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Structure of the periodic table
The primary determinant of an element's chemical properties is its electron configuration, particularly the valence shell electrons.
For instance, any atoms with four valence electrons occupying p orbitals will exhibit some similarity.
The type of orbital in which the atom's outermost electrons reside determines the "block" to which it belongs.
The number of valence shell electrons determines the family, or group, to which the element belongs.
The total number of electron shells an atom has determines the period to which it belongs.
Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order (the Aufbau principle):
Subshell:
S
G
F
D
P
Period
1
1s
2
2s
2p
3
3s
3p
4
4s
3d
4p
5
5s
4d
5p
6
6s
4f
5d
6p
7
7s
5f
6d
7p
8
8s
5g
6f
7d
8p
Hence the structure of the table. Since the outermost electrons determine chemical properties, those with the same number of valence electrons are grouped together.
Progressing through a group from lightest element to heaviest element, the outer-shell electrons (those most readily accessible for participation in chemical reactions) are all in the same type of orbital, with a similar shape, but with increasingly higher energy and average distance from the nucleus.
For instance, the outer-shell (or "valence") electrons of the first group, headed by hydrogen, all have one electron in an s orbital. In hydrogen, that s orbital is in the lowest possible energy state of any atom, the first-shell orbital (and represented by hydrogen's position in the first period of the table).
In francium, the heaviest element of the group, the outer-shell electron is in the seventh-shell orbital, significantly further out on average from the nucleus than those electrons filling all the shells below it in energy. As another example, both carbon and lead have four electrons in their outer shell orbitals.
Note that as atomic number (i.e. charge on the atomic nucleus) increases, this leads to greater spin-orbit coupling between the nucleus and the electrons, reducing the validity of the quantum mechanical orbital approximation model, which considers each atomic orbital as a separate entity.
Because of the importance of the outermost shell, the different regions of the periodic table are sometimes referred to as periodic table blocks, named according to the sub-shell in which the "last" electron resides, e.g. the s-block, the p-block, the d-block, etc.
Regarding the elements Ununbium, ununtrium, ununquadium, etc., they are elements that have been discovered, but so far have not been named.
The periodic table of the chemical elements is a tabular method of displaying the chemical elements.
Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869.
Mendeleev intended the table to illustrate recurring ('periodic') trends in the properties of the elements.
The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.
The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior.
The table has also found wide application in physics, biology, engineering, and industry. The current standard table contains 117 confirmed elements as of October 16, 2006 (while element 118 has been synthesized, element 117 has not).
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