s-Block Position

The s-block occupies the left two columns of the periodic table (Groups 1 and 2) plus helium (He, 1$s^{2}$):

Period 1: H (1$s^{1}$) .......... He (1$s^{2}$) [noble gas]
Period 2: Li (2$s^{1}$), Be (2$s^{2}$)
Period 3: Na (3$s^{1}$), Mg (3$s^{2}$)
Period 4: K (4$s^{1}$), Ca (4$s^{2}$)
Period 5: Rb (5$s^{1}$), Sr (5$s^{2}$)
Period 6: Cs (6$s^{1}$), Ba (6$s^{2}$)
Period 7: Fr (7$s^{1}$), Ra (7$s^{2}$)

Key trends across a period (Group 1 → Group 2):

• Atomic radius: ↓ decreases (more nuclear charge, same shell)
• Ionization enthalpy: ↑ increases (Group 2 > Group 1 in same period)
• Melting point: ↑ increases (Group 2 has stronger metallic bonds from 2 valence electrons)

Why Group 2 has smaller radius than Group 1 in same period: Going from Na (Group 1, [Ne]3$s^{1}$) to Mg (Group 2, [Ne]3$s^{2}$): +1 proton pulls electrons in → smaller radius despite more electrons.