Introduction to s and p-Block Elements
The s-block elements include Groups 1 (Alkali Metals) and 2 (Alkaline Earth Metals), characterized by their ns¹ and ns² valence electron configurations. The p-block elements comprise Groups 13 to 18, with valence electron configurations ns²np¹ to ns²np⁶.
Key Characteristics
- s-block elements are highly reactive metals
- p-block contains metals, metalloids, and non-metals
- Trends in properties follow periodic variations
- Diagonal relationships exist between certain elements
Alkali Metals (Group 1)
Elements
Lithium (Li), Sodium (Na), Potassium (K)
Rubidium (Rb), Caesium (Cs), Francium (Fr)
Francium is radioactive
Electronic Configuration
General: [Noble gas] ns¹
Li: [He] 2s¹, Na: [Ne] 3s¹
K: [Ar] 4s¹, Rb: [Kr] 5s¹
Cs: [Xe] 6s¹, Fr: [Rn] 7s¹
Occurrence
Never found free in nature
Lithium: Lepidolite, Spodumene
Sodium: Rock salt (NaCl), Chile saltpetre
Potassium: Carnallite, Felspar
Physical Properties
| Property | Trend | Explanation |
|---|---|---|
| Atomic/Ionic Radius | Increases down the group | Addition of new shells |
| Density | Increases (except K) | Atomic mass increase predominates |
| Melting/Boiling Point | Decreases down the group | Weakening metallic bonds |
| Ionization Energy | Decreases down the group | Increasing atomic size |
| Electropositive Character | Increases down the group | Decreasing ionization energy |
Chemical Properties
Reaction with Oxygen
Li forms Li₂O (oxide)
Na forms Na₂O₂ (peroxide)
K, Rb, Cs form MO₂ (superoxide)
4Li + O₂ → 2Li₂O
2Na + O₂ → Na₂O₂
K + O₂ → KO₂
Reaction with Water
Vigorous reaction producing hydroxides
Reactivity increases down the group
Li reacts slowly, Cs reacts explosively
2M + 2H₂O → 2MOH + H₂
Reaction with Halogens
Form ionic halides (M⁺X⁻)
Reactivity increases down the group
Lithium halides are covalent
2M + X₂ → 2MX
Anomalous Behavior of Lithium
Differences from Other Alkali Metals
- Harder and higher melting point
- Forms nitride with nitrogen: 6Li + N₂ → 2Li₃N
- Carbonates and bicarbonates less stable
- Hydroxide decomposes on heating
- Chloride is deliquescent and soluble in organic solvents
- Does not form alums
Diagonal Relationship: Li with Mg
|
Similarities
Both are harder and higher melting
Chlorides are covalent and soluble in organic solvents
Carbonates decompose on heating
Form nitrides directly with nitrogen
Bicarbonates are soluble in water
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Differences
Li is group 1, Mg is group 2
Li forms +1 ions, Mg forms +2 ions
Different chemical reactivity patterns
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Alkaline Earth Metals (Group 2)
Elements
Beryllium (Be), Magnesium (Mg), Calcium (Ca)
Strontium (Sr), Barium (Ba), Radium (Ra)
Radium is radioactive
Electronic Configuration
General: [Noble gas] ns²
Be: [He] 2s², Mg: [Ne] 3s²
Ca: [Ar] 4s², Sr: [Kr] 5s²
Ba: [Xe] 6s², Ra: [Rn] 7s²
Occurrence
Be: Beryl, Phenacite
Mg: Magnesite, Dolomite, Carnallite
Ca: Limestone, Gypsum, Fluorapatite
Ba: Barytes, Witherite
Physical Properties
| Property | Trend | Comparison with Alkali Metals |
|---|---|---|
| Atomic/Ionic Radius | Increases down the group | Smaller than corresponding alkali metals |
| Density | Irregular trend | Higher than alkali metals |
| Melting/Boiling Point | No regular trend | Higher than alkali metals |
| Ionization Energy | Decreases down the group | Higher than alkali metals |
