When PbO₂ is converted into Pb(NO₃)₂ valency of Pb changes from 4+ to 2+ since Pb has a variable valency of 2+ and 4+  i.e, the electrons in the outermost shell were 4 at the beginning but now 2 out of 4 electrons have jumped to the sub-shell that lies between the 3rd shell and the 4th shell,therefore only leaving 2 electrons in the outermost shell. NO₃ has the valency 1- and Pb has now a valency of 2+ hence 1 atom of Pb combines with 2 molecules of NO₃,therefore forming Pb(NO₃)₂.The no. of protons in both cases remains the same.

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