Atom

Atom is the smallest practical of an element ,which can take part in a chemical reaction.

Example: He and Ne have atoms, which have independently existence while atoms of Hydrogen ,nitrogen do not exist independently.

Size of Atom : An ordinary optical microscope can measure the size of an object upto 500nm .

Diameter of Atom : X_ray work has shown that the diameter of atoms are 0.2nm

Mass of Atom : Mass of atom is 1.661*10-27 kg.