| Electropositive Character | Increases down the group | Less electropositive than alkali metals |
Chemical Properties
Reaction with Oxygen
Form oxides (MO)
Ba and Ra form peroxides
Reactivity increases down the group
2M + O₂ → 2MO
Ba + O₂ → BaO₂
Reaction with Water
Be - no reaction (protective oxide layer)
Mg - reacts with hot water
Ca, Sr, Ba - react with cold water
M + 2H₂O → M(OH)₂ + H₂
Reaction with Halogens
Form ionic halides (MX₂)
BeCl₂ is covalent
Form hydrated crystals
M + X₂ → MX₂
Anomalous Behavior of Beryllium
Differences from Other Alkaline Earth Metals
- Amphoteric oxide and hydroxide
- Covalent compounds
- Does not react with water
- Forms complexes
- Carbide gives methane on hydrolysis
- Does not impart color to flame
Diagonal Relationship: Be with Al
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Similarities
Both form covalent compounds
Oxides and hydroxides are amphoteric
Chlorides are covalent and soluble in organic solvents
Carbides give methane on hydrolysis
Both form complexes
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Differences
Be is group 2, Al is group 13
Be forms +2 ions, Al forms +3 ions
Different chemical reactivity patterns
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Boron Family (Group 13)
Elements
Boron (B), Aluminium (Al), Gallium (Ga)
Indium (In), Thallium (Tl)
Boron is non-metal, others are metals
Electronic Configuration
General: [Noble gas] ns² np¹
B: [He] 2s² 2p¹, Al: [Ne] 3s² 3p¹
Ga: [Ar] 3d¹⁰ 4s² 4p¹
In: [Kr] 4d¹⁰ 5s² 5p¹
Tl: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
Occurrence
B: Borax, Colemanite, Boric acid
Al: Bauxite, Cryolite, Corundum
Ga, In, Tl: Occur in trace amounts
Physical Properties
| Property | Trend | Notes |
|---|---|---|
| Atomic Radius | Increases down the group | Ga < Al due to poor shielding of d-electrons |
| Melting Point | Decreases B to Ga, then increases | B has very high m.p. due to covalent network |
| Boiling Point | Decreases down the group | Regular trend |
| Ionization Energy | Decreases down the group | Ga > Al due to smaller size |
| Oxidation State | +3 (common), +1 (Tl, In due to inert pair effect) | B shows only +3 oxidation state |
Chemical Properties
Reaction with Oxygen
Form oxides M₂O₃
B₂O₃ - acidic, Al₂O₃ - amphoteric
Ga₂O₃, In₂O₃, Tl₂O₃ - basic
4B + 3O₂ → 2B₂O₃
4Al + 3O₂ → 2Al₂O₃
Reaction with Water
B - no reaction
Al - reacts to form protective oxide layer
Ga, In - not affected
Tl - forms hydroxide layer
Reaction with Halogens
Form trihalides MX₃
Boron halides are covalent Lewis acids
AlCl₃ exists as dimer Al₂Cl₆
2B + 3X₂ → 2BX₃
2Al + 3X₂ → 2AlX₃
Anomalous Behavior of Boron
Differences from Other Group 13 Elements
- Non-metal while others are metals
- Very high melting and boiling points
- Forms only covalent compounds
- Forms electron-deficient compounds (boranes)
- Trihalides exist as monomers
- Oxides and hydroxides are acidic
- Does not decompose steam
Diagonal Relationship: B with Si
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Similarities
Both are non-metals
High melting points
Form covalent compounds
Form volatile hydrides
Chlorides are hydrolyzed by water
Form weak acids (H₃BO₃ and H₂SiO₃)
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Differences
B is group 13, Si is group 14
B forms +3 compounds, Si forms +4 compounds
Different chemical reactivity patterns
