Periodic table

Any systematic study must be based on a good system of classification in chemistry, classification of the elements began with lavoisier, whose classification was essentially into metals and nonmetals. It was the Russian scientists, Dmitri mendeleev who, between 1869-71,developed the periodic law.
The law states that the properties of the elements varied periodically with their relative atomic masses. He arrange the elements in increasing order relative atomic mass.
Mendeleev periodic table had many gaps. he predicted the properties of some of these missing elements, later elements such as germanium, gallium, scandium were discovered, which not only fitted exactly into mendeleev periodic table, but also had the predicted properties.
Therefore, the modern periodic law states that the physical and chemical properties of the elements and periodic function of atomic numbers.
The modern periodic table is divided into eight vertical columns known as groups and Seven horizontal columns known as periods.
Group : the group are numbered from 0 to 7 and constitute element of similar chemical properties. element in the same group have the same number of electrons in the outer most shell of their atoms ,they have the same number of valence electrons. In between group 2 and 3 lie the transistion element
Period : the period consist of those element that are arranged in the horizontal row in the periodic table and are numbered 1 to 7. Element in the same period have the same number of electrons shells, which correspond to the period, elements of period 2 have two electrons shell, those of period 3 have three shells and so on.
The number of valence electrons of the elements in the same period increase progressively by one across the period from left to right. Hence, the properties change from element to elements, example the metallic properties fade and are gradually replaced by non-metalic properties.