s-Block Elements (Class 11): Alkali and Alkaline Earth Metals
Two groups whose entire chemistry follows from one trend — size increases down the group, and almost everything else follows from that.
The trends, and the one cause behind them
Down a group, each element has an additional shell. Size increases, the outer electrons are further from the nucleus and better shielded, so ionisation energy falls. Almost every other trend follows.
| Property | Down the group | Because |
|---|---|---|
| Atomic and ionic radius | Increases | Additional shells |
| Ionisation energy | Decreases | Outer electron further out and better shielded |
| Metallic character | Increases | Electrons lost more readily |
| Reactivity | Increases | Same reason |
| Hydration enthalpy | Decreases | Larger ion has lower charge density |
| Melting point | Generally decreases | Weaker metallic bonding with larger atoms |
Comparing groups 1 and 2, the group 2 elements are smaller, more highly charged and therefore harder, denser and higher melting, with stronger metallic bonding.
Anomalous behaviour of the first element
Concrete consequences that get asked: lithium compounds are more covalent and more soluble in organic solvents than those of the other alkali metals; beryllium compounds are extensively covalent; and both form oxides rather than the peroxides and superoxides their heavier group members form.
The diagonal relationship
Lithium resembles magnesium, and beryllium resembles aluminium, more than either resembles its own group members. The reason is that moving right increases charge and moving down increases size, and the two effects roughly cancel along a diagonal — leaving similar charge density and therefore similar chemistry.
Stating that cancellation explicitly is what a full answer contains; simply listing the similarities is not an explanation.
Reaction with oxygen
The alkali metals show a striking pattern. Lithium forms the normal oxide, sodium forms the peroxide, and the heavier members form superoxides. The explanation is ionic size matching: a large cation stabilises a large anion, so only the larger cations can stabilise the bulky superoxide ion, while the small lithium ion can only stabilise the small oxide ion.
This lattice-energy argument is the expected answer, and it generalises — the same reasoning explains several other stability patterns in the s block.
Solubility trends
Solubility depends on the balance between lattice enthalpy and hydration enthalpy, and which one changes faster down the group decides the trend.
- Group 2 hydroxides become more soluble down the group, because lattice enthalpy falls faster than hydration enthalpy.
- Group 2 sulphates become less soluble down the group, because with a large anion the lattice enthalpy changes little while hydration enthalpy falls significantly.
The opposite directions of these two trends is a favourite question, and the answer is always which enthalpy term dominates — determined by the size of the anion relative to the cation.
Solutions in liquid ammonia
Alkali metals dissolve in liquid ammonia to give deep blue solutions containing ammoniated electrons. These solutions conduct electricity, are strongly reducing, and become bronze and metallic in appearance at high concentration. The blue colour is due to the solvated electrons absorbing in the red region, and this is a standard descriptive question.
Important compounds
- Sodium carbonate, made by the Solvay process, which is worth knowing in outline.
- Sodium hydroxide, from electrolysis of brine.
- Sodium hydrogencarbonate, used as a mild antacid and in baking.
- Calcium oxide and hydroxide, from limestone, widely used in construction.
- Calcium sulphate hemihydrate, plaster of Paris, which sets by rehydrating.
Plaster of Paris is asked most often, and the expected answer names the water content and the setting reaction rather than merely the use.
Frequently asked questions
Why does lithium resemble magnesium?
Because the increase in charge moving right and the increase in size moving down roughly cancel, leaving both ions with similar charge density and hence similar polarising power.
Why do heavier alkali metals form superoxides?
Because a large cation stabilises a large anion through favourable lattice energetics. The small lithium ion cannot, so it forms only the normal oxide.
Why are group 2 hydroxides and sulphates opposite in solubility trend?
Because the anion sizes differ. With a small anion, lattice enthalpy falls faster than hydration enthalpy down the group; with a large anion, hydration enthalpy dominates the change.
Why are alkali metals stored under oil?
Because they react vigorously with both oxygen and moisture in air. The oil excludes both.
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