Give reasons for the following:
Occurrence of characteristic properties of elements takes place at definite intervals in the modern periodic table.

DEAR STUDENT,

The occurrence of characteristic properties of elements takes place at definite intervals in the modern periodic table. This is because:
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.For example:

When we go across the period the no. of shells remains the same but the number of electrons keeps increasing .this results 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 keeps on increasing. this result in less force of attraction.therefore, atomic size increases.

REGARDS,

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