The change in atomic radii across Period 3 (from sodium to argon) exhibits a similar trend to Period 2, but there are some key differences to understand:

Similarities:

* Decreasing atomic radius: Both periods show a decrease in atomic radius as you move from left to right. This is due to the increasing nuclear charge (number of protons). The greater attraction between the nucleus and the electrons pulls the electrons closer, shrinking the atom.

* Electrons added to same energy level: In both periods, the electrons are added to the same principal energy level (n=2 for Period 2, n=3 for Period 3). This means the electrons are generally at the same distance from the nucleus, contributing to the overall size trend.

Differences:

* Shielding effect: In Period 3, the increased number of inner shell electrons (electrons in the n=1 and n=2 levels) provide more shielding from the nucleus for the valence electrons (electrons in the n=3 level). This shielding reduces the effective nuclear charge felt by the valence electrons, causing a slightly larger atomic radius in Period 3 than in Period 2 for corresponding elements.

* Electron repulsion: In Period 3, the increased number of electrons in the same energy level leads to greater electron-electron repulsion. This repulsion pushes the electrons further apart, making the atom slightly larger.

Summary:

While the trend of decreasing atomic radius across Periods 2 and 3 is similar, the increased shielding and electron repulsion in Period 3 result in slightly larger atomic radii compared to the corresponding elements in Period 2.

Key takeaway: The periodic trends in atomic radii are driven by a complex interplay of factors, including nuclear charge, shielding effect, and electron-electron repulsion. These factors influence the size of the atom and create distinct trends across periods and down groups.