Alkali metals have one e^o in their outer most shell. Alkaline earth metals have 2 e^o in their outer most shell. More valence electrons and more positively charged nucleii leads to greater opportunity for metallic bonding.

In Na₂ SO₄.10 H₂O. So the metal will be Na (sodium). The salt is washing soda.

This is due to the smaller size and relatively high charge as Be²⁺ ion. Moreover Beryllium is a non - metal. But Magnesium is a metal. In the case of beryllium the ionisation energy and electronegativity of the halides are almost similar; but there is a vast difference in IE and electronegativity between Mg and halides.

The lattice energy of LiF is higher due to the smaller size of Li⁺ and F^-. So LiF has lower solubility.

Caesium chloride is more readily, formed than sodium chloride, because caesium is more electropositive than sodium. As the electropositive character of the alkali metal increases, the case of formation of the halide increases.

Lithium iodide shows covalent character, as Li⁺ ion, being smaller exerts high polarising power on the iodide anion. Alternatively, the iodide ion being the largest can be polarised to a greater extent by Li⁺ ion. Hence, lithium iodide is covalent.

The reactivity of alkali metals is due to their low ionisation enthalpy. the lonisation enthalpy decreases down the group. Hence the reactivity of alkali metal increases from Li to Cs.
The high reactivity of alkali metals is proved by their reaction with oxygen add halogen which are most electro negative.

The heat in the flame excites the valence electron to a higher energy-level. When it drops back to its actual energy level, the excess energy is emitted as light, whose wavelength is in the visible region. This appears as a characteristic colour to the particular metal.

The atoms of alkali metals have largest size intheir respective periods and therefore valence electrons are loosely bond to the nucleus and hence can be easily removed. i.e., they have low ionisation energies.

2Ca + O₂ ⟶ 2CaO