Periodic table

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The periodic table is an arrangement of chemical elements devised by Antoine Lavoisier in 1789 and revised to its present form by many others, with the most recent revision being made by Dmitri Mendeleev. It is meant to be arranged in a systematic format to illustrate the trends in the properties of the elements. The periodic table is now a vital component of the discipline of chemistry as it provides a framework for the comparison of chemical behaviors.

Arrangement

The earlier arrangement of the elements was done based on the atomic masses of the elements. Mendeleev's periodic table however moved away from this concept as he instead attempted to class the elements according to chemical properties rather than atomic number. This meant that some of the elements were not classed in order of the mass number and that gaps were created for socalled missing elements.

The theory of atomic structure gave definition to Mendeleev's periodic table as it became apparent that the elements had been listed according to increasing atomic number. This is the number of protons in the atomic nucleus. Mendeleev started new rows in his table and elements with similar properties fell into the same vertical columns or groups. It also begame apparent with the modern quantum mechanical theories of electron in atoms that the periods or horizontal rows were synonomous with the filling of a quantum shell of electrons.

Groups and periods

The properties exhibited by the elements in the periodic table vary according to two major directions. These are horizontally or along periods and vertically or along groups. Groups are given names according to the type of elements contained in them. In many groups there are clear trends in properties down the groups. Some groups are the alkali metals, alkali earth metals, transitional metals, halogens and noble gases. Though groups are considered to be the better way to identify trends it is seen that some of the periods similar properties are more significant than in groups.