# The first ionisation enthalpies of Na,Mg,Al and Si are in the order: - Chemistry

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The first ionisation enthalpies of $\ce{Na, Mg, Al}$ and $\ce{Si}$ are in the order:

#### Options

• $\ce{Na < Mg > Al < Si}$

• $\ce{Na > Mg > Al > Si}$

• $\ce{Na < Mg < Al < Si}$

• $\ce{Na > Mg > Al < Si}$

#### Solution

$\ce{Na < Mg > Al < Si}$

Explanation:

The electronic configurations of $\ce{Na}$ and $\ce{Mg}$ are:

$\ce{Na}$ (11): $\ce{[Ne]}$ 3s1 and $\ce{Mg}$ (12): $\ce{[Ne]}$ 3sIn both the atoms, the electron is to be removed from 3s-orbital but nuclear charge in $\ce{Na}$ is less than $\ce{Mg}$. Thus, ionisation energy of $\ce{Na}$ is less than $\ce{Mg (Na < Mg)}$.

The electronic configurations of Mg and Al are:

$\ce{Mg: [Ne]}$ 3s2;  $\ce{Al: [Ne]}$ 3s2 3p1

In $\ce{Mg}$, the electron is to be removed from 3s-orbital while in $\ce{Al}$, it is to be removed from 3p-orbital. Since it is easier to remove an electron from 3p-orbital in comparison to 3s-orbital, the ionization enthalpy of $\ce{Mg}$ is higher than $\ce{Al (Mg > Al)}$.

The electronic configurations of $\ce{Al}$ and $\ce{Si}$ are:

$\ce{Al}$ (13): $\ce{[Ne]}$ 3s2 3p1 $\ce{Si}$ (14): $\ce{[Ne]}$ 3s2 3p2

In both the atoms, the electron is to be removed from 3p-orbital but nuclear charge in $\ce{Si}$ is more than $\ce{Al}$.

Thus, ionisation enthalpy of $\ce{Al}$ is less than $\ce{Si}$.

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Chapter 3: Classification of Elements and Periodicity in Properties - Multiple Choice Questions (Type - I) [Page 27]

#### APPEARS IN

NCERT Exemplar Chemistry Class 11
Chapter 3 Classification of Elements and Periodicity in Properties
Multiple Choice Questions (Type - I) | Q 4 | Page 27

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