[The Scientist (OP)]

Please elaborate upon explanation. Thanks.

[Bored chemist]

It depends.
Some substances like iodine have only one stable isotope so all the atoms in a sample of pure iodine are identical.
Tin, on the other hand has a hat full of different isotopes (ten iirc) so a pure tin sample would contain 10 different sorts of atoms.
In practice you will never get a pure sample of a material.