How does the size of atoms (atomic size) generally vary in going from left to right in a period of the periodic table? Why does it vary this way?
The size of the atoms decreases on moving from left to right in a periodic table. This happens because the number of electrons and protons also increases and the nucleus of the atom becomes more positively charged. The nucleus exerts greater force of attraction on the electrons and pulls them tightly. As the nucleus pulls the outermost electrons towards it, the size of the atom (or the atomic radius) decreases.