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Why gallium has higher ionisation enthalpy than aluminium

Dear Student,
In gallium, due to poor shielding of valence electrons by the intervening 3d electrons.In  Ga, the 10 electrons present in 3d-subshell do not shield the outer electrons from the nucleus effectively. The nuclear charge becomes effective, thus, atomic radius decreases and hence, the ionisation enthalpy of gallium is higher than that of aluminium.
  • As a result effective nuclear charge in Ga increases.
  • So Ionisation energy  of Ga is slightly more than that of Al.(Aluminium)

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Gallium -Ga -outermost valence shell electronic configuration-4s?4p?
Aluminum-Al-outermost valence shell electronic configuration-3s?3p?
?Before valence shell configuration ,Ga 10 electrons are filled in d-orbitals which has poor shielding effect. so effective nuclear charge will increase and hence Ga will be smaller in size than Al.
As nuclear charge is more, it is difficult to remove an electron from outermost shell of Ga, hence ionization energy is more than Al.
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If you look at the periodic table you can see that Ga comes after Zn that is Ga follows the the 3d series . therefore it is due to the poor shielding provided by the d orbital electrons ( order of shielding is s>p>d>f) therefore the outer electrons experience more effective nuclear charge than aluminium and Boron therefore Ga has greater IE value than Al.
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