21. State modern periodic law. What is the cause of periodicity in the propertiesof elements? Explain it with the help of an example.

21. State modern periodic law. What is the cause of periodicity in the

1.	21. State modern periodic law. What is the cause of periodicity in the propertiesof elements? Explain it with the help of an example.
Answer» <p>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. Further an examination of the electronic configuration of the various elements clearly indicates that with a gradual increase in atomic number there occurs a repetition of similar outer electronic configurations after certain regular intervals. By correlating these two observations, we can say that, the cause of periodicity in properties is the repetition of similar outer electronic configurations at certain regular intervals.WHEN we go across the period the no. of shells remains same but the number of electrons keep increasing .this result in more force of attraction among electrons and protons and thus atomic size decreases.but when we go down the group, the no. of valence electrons remains same but the no. of shells kkep increasing.this result in less force of attraction.therefore, atomic size increases.</p>