Chemistry · Periodic Classification Of Properties · NEET
The general rule is: down a group, size increases and IE decreases, because the outer electron is farther from the nucleus and more shielded. This works from B to Al (IE falls: B ~801, Al ~577 kJ/mol). But after Al, the pattern breaks. Ga comes right after the 3d transition metals, and In/Tl come after d (and Tl after 4f) blocks. These d and f electrons are added into inner-ish shells and shield the nucleus very poorly. So the effective nuclear charge (Zeff) on the outer electron stays high, and IE does not drop the way the simple rule predicts.
Aluminium: [Ne] 3s² 3p¹. Gallium: [Ar] 3d¹⁰ 4s² 4p¹. Going from Al to Ga we cross the entire 3d series (10 extra electrons in 3d). The 3d electrons shield the outer 4p electron very poorly. Because of this, the electrons feel a larger effective nuclear charge in Ga, its size does not increase much (Ga is even a bit smaller than Al), so it is NOT easier to remove the electron. Result: IE of Ga (~579 kJ/mol) is about the same as, or slightly greater than, IE of Al (~577 kJ/mol), instead of being clearly lower.
Thallium: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. Between In and Tl we cross both the 4f (lanthanoid) series and the 5d series. The 4f electrons shield the nucleus extremely poorly (this is the lanthanoid contraction) and 5d electrons also shield weakly. So the 6p electron of Tl feels a strong nuclear pull, effective nuclear charge is high, and its IE (~589 kJ/mol) is greater than that of In (~558 kJ/mol). This breaks the smooth 'IE decreases down group' expectation.
The observed first ionization enthalpies (kJ/mol) are roughly: B ~801 > Tl ~589 ≈ Ga ~579 ≈ Al ~577 > In ~558. So the trend is NOT a simple top-to-bottom decrease. B is clearly highest. Then instead of a steady fall, Al, Ga, In, Tl bunch together with Ga and Tl slightly raised. Simple exam version to remember: IE falls B → Al, but from Al onwards it does NOT decrease smoothly because of d and f contraction.
Shielding means inner electrons block the outer electron from feeling the full positive charge of the nucleus. s and p electrons are close to the nucleus and shield well. But d and (especially) f electrons have diffuse, poorly-penetrating shapes, so they do not stand between the nucleus and the outer electron effectively. Because they shield poorly, the outer electron feels a higher effective nuclear charge (Zeff), it is held more tightly, and the ionization enthalpy stays high. This is exactly why Ga (after 3d) and Tl (after 4f + 5d) do not follow the normal decreasing trend.
No — be careful, these are different anomalies. B < Be and N > O are anomalies ACROSS a period (period 2), caused by stable filled 2s² (Be) and stable half-filled 2p³ (N) configurations. The Group 13 anomaly discussed here is DOWN a group, caused by poor shielding of d and f electrons (d-contraction and lanthanoid contraction). NEET mixes these, so know which direction the question is asking.
Which of the following statements are true? A. Unlike Ga, that has a very high melting point, Cs has a very low melting point. B. On the Pauling scale, the electronegativity values of N and Cl are not the same. C. Ar, K⁺, Cl⁻, Ca²⁺ and S²⁻ are all isoelectronic species. D. The correct order of first ionization enthalpies of Na, Mg, Al and Si is Si > Al > Mg > Na. E. The atomic radius of Cs is greater than that of Li and Rb.
The correct order of atomic radii in group 13 elements is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because Ga comes after the 3d block and Tl after the 4f + 5d blocks, and d and f electrons shield the nucleus poorly, so the outer electron feels a strong pull and IE stays high (Ga ≈ Al, Tl > In).
Boron (B), about 801 kJ/mol. It is at the top and smallest, so its outer electron is closest to the nucleus and hardest to remove.
Indium (In), about 558 kJ/mol. Note it is In, not Tl, because Tl's IE is raised by lanthanoid contraction and poor 4f/5d shielding.
Yes. Ga follows the 3d transition series (d-block / scandide contraction). Tl follows the 4f lanthanoids plus 5d, so both lanthanoid contraction and poor d-shielding raise its ionization enthalpy above indium.
Yes — NEET tests it as ordering questions (like NEET 2025 with Al dipping below Mg, and NEET 2018 on Group 13 atomic radii). You must know that after Al the trend goes flat/wavy, not steadily down.
