Group 13 ionisation enthalpy doesn't decrease smoothly (Ga > Al, Tl > In). Is that the inert pair effect or something else?

7 answers & replies

• [+][–]Tushar PawarDropperSample· 1mo ago

  It's the inert pair effect. Electrons in the s orbital don't get involved, that's why ionisation is higher for Ga and Tl.

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  • [+][–]Arjun Nair12thSample· 1mo ago

    Mehak is right to be suspicious. Inert pair effect is about the oxidation state (the $ns^2$ pair not taking part in bonding, so Tl+ is more stable than Tl3+). It doesn't explain the IE anomaly directly. The anomaly comes from poor shielding.

    Between Al and Ga, the ten 3d electrons are added. d electrons are poor at shielding the nuclear charge. So effective nuclear charge felt by the outer electron goes up more than usual, and Ga is slightly smaller than Al (135 vs 143 pm) and holds its electron a bit tighter. Same story between In and Tl, with the added 4f electrons (and 5d), which shield very poorly (lanthanoid contraction).

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    • [+][–]Tushar PawarDropperSample· 1mo ago

      Okay, makes sense, I was mixing the two. Both effects are real, they just answer different questions.

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    • [+][–]Mehak AroraDropperSample· 1mo agoAuthor

      this is exactly what I needed, "poor shielding for IE, inert pair for oxidation state". Two ideas, two questions. Going to sleep now, thank you 🌙

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• [+][–]Kavya NaikDropperSample· 1mo ago

  The full order from NCERT numbers: B > Tl > Ga > Al > In. Write it as B (801), Tl (589), Ga (579), Al (577), In (558). Examiners love "which is the correct order of IE?" and the tricky parts are Tl high and In lowest.

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• [+][–]Rahul DubeyDropperSample· 1mo ago

  Now connecting your other question. Because the three valence electrons are $ns^2 np^1$, the group's common state is +3. Down the group, +1 gets more stable. That's the inert pair effect: the $ns^2$ pair is held tight and doesn't take part in bonding, so Tl+ is more stable than Tl3+. That's why $Tl^{3+}$ is a good oxidising agent (it wants to go to +1) and $Al^{3+}$ is not.

  For boron, which has just 6 electrons in $BCl_3$ (three bonds, no lone pair), the octet is incomplete, so it accepts a lone pair from something like $NH_3$ and forms $BCl_3 \cdot NH_3$. That is why it's a Lewis acid. Boron's small size also makes it show covalent chemistry only, no $B^{3+}$ ions.

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• [+][–]Aman YadavDropperSample· 1mo ago

  Last point: Al is amphoteric. Reacts with both acid and base: $2Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2$. Examiners like this when comparing with group 13 oxide behaviour: $B_2O_3$ is acidic, $Al_2O_3$ and $Ga_2O_3$ are amphoteric, $In_2O_3$ and $Tl_2O_3$ are basic. Acidic to basic as you go down.

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