Mendeleev’s periodic law | مصادر الكيمياء

Mendeleev’s periodic law



Mendeleev’s periodic law

Mendeleev gave a periodic law in 1869. According to Mendeleev’s periodic law, the properties, of elements are periodic functions of their atomic masses. He found that if the elements are arranged in the order of increasing atomic masses, then the properties of elements are repeated after regular intervals or periods.

Important features of Mendeleev’s periodic table:

1 – The table is divided vertically into 8 columns known as groups and horizontally into rows known as periods.

2 – Each group of the table is further sub-divided into two .sub-groups A and B. The resemblances of the properties of the elements within a sub-group one more marked than those between members of the two sub-groups.

Defects of the Mendelen’s perioclic table

1 –  No position for isotopes are available, for example two isotopes of chlorine 35Cl and 37Cl should have different places in the table.

2 –  The position of hydrogen is not properly defined .Hydrogen is placed in IB group, whereas it resemble the elements of group IA (alkali metals) as well as the elements of group VIIA (halogens).

3 –  The elements of higher atomic mass is placed before the clement of lower atomic mass.

4 – Some chemically dissimilar elements arc placed in the same group. For example the noble metals copper (Cu), silver (Ag) and gold (Au) arc placed in group 1 along . with alkali metals (Li, Na, K, Pb, Cs etc). Their properties arc different.



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