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I was studying about the periodic table recently, and was reading a topic associated with oxides of halogens, and came across the following line

The bromine oxides, $\ce{Br2O}$, $\ce{BrO2}$, $\ce{BrO3}$ are the least stable halogen oxides (Middle row anomaly) and exist only at low temperatures. They are very powerful oxidizing agents.

So, I went to search this on Google, and found these lines

It refers to the instability of oxides of bromine as compared to relative stability of oxides of chlorine and Iodine at room temperature, the former being stable only at low temperatures.

But the line above is simply restating what the book had already told. So, is their any specific reason why this happens, or is this only due to the experimental data we have gathered?

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  • $\begingroup$ According to wikipedia diiodine mono-oxide is very unstable and dibromine monoxide is stable below -40 C. So, which is more stable? $\endgroup$
    – Apurvium
    Commented Jun 25, 2022 at 5:36

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This is due to the transition metal contraction.

Bromine has the electron configuration $\ce{[Ar] 4s^{2} 3d^{10} 4p^{5}}. $The 3d orbital has no radial nodes and is therefore quite contracted (close to the nucleus), so there is relatively little repulsion between the 3d electrons and the 4p electrons. This makes it much harder to acheive high oxidation states of bromine (I, IV, VI for the examples you give) because the ionization energies are higher than you might expect from a simplistic approach to periodic trends.

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    $\begingroup$ Why doesn't that argument apply to iodine as well? $\endgroup$
    – ron
    Commented May 9, 2015 at 17:56
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    $\begingroup$ @ron he is correct, it does not apply for iodine as 4d orbitals have 1 radial node, while 3d does not have them. $\endgroup$
    – Rajat Jain
    Commented May 9, 2015 at 23:38
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    $\begingroup$ Your explanation to me seems equivalent. What is shielding other than electron-electron repulsion ? $\endgroup$
    – J. LS
    Commented May 10, 2015 at 11:28
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    $\begingroup$ @orthocresol, If I ask this as a new question, it would be regarded as duplicate. $\endgroup$
    – Nilay Ghosh
    Commented Oct 30, 2015 at 16:32
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    $\begingroup$ @NilayGhosh It would only be marked as a duplicate if there is another question that specifically lists manifestations of the alternation effect, or middle row anomaly, or whatever you want to call it. This question does not deal with examples of the phenomenon, but rather the reason behind it. $\endgroup$
    – orthocresol
    Commented Oct 30, 2015 at 17:10

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