Atomic size gradually decreases from left to right across the periodic table because within a period of elements, all electrons are added to the same shell. However, at the same time, protons are being added to the nucleus, making it more positively charged. The effect of increasing proton number is greater than that of the increasing electron number. Therefore, there is a stronger nuclear attraction. This means that the nucleus attracts the electrons more strongly, pulling the atom's electron shells closer to the nucleus. As a result, the atomic diameter of the atom decreases.
