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  3. Chemistry
  4. In the 3rd period, the first ionisation potential is of the order

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Q. In the $3^{rd}$ period, the first ionisation potential is of the order

JIPMERJIPMER 2014Classification of Elements and Periodicity in Properties

Solution:

Ionisation potential increases across a period with increasing atomic number. But $ Mg \, (1s^22s^22p^63s^2)$ has higher ionisation enthalpy than $Al (1s^22s^22p^63s^23p^1)$ due to stable fully filled orbitals of magnesium. Thus, the correct order is :
1st $I.E (kJmol^{-1}) : \underset{496}{ Na }<\underset{577}{ Al }<\underset{737}{ Mg }<\underset{786}{ Si }<\underset{1011}{ P }$