Alkali metals

• Alkali metals are most reactive metals and thus the do not occur in free state

• They mostly occur as halides, borates, silicates, nitrates and oxides

• Some alkali metals occur abundantly in nature

• Sodium is seventh and potassium is eight abundant element by weight in earth’s crust

All the alkali metals have one valence electron ns1

outside the noble gas core. The loosely held s-electron in the outermost valence shell of these elements makes them the most electropositive metals. They readily lose electron to form monovalent ions. This is the reason why they are never found in free state in nature.

The alkali metal atoms have the largest sizes in a particular period of the periodic table. With increase in atomic number, the atomic and ionic radii of alkali metals increase on moving down the group, i.e. from Li to Cs.

The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs. Hydration enthalpies decrease with increase in ionic sizes.

Li+> Na+ > K+ > Rb+ > Cs+

(i) Reactivity towards air

(ii) Reactivity towards water

(iii)Reactivity towards dihydrogen

(iv)Reactivity towards halogens

(v) Reducing nature

(vi)Solutions in liquid ammonia

(i) Reactivity towards air: The alkali metals tarnish in dry air due to the formation of their oxides which in turn reacts with moisture to form hydroxides. They burn vigorously in oxygen forming oxides.a. 4Li + O2 2Li2O (oxide)b. 2Na + O2 Na2O2(peroxide)c. M + O2 MO2 (superoxide)

(ii) Reactivity towards water: The alkali metal reacts with water to form hydroxide and dihydrogen.2M + 2H2O 2M+ +2OH- + H2(M= an alkali metal)

(iii) Reactivity towards dihydrogen: The alkali metals react with dihydrogen at about 673K (lithium at 1073K) to form hydrides. All the alkali metal hydrides are ionic solids with high melting points.2M + H2 2M+H-

(iv)Reactivity towards halogens: The alkali metals readily react vigorously with halogens to form ionic halides, M+X-. The Li+ ion is very small in size and has high tendency to distort electron cloud around the negative halide ion. Since anion with large size can be easily distorted among halides lithium iodide is the most covalent in nature.

(v)Reducing nature: The alkali metals are strong reducing agents, lithium being the most and sodium the least powerful. The standard electrode potential which measures the reducing power represents the overall change:M(s) M(g)sublimation enthalpyM(g) M+(g) + e-

ionization enthalpyM+(g) + H2O M+(aq) hydration enthalpy

(vi) Solutions in liquid ammonia: The alkali metal dissolves in liquid ammonia giving deep blue solutions which are conducting in nature.M + (x + y)NH3 [M(NH3)x]

+ + [e(NH3)y]

-

The solutions are paramagnetic and on standing slowly librates hydrogen resulting in the formation of amide.M+

(am) + e- +NH3(l) MNH2(am) + 1/2H2(g)

(where ‘am’ denotes solution in ammonia.)

Lithium is used in making alloys . Lithium carbonate is used in making a special kind of glass. Lithium aluminum hydride is used as reducing agent in synthetic chemistry. Along with this Sodium is used in softening of hard water, laundry, and cleaning, it is also used in paints and dyes. Sodium is also used in paper and textiles industries and as a reagent in laboratories.

1. Oxides and Hydroxides:Under appropriate conditions pure compounds M2O, M2O2 and MO2may be prepared on combustion in excess of air. These oxides are easily hydrolyzed by water to form hydroxides.

M2O + H2O 2M+ + 2OH-

M2O2 + 2H2O 2M+ + 2OH- + H2O2

2MO2 + 2H2O 2M+ + 2OH- + H2O2 + O2

2. Halides:

The alkali metal halides, MX (X = halogen) are all high melting, colorless crystalline solids. They can be prepared by the reaction of the appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid (HX).

The melting and boiling points always follow the trend: fluoride > chloride > bromide > iodide. All these halides are soluble in water.

3. Salts of Oxo-Acids:

Oxo-acids are those in which the acidic proton is on a hydroxyl group with an oxo group attach to the same atom. They are generally soluble in water and thermally stable. Their carbonates and hydrocarbonates are highly stable to heat.