JEE Chemistry S Block Elements Notes Set 03

Group 1: General Trends

The alkali metals (Li, Na, K, Rb, Cs, Fr) have one electron in the outer s-orbital, ns1, outside a noble-gas core. This single loosely held electron explains almost everything about them: strong reducing power, only the +1 state, ionic compounds and soft low-melting metals.

Reactions of the Alkali Metals

1. Oxygen: Li gives the normal oxide Li2O, Na gives mainly the peroxide Na2O2, and K, Rb, Cs give superoxides MO2. Bigger cations stabilise bigger anions by lattice energy.
2. Peroxides are diamagnetic and contain O2(2-). Superoxides contain O2(-) with one unpaired electron, so they are paramagnetic and coloured.
3. Water: all react to give hydroxide and hydrogen, with vigour rising down the group. Li is less violent than Na because it does not melt in the heat of reaction.
4. Hydrogen, halogens, sulphur and nitrogen: metals react with hydrogen to give ionic saline hydrides MH; Li alone reacts with nitrogen to give Li3N.
5. Oxides and hydroxides are strongly basic; the hydroxides are the strongest bases and their solubility rises down the group.

Solutions in Liquid Ammonia

Alkali metals dissolve in liquid ammonia to give deep blue solutions that conduct electricity. The blue colour is due to ammoniated electrons, which absorb in the red region. Dilute solutions are paramagnetic. Concentrated solutions turn bronze, become diamagnetic as the electrons pair up, and conduct like a metal. On standing, or with a catalyst, the solution slowly gives the amide and hydrogen. The solvated electron makes the solution a very strong reducing agent.

Anomalous Behaviour of Lithium and Diagonal Relationship with Magnesium

Lithium is unusual because the Li+ ion is tiny and has a very high charge density. It is harder, has a higher melting point, forms the normal oxide, and is the only alkali metal to form a nitride. Its salts have notable covalent character. Li resembles Mg in several ways:

1. Both form normal oxides, nitrides, and hydroxides that decompose on heating.
2. Carbonates of both decompose to the oxide on heating; the other alkali metal carbonates do not.
3. Both nitrates give the oxide, NO2 and O2 on heating; sodium and potassium nitrates give the nitrite.
4. Chlorides of both are deliquescent, soluble in alcohol and crystallise as hydrates; the bicarbonates exist only in solution.

Important Sodium Compounds

Glauber's salt is Na2SO4.10H2O, Chile saltpetre is NaNO3, borax is Na2B4O7.10H2O, hypo is Na2S2O3.5H2O.

Thermal Stability Rules

1. Carbonates: stability rises down the group; Li2CO3 decomposes to Li2O and CO2 while the others resist heating.
2. Nitrates: LiNO3 gives Li2O, NO2 and O2; NaNO3 and KNO3 give the nitrite and oxygen.
3. Hydrogencarbonates exist as solids for Na, K, Rb, Cs only; they decompose on heating to the carbonate, CO2 and water.

Biological Role

Na+ and K+ ions keep the balance of fluids in cells, drive nerve impulses and are carried across membranes by the sodium-potassium pump. Na+ is mainly outside and K+ mainly inside the cell.