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Carbon Family (Group 14)
Elements
Carbon (C), Silicon (Si), Germanium (Ge)
Tin (Sn), Lead (Pb)
C, Si - non-metals, Ge - metalloid, Sn, Pb - metals
Electronic Configuration
General: [Noble gas] ns² np²
C: [He] 2s² 2p², Si: [Ne] 3s² 3p²
Ge: [Ar] 3d¹⁰ 4s² 4p²
Sn: [Kr] 4d¹⁰ 5s² 5p²
Pb: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p²
Occurrence
C: Diamond, Graphite, Carbonates
Si: Silica, Silicates (second most abundant)
Ge: Traces in coal
Sn: Cassiterite (SnO₂)
Pb: Galena (PbS)
Physical Properties
| Property | Trend | Notes |
|---|---|---|
| Atomic Radius | Increases down the group | Regular trend |
| Melting Point | Decreases down the group | C has very high m.p. |
| Density | Increases down the group | Regular trend |
| Ionization Energy | Decreases down the group | C has highest I.E. |
| Oxidation State | +4 (common), +2 (Sn, Pb due to inert pair effect) | C shows only +4 oxidation state |
Chemical Properties
Reaction with Oxygen
Form oxides MO and MO₂
CO₂ - acidic, SiO₂ - acidic
GeO₂ - weakly acidic, SnO₂, PbO₂ - amphoteric
C + O₂ → CO₂
Si + O₂ → SiO₂
Reaction with Water
C - no reaction
Si - reacts with steam at redness
Ge, Sn, Pb - not affected
Si + 2H₂O → SiO₂ + 2H₂
Reaction with Halogens
Form tetrahalides MX₄
PbI₄ and PbBr₄ do not exist
CCl₄ is prepared from hydrocarbons
C + 2Cl₂ → CCl₄
Si + 2Cl₂ → SiCl₄
Anomalous Behavior of Carbon
Differences from Other Group 14 Elements
- Maximum catenation property
- Forms pπ-pπ multiple bonds
- CO₂ is gas while others are solids
- Not affected by alkalies
- Diamond is hardest known substance
- Has highest melting point in the group
Catenation
Catenation is the tendency of an element to form chains or rings by bonding with atoms of the same element. Carbon shows maximum catenation due to strong C-C bonds.
Bond Energies: C-C (348 kJ/mol) > Si-Si (180 kJ/mol)
Catenation Trend
- C: Forms chains of any length
- Si, Ge: Maximum 6 atoms in chain
- Sn, Pb: Maximum 1-2 atoms in chain
Nitrogen Family (Group 15)
Elements
Nitrogen (N), Phosphorus (P), Arsenic (As)
Antimony (Sb), Bismuth (Bi)
N, P - non-metals, As, Sb - metalloids, Bi - metal
Electronic Configuration
General: [Noble gas] ns² np³
N: [He] 2s² 2p³, P: [Ne] 3s² 3p³
As: [Ar] 3d¹⁰ 4s² 4p³
Sb: [Kr] 4d¹⁰ 5s² 5p³
Bi: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p³
Occurrence
N: Atmosphere (78%), Nitrates
P: Phosphorite, Apatite
As, Sb, Bi: Occur as sulphides
Physical Properties
| Property | Trend | Notes |
|---|---|---|
| Atomic Radius | Increases down the group | Regular trend |
| Melting Point | Increases N to As, then decreases | Irregular trend |
| Ionization Energy | Decreases down the group | N has highest I.E. |
| Electronegativity | Decreases down the group | N has highest electronegativity |
| Oxidation State | -3 to +5 | +3 stability increases down the group |
Chemical Properties
Reaction with Oxygen
Form oxides N₂O, NO, N₂O₃, NO₂, N₂O₅
P forms P₄O₆ and P₄O₁₀
As, Sb, Bi form M₂O₃ and M₂O₅
N₂ + O₂ → 2NO
P₄ + 5O₂ → P₄O₁₀
Reaction with Water
N - no reaction
P - reacts with steam to form phosphoric acid
As, Sb, Bi - no reaction
Reaction with Halogens
Form trihalides and pentahalides
N does not form pentahalides
PCl₅ exists but PI₅ does not
2P + 3Cl₂ → 2PCl₃
2P + 5Cl₂ → 2PCl₅
Anomalous Behavior of Nitrogen
Differences from Other Group 15 Elements
- Diatomic gas while others are solids
