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AIIMS 201925 May
Exam
AIIMS
Year
2019
Date
25 May
Subject
Chemistry
Chapter
Classification of Elements and Periodicity in Properties

AIIMS 2019 · 25 May · Chemistry · Classification of Elements and Periodicity in Properties

The correct order of decreasing first ionisation energy is

The correct order of decreasing first ionisation energy is

Options

  1. AC>B>Be>Li\mathrm{C}>\mathrm{B}>\mathrm{Be}>\mathrm{Li}
  2. BC>Be>B>Li\mathrm{C}>\mathrm{Be}>\mathrm{B}>\mathrm{Li}
  3. CB>C>Be>Li\mathrm{B}>\mathrm{C}>\mathrm{Be}>\mathrm{Li}
  4. DBe>Li>B>C\mathrm{Be}>\mathrm{Li}>\mathrm{B}>\mathrm{C}

Correct answer

B. C>Be>B>Li\mathrm{C}>\mathrm{Be}>\mathrm{B}>\mathrm{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⁡(1s22s2)\operatorname{Be}\left(1 s^2 2 s^2\right) has higher ionisation enthalpy than B(1s22s22p1)\mathrm{B}\left(1 s^2 2 s^2 2 p^1\right) because Be has fully filled stable electronic configuration.
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