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What is the value of electron gain enthalpy of Na+ if IE1 of Na=5.1eV?

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Important Questions on Classification of Elements and Periodicity in Properties

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The 1st  Ionisation enthalpy of Na, Mg and Si are 496, 737, 776 kJ mol-1, respectively then what will be the 1st  ionisation enthalpy of Al in kJ mol-1:
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The species Ar, K+ and Ca2+ contain the same number of electrons. In which order do their radii increase?
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Because of lanthanoid contraction, which of the following pairs of elements have nearly the same atomic radii? (Numbers in the brackets are atomic numbers).
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The formation of the oxide ion O2-(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:

O(g)+e-O-(g); ΔfH=-141 kJmol-1

O-(g)+e-O2-(g); ΔfH=+780 kJmol-1

Thus process of formation of O2- in gas phase is unfavourable even though O2- is isoelectronic with neon. It is due to the fact that