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In transition elements shielding effect is observed . Due to this zinc shows abnormality in atomic size. But at the same time we also say that gallium has same similar size as aluminium due to poor shielding effect of d electrons . Why is it that d electrons have good shielding effect in case of zinc but poor shielding effect in case of gallium?

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  • $\begingroup$ Slater's rule may be used for quantitative reasoning $\endgroup$ – Zenix Mar 26 '20 at 7:02
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When we are talking about zinc... We have to think about period trends. So in periods along moving from left to right covalent radius decreases. But in case of transition metal..

Two effects works first is $Z_{eff}$. Another effect is electronic repulsion.At the starting the no of electron in $d-$ orbital are less so $Z_{eff}$ is dominant over electronic repulsion but as we are moving alosng d-orbital electronic effect become dominant factor over $Z_{eff}$, So in the middle redius become almost constant and at the end it increases.

When we are talking about gallium and Aluminium radius comparision thats mean we are talking about group trends ,So in group trend abnormal size is due to poor shielding effect and relativistic effect. Normal trend on going down the size increases from aluminium to gallium size decreases because here a poor shielding of 3-d orbital present so $Z_{eff}$ penetarate and valence shell contract so size decreases.

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  • $\begingroup$ Why shouldn't electronic repulsions taken into account in case of gallium and aluminium size comparison? $\endgroup$ – Akshat Mar 26 '20 at 13:09
  • $\begingroup$ We can compare electronic repulsion in case of zinc and gallium because as both have d10 orbital. And it more clear when we see radius of gallium is 187pm and radius of zinc is 139 pm. But when we are talking about Aluminum there is no d orbital and galium has. So that's can be a possible explaination because we don't have d orbital in case of aluminum and have d orbital in case of galium so basically we can't compare it. I can just mention Galium radius 187 pm and zinc radius 139 pm and Aluminum radius 184 pm. So we can see difference of radius length of zinc and gallium is to $\endgroup$ – Young Kindaichi Mar 26 '20 at 13:46
  • $\begingroup$ Yes the values are correct they are as expected . But we know that there are electronic repulsions in gallium and they aren't in aluminium . So I am asking that why can't gallium atom be bigger than aluminium because of the above reason . $\endgroup$ – Akshat Mar 27 '20 at 3:15
  • $\begingroup$ I mean gallium should be far more bigger. $\endgroup$ – Akshat Mar 27 '20 at 3:16

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