Q.Why alkaline earth metals are harder than alkali metals.

Step 1. Metallic hardness and melting/boiling point both depend on the strength of the metallic bonding holding the lattice of metal cations and delocalised valence electrons together -- the stronger this attraction, the harder and higher-melting the metal.

Step 2. Alkaline earth metal atoms are smaller than the alkali metals of the same period (Section 5.5.1: higher nuclear charge pulls the electrons in more tightly), so their cations pack more closely together in the metallic lattice.

Step 3. Each alkaline earth metal atom also contributes TWO valence electrons to the shared 'sea' of delocalised electrons (ns²), compared to only ONE from each alkali metal atom (ns¹) -- doubling the electron density available to bind the lattice cations together. …

Connect metallic bond strength to two structural factors that differ between the two families -- smaller atomic size and twice the number of valence electrons per atom -- and relate stronge …

• Explaining the difference only via 'higher charge' (+2 vs +1) without connecting it to the actual mechanism -- it is the extra delocalised electron per atom (not just the ionic …