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Q.
The $+1$ oxidation state of thallium is more stable than its $+3$ oxidation state because of
J & K CETJ & K CET 2013The p-Block Elements
Solution:
Inert pair effect due to this thallium is found commonly in $+1$ oxidation state.
It has electronic configuration as
$[X e] 4 f^{14} 5 d^{10} 6 s^{2} 6 p^{1}$
and shows oxidation states $+3$ and $+1$.