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3. Classification of elements and periodicity in properties — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry3. Classification of elements and periodicity in properties1 Mark
MCQ
The correct decreasing order of first ionisation enthalpies of five elements of the second period is
  • A
    $Be > B > C > N > F$
  • B
    $N > F > C > B > Be$
  • ✓
    $F > N > C > Be > B$
  • D
    $N > F > B > C > Be$
✓

Answer

Correct option: C.
$F > N > C > Be > B$
c
As we move along the period, the atomic size decreases due to increase in nuclear charge. Therefore, it is more difficult to remove electron from an atom. Hence the sequence of first ionization enthalpy in decreasing order is $F > N > C > Be > B$ But ionization enthalpy of boron is less as compared to beryllium because first electron in boron is to be removed from $p-$ orbital while in beryllium, it is to be removed from $s-$ orbital. As $s-$ orbital is closer to nucleus in comparison to $p-$ orbital thus energy required to remove an electron from $s-$ orbital is greater.

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