why exact size of an atom can not be determined??
An atom is not a solid object with a well-defined outer boundary. It consists of a small, dense nucleus surrounded by a "cloud" of electrons. Most of the volume of the electron cloud is empty space. We can estimate a radius for the electron cloud but due to the Heisenberg Uncertainty Principle, a specific location for each electron cannot be measured. Hence the atomic radius is just an approximation and not an exact measurement.