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Question

The first ionization enthalpies of Na, Mg, Al and Si are in the order:
  1. Na<Mg>Al<Si
  2. Na>Mg>Al>Si
  3. Si>Al>Mg>Na
  4. Na>Al>Si>Mg

A
Na>Mg>Al>Si
B
Na<Mg>Al<Si
C
Si>Al>Mg>Na
D
Na>Al>Si>Mg
Solution
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As we move across the period, nuclear charge increases, atomic size decreases hence, ionization enthalpy increases. For Al (3s23p1 ), electron has to be removed from partially filled 3p orbital whereas in Mg(3s2), electron has to be removed from stable fully filled 3s 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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