What is the primary reason for the decrease in atomic radius across Period 3?

Confusing Atomic Radius Trends

Students often incorrectly state that atomic radius decreases across Period 3 due to increased electron shielding rather than increased nuclear charge.

To explain the trend in atomic radius across Period 3, use the following reasoning: The atomic radius decreases as you move from sodium (Na) to argon (Ar) because the proton number increases, leading to a greater nuclear charge that pulls the electrons closer to the nucleus, outweighing any effects of electron shielding. Therefore, the atomic radius trend can be summarized as follows: 1. **Formula/Rule**: Atomic radius decreases across a period due to increased nuclear charge. 2. **Substitution**: As you move from Na (11 protons) to Ar (18 protons), the nuclear charge increases from +11 to +18. 3. **Working**: The increased nuclear charge results in a stronger attraction between the nucleus and the electron cloud, leading to a smaller atomic radius. 4. **Answer**: The atomic radius decreases across Period 3. 5. **Units/Conclusion**: The trend is observed in picometers (pm) or nanometers (nm), confirming that atomic radius decreases as you move from left to right across the period.