Chemistry · Classification of Elements and Periodicity in Properties

Consider the following atomic radii values (in pm) for Period 3 elements from Table 3.6(a): Na = 186, Mg = 160, Al = 143, Si = 117, P = 110, S = 104, Cl = 99. Based on these periodic trends, what is the primary factor responsible for the sharp decrease in atomic size from Sodium (\( 186\text{ pm} \)) to Chlorine (\( 99\text{ pm} \))?

1. Addition of a new principal quantum shell at each step
2. Increase in effective nuclear charge within the same valence shell
3. Complete screening of nuclear charge by inner core d-electrons
4. Sharp decrease in the number of valence electrons across the period