Chapter 2: s-Block Elements – Chemistry 2nd Year Notes

This chapter covers the s-Block Elements from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). The s-block contains Group IA (the alkali metals: Li, Na, K, Rb, Cs, Fr) and Group IIA (the alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra). They are the most reactive metals in the periodic table and are never found free in nature.

Here you will learn their general properties and trends, why they are strong reducing agents, their flame colours, the anomalous behaviour of lithium and beryllium, and the industrially important compounds made from them such as caustic soda, washing soda (Solvay process), plaster of Paris and bleaching powder.

Learning Objectives

• Identify the members of Group IA and Group IIA and write their general electronic configuration.
• Explain the general properties of s-block elements and their trends down the group.
• Explain why s-block metals are strong reducing agents and very reactive.
• Describe the reactions of these metals with water, oxygen and hydrogen.
• Explain the flame colours of s-block metals and their cause.
• Describe the anomalous behaviour of lithium and beryllium and the diagonal relationship.
• Explain the manufacture of sodium carbonate by the Solvay process.
• State the preparation and uses of NaOH, Na2CO3, plaster of Paris and bleaching powder.

Key Concepts

The Two Families of the s-Block

Group IA (ns1) are the alkali metals (Li, Na, K, Rb, Cs, Fr); they react with water to form strong alkalis, which is why they are called alkali metals. Group IIA (ns2) are the alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra); their oxides are alkaline and are found in the earth. In both groups metallic character increases down the group.

General Properties and Trends

Because they have only one or two loosely held valence electrons, s-block metals have large atomic sizes and low ionization energies, so they are strongly electropositive and lose electrons easily. Their metallic bonding is weak, making them soft and low-melting with low densities (Li, Na and K float on water). Down the group the size increases, the ionization energy decreases and the reactivity increases. Group IA always shows the +1 oxidation state and Group IIA always shows +2.

Reducing Nature

Since s-block metals lose their outer electrons very easily, they are powerful reducing agents, and their reducing power increases down the group. Lithium is a special case: its very high hydration energy makes it the strongest reducing agent in aqueous solution even though it is at the top of the group.

Important Reactions

With water, alkali metals give a hydroxide and hydrogen gas (2Na + 2H2O gives 2NaOH + H2), the reaction becoming more violent down the group. With oxygen, lithium forms a normal oxide, sodium a peroxide and potassium a superoxide. With hydrogen they form ionic hydrides (2Na + H2 gives 2NaH). Alkali metal nitrates decompose on heating to the nitrite and oxygen, while Be(OH)2 is amphoteric unlike the basic hydroxides of the other Group IIA members.

Flame Colours

When an s-block metal or its salt is heated in a flame, the heat excites its valence electrons; as they fall back they emit light of a definite colour, used to identify the metal. Lithium gives crimson red, sodium golden yellow, potassium lilac or violet, calcium orange-red, strontium crimson red, barium apple green, and rubidium and caesium violet. Beryllium and magnesium give no flame colour because their electrons are too tightly held.

Anomalous Behaviour and the Diagonal Relationship

The first member of each group (lithium in IA and beryllium in IIA) differs from the rest because of its very small size and high polarising power. As a result lithium resembles magnesium and beryllium resembles aluminium; this similarity between an element and the one diagonally placed to it is called the diagonal relationship.

Important Compounds

Caustic soda (NaOH) is a white, corrosive, strongly basic solid made by the electrolysis of brine, used in soap, paper and artificial silk. Sodium carbonate or washing soda (Na2CO3) is manufactured by the Solvay (ammonia-soda) process, in which ammoniacal brine is treated with carbon dioxide to precipitate sodium bicarbonate, which is then heated to sodium carbonate; ammonia and carbon dioxide are recycled. Gypsum (CaSO4.2H2O) on heating gives plaster of Paris (CaSO4.half H2O), which sets hard with water and is used for casts and chalk. Bleaching powder, Ca(OCl)Cl, is made by passing chlorine over slaked lime and is used to bleach and disinfect.

Biological Importance

Sodium and potassium ions control water balance and nerve signals, calcium is the main mineral of bones and teeth and helps blood to clot, and magnesium is the central metal ion in chlorophyll.

Important Definitions

Formulas & Rules

Item	Formula / Fact
Caustic soda	NaOH
Washing soda	Na2CO3.10H2O
Baking soda	NaHCO3
Gypsum / Plaster of Paris	CaSO4.2H2O / CaSO4.half H2O
Bleaching powder	Ca(OCl)Cl
Down a group	size up, ionization energy down, reactivity up

Diagrams & Illustrations

Position of the s-block: a periodic-table schematic highlighting Group IA (alkali metals, ns1) and Group IIA (alkaline earth metals, ns2) on the far left, with the rest of the table shown faded.

