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

Rajasthan BoardEnglish MediumSTD 11 ScienceChemistry3. Classification of elements and periodicity in properties1 Mark
MCQ
Correct increasing order of first ionistion potential is
  • ✓
    $Na < Mg > Al < Si$
  • B
    $Na < Mg < Al < Si$
  • C
    $Na > Mg > Al > Si$
  • D
    $Na < Mg < Al > Si$
✓

Answer

Correct option: A.
$Na < Mg > Al < Si$
a
As we move across the period, nuclear charge increases, atomic size decreases hence, ionization enthalpy increases. For $Al \left(3 s ^2\, 3 p ^1\right)$, electron has to be removed from partially filled $3 p$ orbital whereas in $Mg \left(3 s ^2\right)$, electron has to be removed from stable fully filled $3 s$ orbital.

Removal of an electron from stable, fully filled orbital requires more energy than removal of electron from partially filled orbital. Thus, ionisation enthalpy for $Mg$ is greater than ionisation enthalpy for $Al$. So, the correct order of first ionization enthalpies is: $Na \,<\, Mg\, >\, Al\, <\, Si$

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