What is atomic radius? Why does atomic radius decrease across a period?
Answer – Atomic radius is defined as the distance from the center of the nucleus of an atom to the outermost shell of electrons. The atomic radius decreases along a period. In moving from left to right across the period, the charge on the nucleus increases by one unit (due to increase in atomic number), but the additional electron goes to the same shell. As a result, outer electrons are pulled in closer to the nucleus. This causes the decrease of atomic size.