State as to why
(a) Alkali metals show only +1 oxidation state
(b) Na and K impart colour to the flame but Mg does not.lt
(c) Lithium on being heated in air mainly forms the monoxide and not the peroxide
(d) Li is the best reducing agent in aqueous solution

Explanation

Alkali metals have low jonization enthalpies
They have a strong tendency to lose 1 electron to form unipositive ions. Thus they show an oxidation state of +1 and are strongly electropositive.
(b) Valence electrons of alkali metals like Na and K easily absorb energy from the flame and are excited to higher energy levels. When these electrons return to the ground state, the energy is emitted in the form of light. Magnesium atom has small size so electrons are strongly bound to the nucleus. Thus they need large amount of energy for excitation of electrons to higher energy levels, which is not possible in Bunsen flame.
(c) Due to the small size of `Li^(+)` it has a strong positive field, which attracts the negative charge so strongly that it does not permit the oxide ion `(O^(2-))` to combine with another oxygen atom to form peroxide ion.
(d) Among alkali metals, lithium has the most negative electrode potential`(Etheta -3.04 V)`, so it is the strongest reducing agent in the aqueous solution