Periodic table of chemical elements (PTE)
History1864 the English chemist John Alexander Reina Newlands arranged the chemical elements known at that time in order of ascending atomic weight. He found out, that similar chemical properties recurred at intervals of eight, for which reason he named his discovery the "law of octaves".
Arranging the chemical elements in order of ascending atomic weight and building groups of similar chemical properties was the ambition of the chemists Dmitri Mendelejew and Lothar Meyer in 1869, too. The table they created started with a new row when the characteristics of the elements began to repeat and they used gaps when it seemed that the corresponding element had not yet been discovered. For this reason they were able to predict the properties of the undiscovered elements.
While the atomic models became more and more precise, the ranking was done by the proton number instead of the atomic weight. Mendelejew and Meyer had already swapped the places of some elements to keep the intervals of the chemical properties of their table. The atomic weight of some elements is higher than those of atoms with a lower proton number because of the higher number of neutrons inside the atomic nucleus.
With the development of modern quantum mechanical theories it was possible to give an explanation for the similarities of some chemical elements. The chemical properties of a substance (chemical element, ion or molecule) are caused primarily by the valence electrons (outermost electrons). For this reason chemical elements with the same number of valence electrons are grouped together.
Principles of arrangementLike mentioned before, elements are grouped because of the similarities of chemical properties. A very eye-catching group of elements is those of the noble gases. Elements with a high resistance to corrosion, meaning they do not react with water and oxygen, were called to be noble early in the history of chemistry. Well known are the noble metals like silver, platinum and gold. Noble gases are not only very resistant to corrosion, they have generally a very little tendency in participating in chemical reactions. Only a few noble gas compounds could be prepared up to the present. An explanation for the special behavior of noble gases was given with the development of the shell model of the electron configuration. The outer shell of valence electrons is considered to be filled with the maximum number of electrons possible. Besides the fact that noble gases have a very little tendency to participate in chemical reactions, the theories of atomic structure are able to explain why other elements create chemical bondings: Chemical bondings make it possible for the participating atoms to reach the very stable electron configuration of the noble gases. Shells containing just a few electrons can be "cleared" by emitting the electrons, whereby the electron configuration of the remaining ion consists only of completely filled shells. Other atoms try to get electrons from their surroundings to fill their shells with the maximum number of electrons. By creating a covalent bond, both participating atoms can get the maximum number of electrons inside their shells by sharing the electrons. The number of valence electrons defines the number of electrons having to be emitted, absorbed or shared and for this reason the number of chemical bondings is given.
Construction principlesAt the rows of the periodic table, the proton number is increased by one at each cell from the left to the right, by what the number of electrons in the atomic shell is increasing in the same way. Based on the scheme discussed at the chapter atoms, a new row is starting, whenever the s-subshell of the next shell is filled with the first electron. A table row is called period. A table column, listing elements with the same number of electrons in the accordant subshell, is called group.
Traditionally the elements of the eight main groups are labeled by Roman numerals (I - VIII). At these groups the s- and p-.subshells are "filled" with electrons from the left to the right. The elements of the first main group are called alkali metals, named after the Arabic word for potash, the substance potassium was formerly extracted from. The elements of the second main group are called alkaline earth metals, because they are placed between the alkali metals and the earth metals of the third main group. The elements of the third main group aren't called earth metals but boron family nowadays. The elements of the seventh main group are called halogens (=forming salt), because they produce a salt when forming a compound with a metal.
Today the columns of the periodic table are labeled by using Arabic numerals (1-18), following the rules of the International Union of Pure and Applied Chemistry (IUPAC).
According to the "aufbau principle" of the electron shell, after the second main group of the fourth period, the "filling" of the d-subshell of the fourth shell starts. The maximum number of electrons in the d-subshell is 10, which is why after the 10th element (Gallium, Ga) the filling of the p-subshell of the third shell starts (main group number III). The elements of this "insertion" are called transition elements.
Further "insertions" can be located at the 6st and 7th period, where at the f-subshell gets filled with at most 14 electrons. The inner-transition elements of the 6st period are called Lanthanides and those of the 7th period are called Actinides.
Important is the type of bonding between atoms of an element, hence it is often marked in terms of different colors. It is differentiated between metals, half-metals and nonmetals. Elements forming metallic bonding are good conductors of electricity. Nonmetals are insulators and they are forming covalent bondings among each other. Half-metals can act like a conductor as well as an insulator and can't be clearly allocated to the group of metals or nonmetals. Further explanations about those materials will be given at the chapters about semiconductors. An exceptional position have the noble gases which do not form any bonding among each other but exist as single atoms.
Extract of the periodic table