCl). Methane is composed of one carbon atom and four hydrogen atoms (CH4). Carbon dioxide (CO2) is composed of one carbon and two oxygen atoms. And, finally, acetylene gas (C2H2) is composed of two carbons and two hydrogen atoms.
A compound is held together by chemical bonds of which there are several kinds. But regardless of the kind, they are all based on the tendency of the atom to complete its outer electron shell, and thereby make itself as stable as possible. The different kinds of chemical bonds vary by the process by which the atoms get rid of their incomplete electron shell and become complete. The tendency of elements to form compounds through a shift of electronic structure is known as valence. The valence number of an element is the number of electrons of the element involved in the formation of a compound. There are two basic types of valence: electrovalence and covalence.
The first type of valence, electrovalence, involves ions. An ion is an atom (or group of covalently bonded atoms) that is electrically charged because of an excess or deficiency of electrons. Atoms that tend to lose electrons and thereby become positively charged ions are known as metals. Atoms that gain electrons and become negatively charged ions are nonmetals. A positive and negative ion will exert a mutual electromagnetic attraction for each other. An example is common table salt (NaCl), which is a compound of the metal sodium and the nonmetal chlorine.
A common type of bonding that is abundant in living matter is the type of bonding involving electron transfers. Chemical bonding occurs when the electrons in the outer shell of an atom are shared with or transferred to another atom(s). Remember, only the outer electrons of any atom are free to combine with other atoms in chemical reactions. In order to make itself stable, one atom with an extra electron(s) in its outer shell gives up an electron to another atom that needs an electron(s) to fill its outer shell to make it stable. The resulting compound is held together by the electromagnetic attraction of the opposite positive and negative forces. Covalence is the process of sharing electrons, instead of transferring them. Atoms in covalent compounds share one or more pairs of electrons. There can be single bonds (where one pair of electrons is shared, as in methane, CH4), double bonds (where two pairs of electrons are shared, as in carbon dioxide, CO2), or triple bonds (three pairs of electrons are shared, as in acetylene, C2H2).
Electrovalence involves the actual transfer of electrons from one atom to another. Table salt is an example of an electron of an ion of chlorine (with a negative charge) is transferred to an ion of sodium (with a positive charge).
Now that we know something about chemical compounds, we can build to more complicated compounds. These compounds become so complicated that eventually life arises. Chemistry, however, does not stop there for life develops through chemistry -- the chemistry of genetics being a major component.
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