Periodic Law

Periodic trends are specific patterns that are present in the periodic table that illustrate different aspects of a certain element, including its size and its electronic properties. Major periodic trends include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, metallic character, and ionic radius. Periodic trends, arising from the arrangement of the periodic table, provide chemists with an invaluable tool to quickly predict an element's properties. These trends exist because of the similar atomic structure of the elements within their respective group families or periods, and because of the periodic nature of the elements.

These periodic trends are based on the Periodic Law which states that if the chemical elements are listed in order of increasing atomic number, many of their properties go through cyclical changes, with elements of similar properties recurring at intervals. For example, after arranging elements in their increasing atomic numbers, many of the physical and chemical properties of Lithium are recurred into Sodium such as its vigorous reactivity with water, which again recurs in the next cycle starting with Potassium.

This principle was discovered after number of investigations done by scientists in nineteenth century such as Lothar Meyer and Dmitri Mendeleev. Initially, no theoretical explanation for the Periodic Law was available and it was used only as an empirical principle. But, with the development of electronic theory of atomic structure, it became possible to understand the theoretical basis for the Periodic Law. From the modern periodic table, it is evident that the periodic recurrence of elements with similar physical and chemical properties, when the elements are listed in order of increasing atomic number, results directly from the periodic recurrence of similar electronic configurations in the outer shells of respective atoms.

Discovery of Periodic Law constitutes one of the most singularly important events in the history of chemical science. Almost every chemist makes extensive and continued use of Periodic Law. Periodic Law also led to the development of the periodic table, which is widely used nowadays.

The atomic radius is the distance from the atomic nucleus to the outermost stable electron orbital in an atom that is at equilibrium. The atomic radii tend to decrease across a period from left to right. The atomic radius usually increases while going down a group due to the addition of a new energy level (shell). However, atomic radii tend to increase diagonally, since the number of electrons has a larger effect than the sizeable nucleus. For example, lithium (145 picometer) has a smaller atomic radius than magnesium (150 picometer).

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