- Forms pπ-pπ multiple bonds
- Does not form pentahalides
- Does not show pentacovalency
- Hydride (NH₃) shows hydrogen bonding
- Forms five oxides
- Does not form complexes
Oxygen Family (Group 16)
Elements
Oxygen (O), Sulphur (S), Selenium (Se)
Tellurium (Te), Polonium (Po)
O, S - non-metals, Se, Te - metalloids, Po - metal
Electronic Configuration
General: [Noble gas] ns² np⁴
O: [He] 2s² 2p⁴, S: [Ne] 3s² 3p⁴
Se: [Ar] 3d¹⁰ 4s² 4p⁴
Te: [Kr] 4d¹⁰ 5s² 5p⁴
Po: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴
Occurrence
O: Atmosphere (21%), Oxides, Water
S: Sulphides, Sulphates
Se, Te: Occur as selenides and tellurides
Physical Properties
| Property | Trend | Notes |
|---|---|---|
| Atomic Radius | Increases down the group | Regular trend |
| Melting/Boiling Point | Increases down the group | O₂ is gas, others are solids |
| Ionization Energy | Decreases down the group | O has high I.E. |
| Electronegativity | Decreases down the group | O has highest electronegativity |
| Oxidation State | -2, +2, +4, +6 | O shows -2, -1, +2 oxidation states |
Chemical Properties
Reaction with Oxygen
O - forms O₂ and O₃
S, Se, Te form MO₂ and MO₃
SO₂ - acidic, SeO₂ - acidic, TeO₂ - amphoteric
S + O₂ → SO₂
2SO₂ + O₂ → 2SO₃
Reaction with Water
O - slightly soluble
S - insoluble
Se, Te - no reaction
Reaction with Halogens
Form various halides
O forms OF₂, O₂F₂
S forms S₂F₂, SF₂, SF₄, SF₆
Se, Te form SeF₆, TeF₆
S + 3F₂ → SF₆
Anomalous Behavior of Oxygen
Differences from Other Group 16 Elements
- Diatomic gas while others are solids
- Shows -2, -1, +2 oxidation states
- Forms hydrogen bonds
- Maximum covalency of 2
- Paramagnetic
- Compounds are mainly ionic and polar covalent
Ozone (O₃)
Ozone is an allotrope of oxygen with a bent structure (bond angle 116.8°). It is a powerful oxidizing agent and is formed in the upper atmosphere by the action of UV radiation on oxygen.
3O₂ ⇌ 2O₃ (ΔH = +285.4 kJ)
Uses of Ozone
- Disinfecting drinking water
- Purifying air in crowded places
- Manufacture of KMnO₄, artificial silk, synthetic camphor
- Bleaching agent
Important Trends and Relationships
Periodic Trends
Atomic radius increases down the group
Ionization energy decreases down the group
Electronegativity decreases down the group
Metallic character increases down the group
Reactivity varies with group and period
Diagonal Relationships
Li and Mg
Be and Al
B and Si
Due to similar charge/size ratio
Anomalous Behaviors
| Element | Group | Anomalous Properties |
|---|---|---|
| Lithium | 1 | Harder, higher m.p., forms nitride, covalent compounds |
| Beryllium | 2 | Amphoteric, covalent compounds, forms complexes |
| Boron | 13 | Non-metal, high m.p., electron-deficient compounds |
| Carbon | 14 | Maximum catenation, forms multiple bonds |
| Nitrogen | 15 | Diatomic gas, forms multiple bonds, no pentavalency |
| Oxygen | 16 | Diatomic gas, paramagnetic, forms hydrogen bonds |
JEE Main Weightage
This chapter typically carries 3-4 questions in JEE Main, covering properties, trends, anomalous behaviors, and important compounds of s and p-block elements.
Chapter Weightage in JEE Main
Weightage
High (3-4 questions)
Important Topics for JEE Main
- Anomalous behaviors of first elements
- Diagonal relationships
- Important compounds and their uses
- Trends in physical and chemical properties
- Preparation and properties of important compounds
- Oxidation states and their stability