JEE Chemistry S Block Elements Notes Set 01

General Characteristics of Alkali Metals (Group 1)

Group 1 elements (Li, Na, K, Rb, Cs, Fr) have the configuration ns1. They are soft, low-density metals with low melting points that decrease down the group (Li is the highest, Cs about 302 K). Atomic and ionic radii, metallic character and reactivity increase down the group, while ionisation energy and electronegativity decrease. Density generally increases, with K lighter than Na as a known exception.
They show only the +1 state. Their ions have noble gas configuration, so the compounds are colourless and diamagnetic. The hydrated ion size follows Li+ > Na+ > K+ > Rb+ > Cs+, so ionic conductance in water rises from Li+ to Cs+. Hydration energy is highest for Li+, which is why Li has the most negative standard reduction potential and is the strongest reducing agent in water, while Cs is the strongest in the gaseous state.

Flame Colours and the Photoelectric Effect

Because of their low ionisation energies, the outer electron is easily excited in a flame, giving colours: Li crimson, Na golden yellow, K violet, Rb red-violet, Cs blue. Be and Mg do not colour the flame. Caesium, with the lowest ionisation energy, shows the photoelectric effect most easily and is used in photoelectric cells. Among group 2: Ca brick red, Sr crimson, Ba apple green.

Reaction with Oxygen

Li gives the normal oxide Li2O (and some nitride), Na gives mainly the peroxide Na2O2, and K, Rb and Cs form superoxides MO2. Large cations stabilise large anions. Superoxides have an unpaired electron, so KO2 is paramagnetic and coloured. KO2 absorbs CO2 and releases O2 (4KO2 + 2CO2 gives 2K2CO3 + 3O2), so it is used in breathing apparatus and space craft. Na2O2 also absorbs CO2 and releases oxygen.

Reaction with Hydrogen, Water and Liquid Ammonia

All alkali metals form ionic hydrides MH (H- ion) with hydrogen; stability decreases down the group, and LiH is the most stable. They react with water to give hydroxide and hydrogen, with vigour increasing down the group.
In liquid ammonia, they give blue, conducting, paramagnetic solutions due to ammoniated electrons; concentrated solutions turn bronze due to metal ion clusters and are diamagnetic. On standing, or in the presence of iron as catalyst, the blue colour fades as the metal amide forms with H2 evolved.

Anomalous Behaviour of Lithium

Lithium differs because of its small size and high polarising power, and resembles magnesium (diagonal relationship). It forms Li3N directly, its carbonate, nitrate and hydroxide decompose on heating to the oxide, LiCl is largely covalent and soluble in organic solvents, LiCl is deliquescent and crystallises as a hydrate, LiHCO3 does not exist in the solid state, and it does not form alums or superoxide.