Justify the position of lanthanides and actinides in the periodic table .

Explanation

(i) Different atoms of the same element having same atomic number but different atomic mass number are called isotopes.
(ii) Isotopes were discovered long time after Mendeleev put forth the periodic table.
(iii) As isotopes have the same chemical properties but different atomic masses, a challenge was posed in placing them in Mendeleev.s periodic table.
(iv) Based on chemical properties, they were to be placed in same group, while based on atomic masses they were to be placed in different groups.
(v) Hence, there was no definite place for isotopes.
(vi) This anomaly arose in Mendeleev.s periodic table because the elements were arranged in an increasing order of their atomic masses.
(vii) In 1913 AD., Henry Moseley revealed that atomic number is a more fundamental property of an element than its atomic mass.
(viii) Hence, he modified Mendeleev.s Periodic Table and arranged the elements in an increasing order of their atomic number in the Modem Periodic Table.
(ix) By doing so, the anomaly of Mendeleev.s Periodic Table regarding isotopes was removed.
(x) Therefore, in the Modem Periodic Table the isotopes could be placed in the same group and the radioactive isotopes were placed in two separate series below at the bottom of the periodic table.