It's all due of the variation in atomic radius. The atomic radius of Tin(Sn) is less than the atomic radius of Lead (Pb).
As a result, more Sn molecules can fit in the same space as Pb molecules (because Pb has high atomic radius). This is why there are more Tin (Sn) molecules in Pb.
What is Atomic Radius?
The atomic radius of a chemical element is an indicator of the size of its atom, generally the mean or normal distance from the center of the nucleus to the outermost solitary electron.
Because the border is not a well-defined physical entity, there exist several non-equivalent definitions of atomic radius.
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