s-Block Elements — Alkali and Alkaline Earth Metals

The Most Reactive Metals

The s-block elements — Groups 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) and Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra) — are the most electropositive elements in the periodic table. They lose electrons easily, react vigorously with water, and form strongly basic hydroxides.

This chapter is theory and trend-based. CBSE Class 11 gives 4-5 marks. NEET asks 1-2 questions on properties, anomalous behaviour, and compounds.

graph TD
    A[s-Block Elements] --> B[Group 1: Alkali Metals]
    A --> C[Group 2: Alkaline Earth Metals]
    B --> D[Trends down the group]
    D --> D1[Atomic radius increases]
    D --> D2[IE decreases]
    D --> D3[Reactivity increases]
    D --> D4[Electronegativity decreases]
    C --> E[Trends down the group]
    E --> E1[Similar to Group 1]
    E --> E2[Higher IE than Group 1]
    E --> E3[Form M²⁺ ions]
    B --> F[Anomalous: Li]
    C --> G[Anomalous: Be]
    F --> H[Diagonal with Mg]
    G --> I[Diagonal with Al]

Key Trends and Properties

Anomalous Behaviour

Lithium is different from other alkali metals because of its very small size and high charge density. It shows a diagonal relationship with Mg: both form covalent compounds, both react with N$_2$ to form nitrides, and both carbonates decompose on heating.

Beryllium is different from other alkaline earth metals for similar reasons. Its diagonal relationship with Al: both are amphoteric, both form covalent halides, both chlorides are Lewis acids.

Important Compounds

Sodium Hydroxide (NaOH, Caustic Soda)

Manufactured by Castner-Kellner process (electrolysis of brine with Hg cathode) or membrane cell.

Sodium Carbonate (Na$_2$CO$_3$, Washing Soda)

Made by Solvay process. Used in glass manufacture and water softening.

Calcium Oxide (CaO, Quicklime)

$\text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2$. Highly exothermic reaction with water.

Plaster of Paris (CaSO$_4 \cdot \frac{1}{2}$H$_2$O)

Made by heating gypsum (CaSO$_4 \cdot 2$H$_2$O) to 120°C. Sets hard when mixed with water.

Common Mistakes to Avoid

Practice Questions

Q1. Why is Li different from other alkali metals?

Q2. Arrange the hydroxides of Group 2 in order of increasing basicity.

Q3. Why does BeSO$_4$ dissolve in water but BaSO$_4$ does not?

Q4. What happens when Na reacts with water?

FAQs

Why do alkali metals not occur free in nature?

They are extremely reactive — they react with oxygen, water, and most non-metals. They always exist as compounds (NaCl, KCl, etc.).

Why do alkali metals give characteristic flame colours?

Electrons are excited to higher energy levels by the flame, and when they return, they emit light of characteristic wavelengths. The low IE of alkali metals makes this excitation easy.

Why is Na$_2$CO$_3$ stable but Li$_2$CO$_3$ decomposes on heating?

Li$^+$ is small with high polarizing power — it distorts the carbonate ion and promotes decomposition. Na$^+$ is larger and cannot polarize as effectively, so Na$_2$CO$_3$ is stable.

Deeper Concepts

Oxides — what forms when you burn alkali metals

The type of oxide formed depends on the size of the cation:

Why? Larger cations stabilise larger anions. The superoxide ion O$_2^-$ is large and needs a large cation (K$^+$, Rb$^+$, Cs$^+$) to stabilise the crystal lattice. Li$^+$ is too small to stabilise anything bigger than O$^{2-}$.

Thermal stability of carbonates and hydroxides

Group 1 carbonates: All are thermally stable except Li$_2$CO$_3$. Stability increases down the group because the larger cation has lower polarising power.

Group 2 carbonates: All decompose on heating. Stability order: BeCO$_3$ (least) < MgCO$_3$ < CaCO$_3$ < SrCO$_3$ < BaCO$_3$ (most). Again, larger cation = lower polarising power = higher thermal stability.

The same trend holds for hydroxides and nitrates.

Reaction with water — the trend

Reactivity with water increases down both groups:

• Li reacts slowly with cold water
• Na reacts vigorously, melts into a ball
• K catches fire (lilac flame)
• Rb and Cs explode on contact

Group 2 metals react more slowly than their Group 1 neighbours. Be does not react with water at all. Mg reacts only with hot water or steam. Ca onwards react with cold water.

Worked Example — Predicting products

Additional Solved Examples

Example 4 (JEE Main): Arrange BeSO$_4$, MgSO$_4$, CaSO$_4$, SrSO$_4$, BaSO$_4$ in decreasing order of solubility.

$\text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4$

Sulphate solubility decreases down Group 2 because hydration energy decreases faster than lattice energy as cation size increases.

Example 5 (NEET): Dead burnt plaster is obtained by heating gypsum above 200°C. What is it?

Heating gypsum (CaSO$_4 \cdot 2$H$_2$O) at 120°C gives Plaster of Paris (CaSO$_4 \cdot \frac{1}{2}$H$_2$O). Heating above 200°C gives anhydrous CaSO$_4$ — dead burnt plaster. It does not set with water because the crystal structure is destroyed. This is why temperature control matters in plaster manufacture.

Additional Practice Questions

Q5. Why is BeCl$_2$ covalent while MgCl$_2$ is ionic?

Q6. What is washing soda? How is it prepared commercially?

Q7. Why is KO$_2$ used in space capsules?

Q8. Cement is a mixture involving CaO. Name the two key compounds in Portland cement.

Exam Weightage

Biological importance of s-block elements

• Na$^+$ and K$^+$ maintain osmotic balance and nerve impulse transmission. The Na$^+$/K$^+$ pump is essential for every cell.
• Ca$^{2+}$ forms bones and teeth (as hydroxyapatite), triggers muscle contraction, and is essential for blood clotting.
• Mg$^{2+}$ is at the centre of chlorophyll. Without Mg, photosynthesis stops.

Industrial applications

FAQs

Why is sodium stored under kerosene?

Sodium reacts vigorously with moisture in air, producing NaOH and H$_2$. It can catch fire. Kerosene is a non-reactive hydrocarbon that keeps sodium away from air and moisture.

Why is Be amphoteric while other Group 2 elements are basic?

Be$^{2+}$ has very high charge density (small size, +2 charge), which gives BeO and Be(OH)$_2$ significant covalent character. This makes them amphoteric — they react with both acids and bases. Mg onwards, the hydroxides are purely basic.

What is the Solvay process?

An industrial method to produce Na$_2$CO$_3$ (washing soda) from NaCl, NH$_3$, and CO$_2$. The intermediate NaHCO$_3$ precipitates out (less soluble) and is heated to give Na$_2$CO$_3$. NH$_3$ is recovered and recycled. The process cannot be used for K$_2$CO$_3$ because KHCO$_3$ is too soluble to precipitate.