Which of the following pairs of properties of alkali metals decreases down the group?

Analyze the question: We need the pair in which both properties decrease as we go down Group 1 (Li → Na → K → Rb → Cs).

Concept: Going down the group, the number of electron shells increases, so the atomic radius increases. The outer electron is farther from the nucleus and better shielded, so it is held less strongly.

Check each property:

• Atomic radius: increases
• First ionization energy: decreases (the outer electron is easier to remove)
• Electronegativity: decreases (less attraction for bonding electrons)
• Reactivity: increases (metals lose their outer electron more easily)
• Melting point: decreases (metallic bonds weaken as the atoms get larger, e.g. Li 181°C, Na 98°C, K 63°C, Cs 29°C)

Check each option:

• A. First ionization energy decreases, but reactivity increases. ✗
• B. Melting point decreases, but atomic radius increases. ✗
• C. Reactivity increases, but electronegativity decreases. ✗
• D. First ionization energy decreases and melting point decreases. ✓

**Answer: D.

Tip: For alkali metals going down, remember "Reactivity and radius go UP; almost everything else (ionization energy, electronegativity, melting point, density trend aside) goes DOWN." Any option pairing reactivity or radius with a decreasing property is a trap.