s-Block Elements (Alkali and Alkaline Earth Metals)NEET Chemistry · Class 11 · NCERT Chapter 11

Lithium is the smallest and has the highest charge density (charge/size ratio) in Group 1. Its small size gives it a high polarising power, so many of its compounds are covalent rather than ionic (e.g., LiCl is more covalent than NaCl). Li forms a nitride (Li₃N) with nitrogen directly, which other alkali metals do not. It also forms only the monoxide (Li₂O) on burning in air, whereas Na forms Na₂O₂ (peroxide) and K forms KO₂ (superoxide). Lithium's properties resemble magnesium more than sodium — this is the diagonal relationship.

Beryllium (Be) has the smallest size and highest charge density in Group 2. Its compounds are largely covalent (e.g., BeCl₂). It does not react with water at room temperature or even at 100 °C (other Group 2 metals do). Be does not react with nitrogen. It dissolves in both acids and alkalis (amphoteric), unlike other Group 2 metals which react with acids only. Be follows a diagonal relationship with aluminium (Al). BeO and Al₂O₃ are both amphoteric oxides.

Group 1: All react with water, and reactivity increases down the group: Li reacts slowly, Na vigorously (melts and moves on water), K burns with violet flame, Rb and Cs react explosively. Group 2: Reactivity also increases down the group, but more gently than Group 1. Be does not react with water even at high temperature (due to oxide layer). Mg reacts only with hot water or steam. Ca, Sr, Ba react with cold water, with increasing vigour down the group.

A diagonal relationship refers to the similarity between an element in one group and the element in the next group of the next period (diagonally adjacent). This happens because moving one period down increases atomic size, while moving one group to the right in the same period decreases it; these two changes roughly cancel out, giving similar charge density and polarising power. Examples: Li resembles Mg; Be resembles Al; B resembles Si.

Plaster of Paris is calcium sulphate hemihydrate: CaSO₄·½H₂O. It is made by heating gypsum (CaSO₄·2H₂O) to 120–130 °C: 2CaSO₄·2H₂O → 2CaSO₄·½H₂O + 3H₂O. When Plaster of Paris is mixed with water, it reabsorbs water and sets (hardens) to form gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum). The slight expansion during setting makes it ideal for moulds and for setting bones.

Sodium is a very reactive metal. It reacts vigorously with atmospheric moisture (water) to form NaOH and H₂ gas (a fire hazard), and it also reacts with oxygen to form Na₂O₂. To protect it from both water vapour and oxygen, sodium is stored under kerosene oil. Kerosene is a non-polar liquid that does not react with sodium and provides a non-aqueous, non-reactive barrier.