The elements

The scientists classified the elements according to their properties to facilitate their study and to find a relationship between the elements ,  and their chemical and physical properties .

Mendeleev’s periodic table

There were only 67 elements that were known in Mendeleev’s periodic table , Mendeleev classified the elements in an ascending order according to their atomic weights .

Mendeleev left gaps in his table predicting the discovery of the new elements , And he corrected the atomic weights of some elements which were estimated wrongly .

But Mendeleev had to make a disturbance in the ascending order of the atomic weights of some elements to put them in the  groups that suit their properties .

Mendeleev had to deal with the isotopes of one element as different elements due to the difference in their atomic weights . So , He had to put more than one element in one cell of his table .

Moseley’s  periodic table

Moseley classified the elements in an ascending order according to their atomic numbers , He added zero group which includes the inert gases to the table , And he specified a place below the table for lanthanides and actinides elements .

The modern periodic table

There are 116 elements in the modern periodic table .

The elements are classified in an ascending order according to their atomic numbers , And the way of filling the energy sublevels  with the electrons .

The number of the known elements in the modern periodic table till now is 116 elements , 92 elements are available in the Earth’s crust , while the rest are prepared artificially .

There are 7 horizontal periods , and 16 vertical groups ( 18 vertical columns ) , And the elements are classified into 4 blocks ( s , p , d and f ) .

The elements of the same group are similar in the number of the electrons in the outermost energy level . So , they are similar in their chemical properties , And they are different in the number of the energy levels  occupied by the electrons .

The elements of the same period are different in the number of the outermost electrons .  So , they differ in their chemical properties . And they are similar in the number of the energy levels occupied by the electrons .