This video lecture from Chemical Bonding (F.Sc. First Year Chemistry) covers the concept of atomic sizes, why we consider the sizes as Radii and why we cannot measure the precise atomic radii. The trend of atomic sizes in the groups and periods of periodic table are also discussed in the video. Find more e-learning material and educational video lectures in Urdu at maktab.pk. These videos are free to use for promotional and commercial purpose by keeping the credits to Maktab.