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I have read answer to this question on various sites, but was not satisfied by the answers, because I am not able to understand, that if we remove initially 1 electron from scandium, then 4s would be ome unstable, but if we remove 1 more electron, there remains only one electron in 3d subshell. But we have already observed in many compounds, that heavier metallic elements and highly electronegative atoms force stability of unstable configurations. Also, I read somewhere that in [Sc (H20)6 ]2+ Scandium possesses +2 oxidation state, but when I tried to research more on it, some sites displayed the same compound but a +3 complex instead of +2. I am confused on all of this. Please throw some light on this, and please explain why Scandium cannot exhibit +2 oxidation state, is it just because of instability of 3d subshell? And what are the reasons for instability of 3d subshell, is it only the presence of a single electron? Please detail about this.

P.S. : It was also mentioned on some sites that after removal of one electron from 4s subshell of Sc, very high amount of energy is required for removal of second electron (i.e. I.E. 2 >> I.E. 1). How is this possible?

TheUnknownOfficial
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The claim that the second ionization energy of scandium far exceeds the first is simply not true, or as least not specifically true of scandium. A quick look at the Wikipedia page reveals that the first ionization energy is 633 kJ/mol and the second is 1235, a ratio of about two to one. This is quite normal for metals with two outer $s$ electrons and the figures are actually lower than those for magnesium. Moreover, the third ionization energy is low enough for that to occur readily (unlike magnesium and all other alkaline earth metals), explaining why +3 is the most common (but not exclusive, as evident from the Wikipedia article) oxidation state for scandium.

Scandium in oxidation states lower than +3 is known primarily in organometallic compounds, where most other transition metals also tend to exhibit lower than "normal" oxidation states. One inorganic compound reported by Wikipedia is $\ce{KScCl3}$, in which the scandium has oxidation state +2 but does not consist of separate $\ce{Sc^{2+}}$ ions. Instead this compound retains scandium-scandium metallic bonding. The $\ce{Sc(H2O)6^{2+}}$ ion suggested in the question is probably a misprint of $\ce{Sc(H2O)6^{3+}}$.

Oscar Lanzi
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  • The +3 oxidation state for s2, p or dx seems to be the sweet spot for high lattice energies and or solvation energies in aqueous solution. As p or d electrons are added +2 becomes more stable. The higher ox. nos. involve increased covalent character and charge feedback giving polyatomic ions. Intermediate oxidation states almost certainly are involved in reaction mechanisms actually involving short lived intermediates. This raises the likelihood of more stable compounds of these. Mn, Cr, N and Cl are good examples of this. – jimchmst Sep 09 '22 at 23:04