In my chemistry lab we decomposed lead (iv) oxide by heating it and then placing a wooden splint(test the evloved gas) into the mouth of the test tube. The color changed into red lead ( Pb304).Then we reheated the sample again and it turned to PbO. I do not understand why the compound accumulated more atoms and then lost atoms. PbO2---->Pb304----->PbO

Answer 1

The two-step decomposition you described is:
(a) PbO2 --> Pb3O4 + O2
Balancing it,
3PbO2 ---> Pb3O4 + O2

(b) Pb3O4 ---> PbO + O2
or, balanced,
2Pb3O4 ---> 6PbO + O2

In each step, all atoms are accounted for. However the first step must be doubled to reconcile it with the 2nd step.