What is the trend in atomic radius of the elements across Period 3 and why does this occur?

The atomic radius of the elements decreases from sodium to argon. This is because the number of protons increases (sodium has 11, argon has 18) so the nuclear charge increases. Therefore, the attraction between the positive nucleus and negative electrons in the outer shell increases, so the atomic radius (the distance between the nucleus and the outer shell) decreases. The electrons that are added from sodium to argon are all added into the same outer shell, so there is no difference in shielding.