Alkali metals have only one valence electron per metal atom. As a result, the binding energy of alkali metal ions in the close-packed metal lattices are weak. Therefore these are soft and have low melting points.

This is due to abnormal increase in the atomic size of potassium.

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.

Alkali metals have low ionization 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.

Sodium ions:
(i) Na⁺ ions participate in the transmission of nerve signals, in regulating the flow of water across cell membranes.
(ii) In the transport of sugars and amino acids into cell.
Potassium ions:
(i) They activate many enzymes.
(ii) Participate in the oxidation of glucose to produce ATP.
Magnesium ions:
(i) All enzymes that utilise ATP in phosphate transfer require magnesium as a cofactor.
(ii) Mg is the main pigment for the absorption of light in plants.
Calcium:
(i) Ca²⁺ ions are present in bones.
(ii) plays important roles in neuromuscular function

Sodium carbonate It is obtained by Solvay ammonia process 
when CO₂ is passed through a concentrated solution of brine (aqeuous solution of NaCl) saturated with ammonia, NaHCO₃ gets precipitated.NaHCO₃ on Subsequent hetaing gives Na2CO₃.
NaCl + NH₃ + H₂O\(\xrightarrow{\Delta}\) NaHCO₃ + NH₄Cl
2NaHCO₃\(\xrightarrow{\Delta}\)  Na₂CO₃ + CO₂ + H₂O

Because green leafy vegitables are rich in iron and magnesium.

This is because of high solubility of potassium hydrogen carbonate due to which it does not precipitate out when CO₂ is pased through an ammoniated solution of KCl.

Kala chana , soya bean and sprouted lentils are the sources of iron potassium and proteins

No . it is useful our body

alkalin earth metals have smaller size and higher nuclear charge de to which they have a higher tendency to attract H₂O
moleculea as compare to alkali metals. Therefore, they contain more water of crystallisation e.g. LiCl.2H₂O and MgCl₂ 6H₂O.