Study Notes: The p-Block Elements (Groups 13 and 14) – Boron and Carbon Families

THE p-BLOCK ELEMENTS

Introduction to p-Block Elements

The p-block elements are those in which the last electron enters the outermost p orbital. There are six groups of p-block elements in the periodic table, numbered from 13 to 18. These groups are headed by boron, carbon, nitrogen, oxygen, fluorine, and helium. Their general valence shell electronic configuration is ns2np1-6 (except for helium, which is 1s2).

• Variation in Properties: The inner core electronic configuration varies among these elements, influencing their physical and chemical properties, such as atomic and ionic radii, ionisation enthalpy, and electronegativity.

• Oxidation States: The maximum oxidation state is equal to the total number of valence electrons (s + p electrons). Other oxidation states, usually differing by two units, are also common due to the inert pair effect.

• Non-metals, Metalloids, and Metals: The p-block contains all three types. Non-metallic character decreases down the group, with the heaviest element being the most metallic.

• Nature of Compounds: Non-metals form covalent compounds, while metals form ionic compounds. The nature of oxides changes from acidic (non-metals) to basic (metals).

General Electronic Configuration and Oxidation States of p-Block Elements

Group	13	14	15	16	17	18
General Electronic Configuration	ns2np1	ns2np2	ns2np3	ns2np4	ns2np5	ns2np6 (1s2 for He)
First Member	B	C	N	O	F	He
Group Oxidation State	+3	+4	+5	+6	+7	+8
Other Oxidation States	+1	+2, –4	+3, –3	+4, +2, –2	+5, +3, +1, –1	+6, +4, +2

GROUP 13 ELEMENTS: THE BORON FAMILY

Occurrence and General Characteristics

• Boron (B): A typical non-metal, rare in nature, found as orthoboric acid, borax, and kernite. Two isotopes: 10B (19%) and 11B (81%).

• Aluminium (Al): Most abundant metal in the earth’s crust. Important minerals: bauxite (Al2O3·2H2O), cryolite (Na3AlF6).

• Gallium (Ga), Indium (In), Thallium (Tl): Less abundant, more metallic in character down the group.

Electronic Configuration

• General: ns2np1

• B and Al: Noble gas core; Ga and In: Noble gas + 10 d-electrons; Tl: Noble gas + 14 f-electrons + 10 d-electrons.

Atomic and Physical Properties

Property	Boron (B)	Aluminium (Al)	Gallium (Ga)	Indium (In)	Thallium (Tl)
Atomic Number	5	13	31	49	81
Atomic Mass (g/mol)	10.81	26.98	69.72	114.82	204.38
Electronic Configuration	[He]2s22p1	[Ne]3s23p1	[Ar]3d104s24p1	[Kr]4d105s25p1	[Xe]4f145d106s26p1
Atomic Radius (pm)	88	143	135	167	170
Ionization Enthalpy (kJ/mol)	801	577	579	558	589
Electronegativity	2.0	1.5	1.6	1.7	1.8
Density (g/cm3)	2.35	2.70	5.90	7.31	11.85
Melting Point (K)	2453	933	303	430	576
Boiling Point (K)	3923	2740	2676	2353	1730

Chemical Properties

• Oxidation States: +3 is common; +1 becomes more stable down the group due to the inert pair effect.

• Lewis Acidity: Electron-deficient trihalides (e.g., BF3, BCl3) act as Lewis acids.

• Reactivity with Air: Boron is unreactive; Al forms a protective oxide layer; on heating, both form oxides (B2O3, Al2O3).

• Reactivity with Acids and Alkalis: Boron is unreactive; Al dissolves in acids and alkalis, showing amphoteric character.

• Reactivity with Halogens: Forms trihalides (EX3), which are generally covalent and hydrolysed in water.

Important Compounds of Boron

• Borax (Na2B4O7·10H2O): Used in glass manufacture, as a flux, and in laboratory tests (borax bead test).

• Orthoboric Acid (H3BO3): Weak monobasic acid, acts as a Lewis acid, forms by acidifying borax or hydrolysing boron compounds.

• Diborane (B2H6): Simplest boron hydride, contains two bridging hydrogen atoms (three-centre two-electron bonds), highly reactive and flammable.

Uses of Boron and Aluminium

• Boron: Used in bullet-proof vests, aircraft materials, nuclear industry (as neutron absorber), and glass manufacture.

• Aluminium: Used in alloys, packaging, construction, and electrical transmission due to its low density and high conductivity.

GROUP 14 ELEMENTS: THE CARBON FAMILY

Occurrence and General Characteristics

• Members: Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn), Lead (Pb).

• Abundance: Carbon is widely distributed; silicon is the second most abundant element in the earth’s crust.

• Isotopes: Carbon has three isotopes: 12C, 13C (stable), and 14C (radioactive, used in radiocarbon dating).

Atomic and Physical Properties

Property	Carbon (C)	Silicon (Si)	Germanium (Ge)	Tin (Sn)	Lead (Pb)
Atomic Number	6	14	32	50	82
Atomic Mass (g/mol)	12.01	28.09	72.60	118.71	207.2
Electronic Configuration	[He]2s22p2	[Ne]3s23p2	[Ar]3d104s24p2	[Kr]4d105s25p2	[Xe]4f145d106s26p2
Covalent Radius (pm)	77	118	122	140	146
Ionization Enthalpy (kJ/mol)	1086	786	761	708	715
Electronegativity	2.5	1.8	1.8	1.8	1.9
Density (g/cm3)	3.51 (diamond)	2.34	5.32	7.26	11.34
Melting Point (K)	4373	1693	1218	505	600
Boiling Point (K)	–	3550	3123	2896	2024

Chemical Properties

• Oxidation States: +4 and +2 are common; carbon can also show negative oxidation states. The stability of +2 increases down the group due to the inert pair effect.

