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AP EAMCET2009ChemistryClassification of Elements and Periodicity in Properties

Which one of the following order is correct for the first ionisation energies of the elements?

Options

  1. AB < Be < N < O
  2. BBe < B < N < O
  3. CB < Be < O < N
  4. DB < O < Be < N

Correct answer

C. B < Be < O < N

Step-by-step solution

First ionisation energy increases in a period. Thus, the first IE of the elements of the second period should be as follows Be < B < N < O But in practice, the elements do not follow the above order. The first IE of these elements is B < Be < O < N The lower IE of B than that of Be is because in B (1 s^2, 2 s^2 2 p^1 ) , electron is to be removed from 2 p which is easy while in Be (1 s^2, 2 s^2 ) , electron is to be removed from 2 s which is difficult. The low IE of O than that of N is because of the half-filled 2

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