CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 18

Borderline elements: any two of boron, silicon, germanium, arsenic, antimony, tellurium and polonium, for example silicon and germanium. The elements on the right-hand side of the zigzag line are smaller in atomic size than the borderline elements of the same period (in period 3: P $110$ pm, S $104$ pm and Cl $99$ pm, against Si $118$ pm). Reason: across a period the nuclear charge increases by one unit at each step while the electrons are added to the same outermost shell, so the nucleus pulls the electrons closer and the atomic size decreases.