p-Block Elements-I: One Mark Questions, Book Back Answers and Notes

About p-Block Elements-I

Medium ~120 min study

The p-block runs from group 13 to group 18, and it is the only block that contains metals, non-metals and metalloids together. In this unit you study the first two of those groups, the boron family and the carbon family. Their chemistry depends on how many p electrons are present and how tightly the outer s electrons are held.

Two ideas come up again and again. The first is the inert pair effect: going down the group, the ns² electrons take part in bonding less and less, so thallium prefers +1 over +3 and lead prefers +2 over +4. The second is the anomalous behaviour of boron and carbon. Both are small and have no d orbitals in their valence shell, so their maximum covalency is four and they behave quite differently from aluminium and silicon.

Expect questions on the bonding in diborane, the structures of boric acid and borax, the allotropes of carbon, and the classification of silicates by how many oxygen atoms each SiO₄ unit shares. These appear both as MCQs and as short-answer questions, and most of them test whether you can connect a structure to a property.

What you'll learn

• Explain how oxidation states change down groups 13 and 14.
• Describe the bridge bonds in diborane and why the molecule needs them.
• Compare diamond and graphite in terms of hybridisation, structure and conductivity.
• Classify a silicate from the number of shared oxygen atoms per tetrahedron.
• Use the inert pair effect to predict which oxidation state of Tl, Sn or Pb is more stable.
• State the uses of zeolites and alums.

Before you start

• Electronic configurations of elements.
• Periodic trends such as atomic radius and ionisation enthalpy.
• Covalent and coordinate bonding.

p-Block Elements-I explained

Groups 13 and 14 in Detail

General Trends

The general valence shell configuration of the p-block is \(ns^2 np^{1-6}\). Across a period, atomic radius decreases and ionisation enthalpy and electronegativity increase, so the elements become more non-metallic. Down a group, size increases, ionisation enthalpy falls and metallic character increases. Down groups 13 and 14 the inert pair effect also appears. The ns² electrons are poorly shielded by the filled d and f orbitals below them and are held tightly, so the lower oxidation state becomes more stable: \(+1\) for thallium and \(+2\) for lead.

Group 13: The Boron Family

Boron is a metalloid; aluminium, gallium, indium and thallium are metals. The unit describes how boron is obtained and how it reacts, then its main compounds. Borax is used in making glass and as a flux, and boric acid is a mild antiseptic. Boron trifluoride has only six electrons around boron, so it acts as a Lewis acid. Diborane, B₂H₆, is electron-deficient and is held together by two three-centre two-electron B–H–B bridge bonds.

Aluminium and Alums

Aluminium is the most abundant metal in the earth's crust and is used widely in alloys. Its oxide is amphoteric, dissolving in both acids and alkalis. Alums are double salts with the general formula \(M'_2SO_4 \cdot M''_2(SO_4)_3 \cdot 24H_2O\), where M' is a monovalent cation and M'' a trivalent one. Potash alum is the familiar example, used to purify water and as a mordant in dyeing.

Group 14: The Carbon Family

Carbon shows catenation more than any other element, forming long chains and rings; this is why organic chemistry exists. Its allotropes differ in structure: diamond is a rigid three-dimensional network of tetrahedral carbon atoms, while graphite is made of flat layers that slide over each other. Fullerenes and graphene are newer forms. Silicon and germanium lie further down the group and are used as semiconductors.

Oxides and Halides of Carbon and Silicon

Carbon forms two gaseous oxides. Carbon monoxide is poisonous because it binds to haemoglobin far more strongly than oxygen does, and carbon dioxide is a greenhouse gas. Silicon dioxide, by contrast, is a solid network. Silicon tetrachloride is prepared from silicon and chlorine and is hydrolysed by water, while carbon tetrachloride is not, because silicon can use its vacant d orbitals to accept a lone pair from water.

Silicates and Zeolites

Silicates make up most rocks and minerals. Their basic unit is the \(SiO_4^{4-}\) tetrahedron, and they are classified by how many oxygen atoms each tetrahedron shares: isolated units in orthosilicates, pairs in pyrosilicates, rings in cyclic silicates, then chain, sheet and three-dimensional silicates. Zeolites are three-dimensional aluminosilicates with small pores. They act as molecular sieves, as ion exchangers for softening hard water, and as catalysts in the petrochemical industry.

Common mistakes to avoid

• Treating the inert pair effect as a separate force. It arises because the filled d and f orbitals shield the outer s electrons poorly, so those electrons are held tightly.
• Assuming carbon and silicon form similar oxides. CO2 is a discrete gaseous molecule, while SiO2 is a solid network.
• Drawing diborane with ordinary B–B or two-electron B–H bonds only, and missing the two three-centre bridge bonds.
• Counting the shared oxygen atoms wrongly when classifying chain and sheet silicates.
• Ignoring the anomalous behaviour of boron and carbon when comparing them with the rest of their groups.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the inert pair effect?

It is the reluctance of the two outer s electrons to take part in bonding. It becomes stronger down the group in heavier p-block elements, so they show a stable oxidation state two units below the group valency, such as +1 for thallium and +2 for lead.

Why is diborane called electron-deficient?

Diborane has only 12 valence electrons, not enough to form a normal two-electron bond between every pair of bonded atoms. It uses four ordinary B–H bonds and two three-centre two-electron bridge bonds, in which one pair of electrons holds two boron atoms and one hydrogen together.

How do diamond and graphite differ?

In diamond each carbon is \(sp^3\) hybridised and bonded to four others in a rigid three-dimensional network, so it is very hard and does not conduct electricity. In graphite each carbon is \(sp^2\) hybridised and bonded to three others in flat layers. The fourth electron is delocalised, so graphite conducts, and the layers slide over each other, making it soft.

What are the main types of silicates?

Silicates are classified by how the \(SiO_4^{4-}\) units are linked. Orthosilicates have isolated tetrahedra, pyrosilicates have pairs, cyclic silicates form rings, and then come chain silicates, sheet silicates, and three-dimensional silicates in which all four oxygen atoms of every tetrahedron are shared.

What are zeolites used for?

Zeolites are porous aluminosilicates. Their pores let only molecules of a certain size pass, so they work as molecular sieves. They are used as catalysts in the petrochemical industry, for example ZSM-5 in converting alcohols to petrol, and as ion exchangers to soften hard water.

Why does boron behave differently from aluminium?

Boron is very small and has a high ionisation enthalpy, so it forms covalent compounds rather than B3+ ions. It also has no d orbitals in its valence shell, so its maximum covalency is four, whereas aluminium can reach six, as in [AlF6]3-.

What is an alum?

An alum is a double salt of a monovalent metal sulphate and a trivalent metal sulphate that crystallises with 24 molecules of water, for example potash alum, K2SO4·Al2(SO4)3·24H2O. Alums dissolve in water and are used to purify water, in tanning leather and as a styptic to stop bleeding from small cuts.