Modern periodic law states that physical and chemical properties of the elements are periodic function of the atomic numbers i.e. if the elements are arranged in order of their increasing atomic numbers, the elements with similar properties are repeated after certain regular intervals.
Atomic mass depends upon the number of protons and neutrons in the nucleus whereas atomic number implies the number of electrons in the extranuclear part or the number of protons in the nucleus. Since the nucleus is deep seated in an atom and is also shielded by electrons in the extra nuclear part so atomic mass has little effect on the chemical properties of the elements. Electrons are exposed to the environment and hence can interact with the reagent. As a result, the physical and chemical properties of the elements depend upon their atomic numbers rather than atomic masses. The physical and chemical properties of elements could be different depending upon the number of electrons and the electronic configuration in any atoms.
Cause of periodicity
According to the Modern periodic law, the properties of the elements are repeated after certain regular intervals when these elements are arranged in order of their increasing atomic numbers.
The cause of periodicity in properties is the repetition of similar outer electronic configuration after certain regular intervals.
All the elements of group A or 1 i.e. alkali metals have the similar outer electronic configuration, i.e. ns1 where n refers to the number of outermost principal shells. It is because of similarity in electronic configuration that all the elements have similar properties.
All the halogens i.e. elements of group A or 17 have similar outer electronic configuration i.e. ns2 np6 and hence possesses similar properties.
All the elements of group 0 or 18 have similar electronic configuration, i.e ns2 np5 and hence posses similar properties.
Long Form of Periodic Table
The table which is based upon the electronic configuration of the elements is called the present form or the long form of periodic table.
The long form of periodic table consists of 18 vertical columns and 7 horizontal rows. These have been obtained by arranging the elements in order of their increasing atomic numbers in such a way that the elements with similar electronic configuration are placed under each other in the same vertical column. These vertical columns are known as groups while the horizontal rows are called periods.
1) The long form of the periodic table consists of 18 vertical columns called groups.
2) Each group consist of a number of elements having the same electronic configuration of the outermost shell.
3) These groups are numbered from 1 to 18.
1)The long form of periodic table consists of 7 periods.
2)These are numbered as 1,2,3,4,5,6 and 7 from top to bottom.
3)The first period consists of two elements.
4)The 2nd and 3rd period consists of 8 elements each, 4th and 5th consists of 18 elements each while the sixth period consists of 32 elements.
5)The 7th period is incomplete and like the 6th period would have a maximum of 32 electrons.
In the long form of the periodic table ,14 elements of 6th period called the lanthanoids and 14 elements of the seventh period called the actinoids are placed in separate panel at the bottom.