1. Tardigrade
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  3. Chemistry
  4. The decreasing order of the first ionisation energy (in kJ mol -1 ) of He , Mg and Na is He > Mg > Na. The increasing order of the 2nd ionisation energy (. in .kJ mol -1) of these elements will be

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Q. The decreasing order of the first ionisation energy (in $kJ\, mol ^{-1}$ ) of $He , Mg$ and $Na$ is $He > Mg > Na$. The increasing order of the $2nd$ ionisation energy $\left(\right.$ in $\left.kJ\, mol ^{-1}\right)$ of these elements will be

AMUAMU 2012Classification of Elements and Periodicity in Properties

Solution:

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In case of sodium metal removal of $2^\text{nd}$ electron takes place from $2p$ subshell which is more closer to nucleus arid thus more energy is required to remove the electron, whereas removal of $2^\text{nd}$ electron from magnesium takes place from $3s$ subshell.
$He\xrightarrow[\text{removal of} 1s^2 e^-]{IE_1} \underset{(1s^1)}{He^+} \xrightarrow[\text{removal} 1s^1 e^-]{IE_{2}} \underset{1 s^0}{He^{2+}}$
Noble gases have the highest ionisation energies in their respective periods. In helium, removal of $2^\text{nd}$ electron takes place from $1 s^{1}$ subshell which is nearest to nucleus.
Therefore, the increasing order of $I E_{2}$ (in $kJ\, mol ^{-1}$ ) of these elements is as follows $Mg < Na < He$