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NEET2019ChemistryClassification of Elements and Periodicity in PropertiesActual

The correct order of decreasing first ionisation energy is

Options

  1. AC > B > Be > Li
  2. BC > Be > B > Li
  3. CB > C > Be > Li
  4. DBe > Li > B > C

Correct answer

B. C > Be > B > Li

Step-by-step solution

In general as we move from left to right in a period, the ionisation enthalpy increases with increasing atomic number. Be (1 s^2 2 s^2 ) has higher ionisation enthalpy than B (1 s^2 2 s^2 2 p^1 ) because Be has fully filled stable electronic configuration.

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