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Chemistry: Standard Periodic Table

Periodic Table of Chemical Elements

Every substance on earth (or in the known Universe) is either a pure "element," or is a combination of chemical elements that is grouped together in varying amounts and/or combinations - in much the same way as a cake recipe (i.e., 2 parts flour, 1 part sugar, 4 parts water, etc.).

In a chemical "recipe" for a given mineral the chemical elements are expressed by their two letter "atomic symbols," and the quantities of those elements are expressed in a lower-case number that follows. As an example, the "recipe" for the gemstone ruby is 1 part Beryllium, 2 parts Aluminum, and 3 parts Oxygen, which is expressed as BeAl₂O₃.

The "periodic table of chemical elements" is a tabular method of displaying the chemical elements that was created in 1869, by a Russian chemist named Dmitri Mendeleev. In devising the table Mendeleev's intention was to illustrate recurring, or "periodic" trends in the physical properties of the elements. The layout of the periodic table has been refined and extended since its development, as new elements, and methods for describing there behavior have been discovered.

Layout of the Periodic Table

Each element's "atomic number" (the small number in the upper-left corner of each box) expresses the number of protons in their atomic nucleus. Each row is arranged so that elements with similar properties fall into the same vertical columns known as "groups" or "families." The groups consist of alkali metals, alkaline earth metals, halogens and noble gases.

Each horizontal row in the periodic table is referred to as a "period" which corresponds to the filling of a quantum shell of electrons in accordance with quantum mechanical theories of electron configuration within atoms. The horizontal rows (periods) are progressively longer the further down the table you go, grouping the elements into s, p, d and f-blocks to reflect their electron configuration.

All eighteen vertical "groups" in the table are divided into four groupings, or "blocks," which reflect their electron configurations. These four blocks are known as the s-block, d-block, p-block, and f-block.

D-Block Chemical Elements

Groups 3 through 12 consist of the "transition metals," for which the horizontal trends across the 7 periods are often as important as the vertical trends in the 10 groups that comprise the d-block.

P-Block Chemical Elements

Groups 13 through 16 comprise the "non-metals," "metalloids," and "other metals. Nearly every chemical element in the periodic table can be defined as either a "metal" or a "non-metal," but several elements that posses intermediate properties are referred to as "metalloids." These seven metalloids include Antimony, Arsenic, Boron, Germanium, Polonium, Silicon, and Tellurium.

The elements that are referred to as "non-metals" include the gases hydrogen (H), nitrogen (N), and oxygen (O), as well as the halogens and noble gases, but the non-metals also include carbon (C), phosphorus (P), and the chalcogens: sulphur (S), selenium (Se).

Group 17 consist of the "halogens" which are elements that are missing one electron each to fill their shells, so in chemical reactions these elements tend to acquire electrons, called "electronegativity."

Group 18 consist of the "noble gases," which do not need to react with other elements to attain a "full shell" (valence shell, valence electrons), and are therefore much less reactive than other groups.

F-Block Chemical Elements

The actinoid series derives its name from the first element in the series, actinium, and ultimately from the Greek word aktis, or "ray," indicating the radioactivity of 15 these elements. The lanthanoids (lanthanide) are f-block elements, corresponding to the filling of the 4f electron shell, except for lutetium (Lu) which is a d-block lanthanoid.

Bibliography on Birefringence in Gemstones

1. John G. Delly, Michel-Lévy Interference Color Chart . www.modernmicroscopy.com

2. Stanford University, Refraction in Gems . www.stanford.edu

3. Wuerzburg Uniaxial Minerals . www.geographie.uni-wuerzburg.de

4. Stephen A. Nelson Introduction to Uniaxial Minerals . www.tulane.edu

5. Douglas B. Murphy Optical Birefringence . www.micro.magnet.fsu.edu

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