Answer: The atomic size decreases from left to right within a horizontal row of the periodic table due to an increase in nuclear charge and the addition of electrons in the same shell.
Explanation: The atomic size generally decreases from left to right within a horizontal row (period) of the periodic table. This variation arises because, on moving from left to right in a period, the nuclear charge gradually increases by one unit and one electron is also added in the electron shell. Due to this, the electrons get attracted more and more towards the nucleus, consequently, the atomic radii decrease.