Flame test colours: a chart of coloured swatches showing lithium crimson red, sodium golden yellow, potassium lilac, calcium orange-red, strontium crimson red, barium apple green and rubidium or caesium violet.

Solvay process: a flow chart for the manufacture of sodium carbonate: ammoniacal brine plus carbon dioxide gives sodium bicarbonate in the carbonating tower, which on heating gives sodium carbonate, while ammonia and carbon dioxide are recovered and recycled.

Trends in the alkali metals: graphs showing ionization energy decreasing and reactivity increasing down Group IA (Li to Cs).

Solved Examples & Numericals

Example 1: Reactivity order

Which is more reactive, sodium or potassium? Potassium, because going down Group IA the atom becomes larger and its ionization energy decreases, so potassium loses its outer electron more easily.

Example 2: Identifying a metal by flame

An unknown salt gives an apple-green flame. Which metal is present? Barium, whose characteristic flame colour is apple green.

Example 3: Reaction with water

Write the products when sodium reacts with water. Sodium gives sodium hydroxide and hydrogen gas: 2Na + 2H2O gives 2NaOH + H2. The solution turns basic.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the general characteristics of the s-block elements.

The s-block elements are the metals of Groups IA and IIA, whose last electron enters an s-orbital. They have one or two loosely held valence electrons, and almost all their properties follow from this. They have large atomic sizes and low ionization energies, so they are strongly electropositive and lose electrons easily to form +1 or +2 ions. Their metallic bonding is weak, which makes them soft, light and low-melting. Because they lose electrons so readily they are powerful reducing agents and among the most reactive metals known, reacting with water to give hydrogen and a hydroxide, and burning in oxygen to form basic oxides. They give characteristic flame colours, and their reactivity increases down each group as size increases and ionization energy falls. Owing to this high reactivity they are never found free in nature.

Q2: Explain the manufacture of sodium carbonate by the Solvay process.

In the Solvay (ammonia-soda) process, sodium carbonate is made from cheap raw materials: brine, limestone and ammonia. Limestone is heated to give quicklime and carbon dioxide. The carbon dioxide is passed up a carbonating tower through ammoniacal brine, precipitating sparingly soluble sodium bicarbonate: NaCl + NH3 + CO2 + H2O gives NaHCO3 + NH4Cl. The bicarbonate is filtered and heated to give sodium carbonate, water and carbon dioxide: 2NaHCO3 gives Na2CO3 + H2O + CO2. The quicklime is slaked to calcium hydroxide and used to recover ammonia from ammonium chloride. Because both the ammonia and the carbon dioxide are recycled, the process is economical and produces little waste.

Q3: Discuss the anomalous behaviour of lithium and its diagonal relationship with magnesium.

Lithium, the first member of Group IA, differs from the other alkali metals because of its very small size and high charge density. Unlike the others, it forms a normal oxide rather than a peroxide or superoxide, its nitrate decomposes to the oxide instead of the nitrite, and many of its compounds are covalent. These features make lithium resemble magnesium, the element diagonally placed to it: both form normal oxides, both have nitrates that give the oxide on heating, both form covalent hygroscopic compounds and both react only slowly with water. This close similarity is known as the diagonal relationship and arises because the two elements have similar charge-to-size ratios.

MCQs with Answers

Quick Revision Summary

• s-block = Group IA (alkali, ns1, +1) and Group IIA (alkaline earth, ns2, +2).
• Large size, low ionization energy give very reactive, powerful reducing agents.
• Soft, light, low melting; reactivity increases down the group.
• Flame colours: Li crimson, Na yellow, K lilac, Ca orange-red, Ba apple-green.
• Anomalous first members: Li (like Mg) and Be (like Al) show the diagonal relationship.
• Solvay process makes Na2CO3; ammonia and carbon dioxide are recycled.
• Caustic soda = NaOH; gypsum = CaSO4.2H2O; plaster of Paris = CaSO4.half H2O.

Exam Tips

• Learn the flame colours cold; they appear almost every year.
• For any trend-down-the-group answer, give both the change and the reason (size up, ionization energy down).
• Memorise the four Solvay reactions in order; a labelled flow diagram earns full marks.
• Do not confuse washing soda (Na2CO3), baking soda (NaHCO3) and caustic soda (NaOH).
• Remember the exceptions: Li forms a normal oxide, Be(OH)2 is amphoteric, Be and Mg give no flame colour.