Atom is the basic or fundamental unit of the any matter in the universe. Size of an atom is too low. Atom is made up of three particles named Electron, proton and Neutron.
Electrons are the extremely lighter than the proton and neutron, that is why proton and neutrons are situated at the centre called nucleus of the atom and electron revolves around it in a circular orbit.
Size of the atom is depends on the position of electrons and how much space the electrons takes, If the electrons are very close to the nucleus, the size of atom will be very small but if the electrons are far away from the nucleus, size will be higher.
Therefore the size of atom depends on the atomic radii.
Atomic size can be calculated by the help of Avogadro’s number along with the atomic mass as well as the density of solid material.
Atomic volume = (molar mass in gm)/(density in gm/cm^3)(Avogadro number).
Example: Compare the atomic size of Li, C and F.
Solution: All three elements belong to the same period. So it depends on the nuclear charge that attracts the electron towards the nucleus. Li has larger atomic size as it has less nuclear charge and hence nucleus pulls the electrons less. However in F, It has large atomic size because of the large nuclear charge which pulls electrons more to the nucleus.