Elements & the Periodic Table:

The periodic table is one of the most fundamental concepts in chemistry. Elements are arranged in it by properties. To show this, let us look first at a large image of the periodic table.

Here, we see the elements arranged according to properties. Also, we see them arranged as "groups" or "families." There are two ways in which these are currently understood:

Groups are listed as 1-18. This method has been suggested as the way to do things but, in some respects, leaves much to be desired.
A way which most chemists still prefer is to list the groups according to "A" and "B."

Type A elements are often called "representative elements." These have quite similar chemistries within a column. In fact, most of what is learned about in elementary chemistry concerns these. To point this out, we note that several of the families have names.

Elements in group 1A (Li, Na, K, Rd, Cs, Fr) are all extremely reactive metals, called the alkali metals; none of these is found in its free state in nature. With only a few (important) exceptions, all these elements have almost identical chemistries.
Elements in group 2A (Be, Mg, Ca, Sr, Ba, Ra) are called the alkaline earths. These are also quite reactive (but there are some differences as we go down the column). The elements Be and Mg are somewhat different in their behaviors vs. the elements Ca-Ra. The latter are much more like each other than are the first two. The first two, Be and Mg, are quite different from the others and from each other.

Be compounds are very deadly poisons. Also, as we shall see, Be tends to form covalent bonds rather than ionic bonds.
Mg compounds are usually nontoxic. The sulfate of magnesium (MgSO₄) is quite soluble in water whereas the sulfates of Ca-Ra are insoluble or only slightly soluble.

Elements in group 6A vary somewhat strangely down their column. O is an extremely reactive non-metal and is in a class by itself. Po is a metal and has a quite simple chemistry (however, it is radioactive which is one of many properties which sets it apart from the others in the group). The three remaining elements in this family--S, Se, and Te--are called the chalcogens. They have similar chemistries. S is essential to life and is an important component of proteins. Trace amounts of Se are also essential (for instance, Se helps prevent the formation of cataracts in your eye as you age). Te, on the other hand, seems to be useless for life forms.
Elements in group 7A are called halogens. These are all highly reactive and corrosive nonmetals. F and Cl are gases. Br is a liquid at room temperature. I is a solid. (Little need be said about At since it is highly radioactive and does not persist long enough to be of any chemical importance.) F has a very simple chemistry, however, compared to its siblings. Cl, Br, and I can bond in ways much more complex than are available to F. More will be said about these things later.
One thing will be noted now: The chemistries of the elements in the second row of the periodic table (Li-Ne) vary much more from their family "norms" than do the elements in the succeeding rows. For instance, in group 4A, C stands out by itself. It can form a much wider variety of compounds than can Si, Ge, Sn, or Pb. As we shall see later, this is because C can form double bonds and triple bonds whereas Si-Pb cannot. Another example is a comparison of H₂O and H₂S. The former is "water," a relatively high-boiling liquid of extreme importance to living systems. The latter is a stinky, smelly, and extremely toxic gas. The two other compounds we could form in that family--H₂Se and H₂Te are, if anything, even more objectionable than H₂S!

Type B elements are all metals and are most often called the transition metals. There are similarities among members of each column. Also, according to the numbering, there are some similarities among these and their type-A counterparts (at least for the lower numberings). For instance, the 1B metals (Cu, Ag, and Au) have some similarities to the 1A metals. Also, the 2B metals (Zn, Cd, Hg) have some things in common with the 2B metals. Finally, 3B metals (Sc, Y, and all the lanthanides and actinides) have much in their chemistries similar to the 3A element, Al. After this, however, we are really stretching things if we try to compare elements in the families 4B-8B with their "A" counterparts. In fact, things get almost out of hand when we note that three columns of the periodic table encompass the 8B elements. These three sets, Fe-Co-Ni, Ru-Rh-Pd, and Os-Ir-Pt are often referred to as the "triads." There is a lot of interesting chemistry which results when one compares what goes on with the 8B elements both in their rows and in their columns.
Two elements have not been mentioned. These are H and He. H really is in a class by itself. Nominally, it is put in group 1A by many people but, obviously, it is most definitely different in its chemistry than Li-Fr! He, on the other hand, fits well into group 8A. These are quite unreactive and are termed the noble gases. He, Ne, and Ar form no known chemical compounds. Kr, Xe, and Rn do form compounds--but only reluctantly (in fact, it was not until the early 1960's that compounds of these latter elements were synthesized).

One other thing to note:

We now discuss one last thing of great importance. First, let us look at the periodic table again:

Here, we put forth a smaller picture which stresses colors.

The green elements are metals. That is, they share properties which we commonly call metallic. Most of these are "silvery" in sheen. (The only major exceptions here are Cu and Au--copper and gold--which have quite different colors to them.)

The purple elements are nonmetals. These lack a sheen or luster and come in quite varied colors. For instance, F (fluorine) is a yellow gas--and it is actually "F₂" in composition--whereas Ar (argon) is a colorless gas. S (sulfur, occurring as S₈) is a yellow solid and Br (occurring as Br₂) is a red-orange liquid.

The sky-blue elements are metalloids or semi-metals. In their free state, they look like metals but are much more brittle. Their chemistries are, however, somewhat nonmetallic.

The above discussion shows just the tip of the "chemical iceberg" as far as the subtleties of the periodic table are concerned. In the next document, we shall discuss some aspects of things in terms of the first set of assigned problems to this course.