• Catenation: Carbon forms strong C–C bonds, leading to chains and rings. The tendency decreases down the group: C >> Si > Ge ≈ Sn > Pb.

• Reactivity with Oxygen: Forms monoxides (MO) and dioxides (MO2). Dioxides of lighter elements are acidic; those of heavier elements are amphoteric or basic.

• Reactivity with Halogens: Forms tetrahalides (MX4) and dihalides (MX2); stability of dihalides increases down the group.

Allotropes of Carbon

• Diamond: Each carbon atom is sp3 hybridised, forming a rigid 3D network. Hardest known substance, high melting point, used as abrasive and in jewellery.

• Graphite: Layered structure with sp2 hybridised carbons in hexagonal rings. Good conductor of electricity, used as lubricant and in electrodes.

• Fullerenes (e.g., C60): Cage-like molecules, discovered in 1985. Each carbon is sp2 hybridised, forming a soccer-ball structure (Buckminsterfullerene).

Important Compounds of Carbon and Silicon

• Carbon Monoxide (CO): Prepared by limited oxidation of carbon or dehydration of formic acid. Powerful reducing agent, forms metal carbonyls, highly poisonous due to strong binding with haemoglobin.

• Carbon Dioxide (CO2): Produced by complete combustion of carbon. Weakly acidic, forms carbonates and bicarbonates, involved in photosynthesis, contributes to greenhouse effect.

• Silicon Dioxide (SiO2): Major component of earth’s crust, forms a 3D covalent network. Used in glass, ceramics, and as a drying agent.

• Silicones: Organosilicon polymers with repeating (R2SiO) units. Water-repellent, thermally stable, used as sealants, lubricants, and in medical implants.

• Silicates: Minerals with SiO44– tetrahedral units. Found in feldspar, mica, asbestos, and zeolites. Zeolites are used as catalysts and ion exchangers.

Summary Table: Classification of Oxides

Oxide	Nature
CO	Neutral
B2O3	Acidic
SiO2	Acidic
CO2	Acidic
Al2O3	Amphoteric
PbO2	Amphoteric
Tl2O3	Basic

Key Concepts and Definitions

• Inert Pair Effect: The tendency of the s-electrons in heavier p-block elements to remain non-bonding or inert, leading to lower oxidation states.

• Allotropy: The existence of an element in more than one form, differing in physical structure but not in chemical composition (e.g., diamond, graphite, fullerenes for carbon).

• Catenation: The ability of an element to form chains of identical atoms (strongest in carbon).

• Lewis Acid: A species that can accept an electron pair (e.g., BF3, BCl3).

Selected Equations and Reactions

• Formation of Borax Bead: $\mathrm{Na}_2\mathrm{B}_4\mathrm{O}_7 \cdot 10\mathrm{H}_2\mathrm{O} \xrightarrow{\Delta} \mathrm{Na}_2\mathrm{B}_4\mathrm{O}_7 \xrightarrow{\Delta} 2\mathrm{NaBO}_2 + \mathrm{B}_2\mathrm{O}_3$

• Preparation of Diborane: $4\mathrm{BF}_3 + 3\mathrm{LiAlH}_4 \rightarrow 2\mathrm{B}_2\mathrm{H}_6 + 3\mathrm{LiF} + 3\mathrm{AlF}_3$

• Hydrolysis of SiCl4: $\mathrm{SiCl}_4 + 4\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Si(OH)}_4 + 4\mathrm{HCl}$

• Photosynthesis: $6\mathrm{CO}_2 + 12\mathrm{H}_2\mathrm{O} \xrightarrow{\text{chlorophyll, } h\nu} \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 + 6\mathrm{H}_2\mathrm{O}$

• Bond Enthalpy (Catenation):

  Bond	Bond Enthalpy (kJ/mol)
  C–C	348
  Si–Si	297
  Ge–Ge	260
  Sn–Sn	240

Structural Features

• Diamond: Each C atom is tetrahedrally bonded to four others (sp3 hybridisation), forming a rigid 3D network.

• Graphite: Planar sheets of hexagonal rings (sp2 hybridisation), layers held by van der Waals forces, delocalised electrons allow electrical conductivity.

• Fullerenes (C60): Spherical molecules with 20 hexagons and 12 pentagons, each C atom is sp2 hybridised.

• Diborane (B2H6): Contains four terminal B–H bonds (2-centre-2-electron) and two bridging B–H–B bonds (3-centre-2-electron, 'banana bonds').

Applications and Uses

• Boron: Bullet-proof vests, nuclear industry, glass manufacture.

• Aluminium: Alloys, packaging, construction, electrical transmission.

• Carbon: Graphite in electrodes and lubricants, diamond in abrasives and jewellery, fullerenes in materials science.

• Silicon: Semiconductors, glass, ceramics, silicones in sealants and medical devices.

Additional info:

• p-Block chemistry is foundational for understanding organic chemistry, as carbon and its compounds are central to organic molecules.

• The inert pair effect and catenation are important for predicting reactivity and compound formation in heavier p-block elements.