In terms of electronic configurations, explain the variation in the size of the atoms of the elements belonging to the same period and same group ?
The size of the atom increases on moving from top to bottom in the same group because at every step, new shells of electrons are added.
In a period, the size of the atom decreases on moving from left to right. This is because the atomic number of the elements increases on moving from left to right. Thereby, the number of electrons and protons also increases. Due to the increment in the number of protons, the nuclear charge also increases and the electrons get held more tightly to the nucleus.