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The first ionisation enthalpy (Delta(i)H...

The first ionisation enthalpy `(Delta_(i)H)` values of the third-period elements, Na, Mg and Si are respectively 496, 737 and 786 kJ `mol^(-1)`. Predict whether the first `Delta_(i)H` value for Al will be more close to 575 or 760 kJ `mol^(-1)`. Justify your answer.

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As we go from left to right in a period, first ionisation enthalpy increases. Also ionisation enthalpy of group 2 is higher than group 13 elements. This is because s-electrons have more penetration and less shielding effect. Thus correct order of ionisation enthalpy is
`{:(NaltAlltMgltSi),(496lt?737lt786):}`
So correct option for ionisation enthalpy of Al is 575 kJ `"mol"^(-1)` which is less than 737 kJ `mol^(-1)`
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