s block elements group IA
s block elements
ü Alkali (because they form strongly basic hydroxides on reaction with water) and alkaline Earth metals (coz their oxides and hydroxides are alkaline and oxides are found in the earth’s crust) are the most violently active of all the metals.
ü These occur in the combined form with halide, sulphate, carbonate, silicate ions, etc. and are not found in the
ü The diagonal relationship is due to the similarity in ionic sizes and/or charge/radius ratio of the elements.
ü Na+, K+, Mg2+, Ca2+ are found in biological fluids. These maintain ion balance and conduct nerve impulses.
Group 1 (IA)
ü Largest sizes in their respective periods and size increases down the group.
ü IE down the group
ü Hydration enthalpy down the group: Li+ > Na+> K+> Rb+ > Cs+. Li has max. degree of hydration and for this reason Li salts are mostly hydrated, e.g. LiCl.2H2O
ü Silvery white, soft, light and malleable. Low density because large size. density down the group
(But density of K< Na)
ü They and their salts impart characteristic colour to an oxidizing flame coz the flame excites the e . when this e returns to the ground level, it emits radiation in visible region:
ü In flame test, conc. HCl is added to convert the given salt into volatile chloride
ü When irradiated with light, the light energy absorbed may be sufficient to ionize them. Hence Cs & K are useful as electrodes in photoelectric cells.
ü Compounds of sodium and potassium are the most abundant, while those of remaining alkali metals occur in nature rarely. Francium is radioactive element and short lived
ü Lithium is kept wrapped in paraffin wax.
ü These are good conductors of heat and electricity.
ü Li to Cs, there is more repulsion from the non-bonding electrons, resulting in decrease in cohesive energy and an increased softness.
ü Alkali metals have only one valence electron. Thus their metallic bonding is weak. They are, therefore, soft metals of low density and have low melting and boiling points. They also have low enthalpies of fusion, vaporization and atomization.
ü Highly reactive due to large size and low IE. Reactivity down the group. Caesium is the most electropositive element in the periodic table.
ü Reactivity towards air:The alkali metals must be protected from exposure to air because they tarnish in dry air due to formation of their oxides which in turn react with moisture to form hydroxides. They burn vigorously in oxygen forming oxides. Lithium forms monoxide (Li2O), sodium forms monoxide in limited O2 and peroxide in excess O2. Other alkali metals form largely the super oxides. In the presence of atmospheric oxygen we will get the mixture of oxides.
4Li + O2 2Li2O 2Na + O2 Na2O2 M + O2 MO2 (M=K, Rb, Cs) Li can react with N2 present in air to form Li3N
ü The reactions of alkali metals with water and oxygen are so exothermic that they can cause a fire or even an explosion. Therefore, they are kept in kerosene oil.
ü Reactivity towards water: They are easily and readily oxidized by water. The reaction with water is so exothermic that the heat generated often ignites the H2 (g) produced, if air is present. These metals replace hydrogen from water:
2M + 2H2O 2M+ + 2OH- + H2
ü Reaction of Li is less vigorous due to its small size and high H.E. other metals react explosively with water.
ü They also react with proton donors such as alcohols, ammonia gas and alkynes.
ü With dihydrogen: react with H at 673 K (Li at 1073 K)
2M + H2 2M+H- these hydrides are ionic solids with high M.P.
ü With halogens: The metals form compounds mainly with ionic bonds, because they readily give up the s-electron from the outermost shell. Small highly polarizing ion such as Li+ tends to form covalent compounds.
ü Because of strong tendency to lose the s-electron, the alkali metals are powerful reducing agents. Lithium in aqueous solution is a strong reducing agent. This is probably due to high hydration energy of small lithium ion, which compensates for high ionisation energy. The hydration energy of alkali metal ions follows the order: Li+ > Na+ > K+ > Rb+ > Cs+ . Due to extensive hydration, Li+ ion has the highest hydration energy, as a result of which reduction potential of Li is higher than other alkali metals. Thus most powerful reducing agent in solution is lithium. Na is least reducing.
ü Solutions in liquid ammonia: these dissolve in liquid ammonia giving conducting deep blue solutions:
M + (x+y) NH3 [M (NH3)x]+ + [e(NH3)y]-
The blue colour is due to the ammoniated e which absorbs energy in visible region. The solution is paramagnetic and on standing slowly liberates H2 resulting the formation of amide:
M + e- +NH3(l) MNH2 + ½ H2(g)
In concentrated solution, blue colour changes to bronze and becomes diamagnetic.
COMPOUNDS of ALKALI METALS
1. Oxides: the stability of peroxides and superoxides down the group due to stabilization of large anions by large cations through lattice energy effects. Oxides and peroxides are colourless when pure. Superoxides are yellow or orange and paramagnetic. These are easily hydrolysed by water to form hydroxides:
M2O + H2O 2M+ + 2OH-
M2O2 + H2O 2M+ + 2OH- + H2O2
2MO2 + H2O 2M+ + 2OH- + H2O2 + O2
2.Hydroxides: white crystalline solids. Strongest of all bases and dissolve freely in water with evolution of much heat on account of instant hydration.
3.Halides: high melting, colourless crystalline solids. M.P. : F>Cl>Br>
Low solubility of LiF is due to high L.E. while that of CsI is due to small H.E. Other Li halides are soluble in ethanol, acetone & ethylacetate. LiCl is soluble in pyridine also.
4. salts of oxo-acids: generally soluble in water but stable towards heat. Stability of carbonates and bicarbonates towards heat down the group. But Li2CO3 is not so stable due to its covalent character. Its hydrogen carbonate does not exist as solid.