# How does ionic radius change across the periodic table?

You know that ATOMIC size decreases across the Period, from left to right as we face the Table. Why? Well, atomic size is a function of (i) the nuclear charge $Z$, and (ii) shielding by other electrons. Across the Period, where the electrons fill the SAME valence shell, nuclear charge wins in that incomplete electronic shells shield the nuclear charge very poorly, and atomic size decreases across the Period in the given direction.
Units are in $\text{pm} \equiv 1 \times {10}^{-} 12 \cdot m$ (not an ideal unit, but it will have to do). And do the data agree with the given argument?