Class 12 Chemistry · Chapter 4
Short answer:
Ye chapter NCERT Chemistry Part I ka Chapter 4 hai — current rationalised syllabus me directly d-block se shuru hota hai (metallurgy wala purana chapter hata diya gaya hai). Textbook me 30 exercise questions hain; yahan 18 important cover kiye gaye hain — electronic configuration likhna, Cr/Cu ki anomalous configuration, Zn/Cd/Hg transition element kyun nahi hain, Mn2+ vs Fe2+ stability, colour aur magnetic moment ka reasoning, KMnO4 aur K2Cr2O7 ki preparation-properties, aur lanthanoid contraction ke consequences.
d- aur f-block elements wahi elements hain jinke atoms (ya unke common ions) me (n-1)d ya (n-2)f orbitals partially filled hote hain. Ye "transition" elements isliye kehlate hain kyunki inki properties s-block (bahut reactive metals) aur p-block (kam metallic) ke beech ek transition dikhati hain — variable oxidation states, coloured compounds, catalytic activity, aur alloy banane ki tendency in sabki pehchaan hai.
Chapter 4 Summary — 5 Minute Revision
1. d-Block me kaun aata hai
Periodic table ke groups 3 se 12 tak — 4 series: 3d (Sc–Zn), 4d (Y–Cd), 5d (La/Lu–Hg), 6d (Ac/Lr–Cn, incomplete). General electronic configuration: (n-1)d1-10 ns0-2.
Transition element ki definition: jiska atom ya common ion (n-1)d orbital partially filled rakhta hai (d1 se d9 tak, dono ends exclude).
| Element | Z | Ground state configuration |
|---|---|---|
| Sc | 21 | [Ar] 3d1 4s2 |
| Ti | 22 | [Ar] 3d2 4s2 |
| V | 23 | [Ar] 3d3 4s2 |
| Cr | 24 | [Ar] 3d5 4s1 (exception — half-filled d aur s dono stable) |
| Mn | 25 | [Ar] 3d5 4s2 |
| Fe | 26 | [Ar] 3d6 4s2 |
| Co | 27 | [Ar] 3d7 4s2 |
| Ni | 28 | [Ar] 3d8 4s2 |
| Cu | 29 | [Ar] 3d10 4s1 (exception — fully-filled d extra stable) |
| Zn | 30 | [Ar] 3d10 4s2 |
↔ Table ko side me swipe karein
Cr aur Cu me exception isliye aata hai kyunki half-filled (d5) aur fully-filled (d10) configurations symmetrical electron distribution ki wajah se extra stable hote hain — inme exchange energy zyada hoti hai.
2. Zn, Cd, Hg transition elements kyun NAHI hain
Zn, Cd, Hg ka ground state configuration (n-1)d10 ns2 hai aur inke common ions (Zn2+, Cd2+, Hg2+) me bhi d10 fully-filled configuration rehta hai — kabhi partially filled d nahi hota. Isliye definition follow nahi karte aur inhe transition elements nahi maana jaata (inhe group 12 / d-block ka "non-transition" hissa kehte hain).
3. General trends
- Metallic character: sabhi d-block elements metals hain — hard, high melting/boiling point, good conductors — kyunki d-electrons bhi metallic bonding me participate karte hain (strong metallic bonding).
- Atomic aur ionic radii: ek series me left se right jaate hue radius pehle ghatta hai (increasing nuclear charge), phir beech me roughly constant ho jaata hai (d-electrons ka screening s-electrons ke against balance kar deta hai), aur end me thoda badhta hai (electron-electron repulsion d10 config me).
- Ionisation enthalpy: ek series me left se right general trend badhne ka hai (irregular, kyunki d-orbital half-filled/fully-filled stability beech-beech me values disturb karti hai) — s-block se zyada, p-block se kam.
- Oxidation states: variable oxidation states dikhate hain kyunki (n-1)d aur ns electrons ki energy close hoti hai, dono bonding me part le sakte hain. Group 7 (Mn) tak highest oxidation state group number ke barabar badhti hai (Mn me +7, KMnO4 me), uske baad ghatne lagti hai.
- Colour: d-block compounds me colour isliye dikhta hai kyunki unpaired d-electrons d-d transition kar sakte hain — ek lower energy d-orbital se higher energy d-orbital me electron jump karta hai visible light absorb karke, aur jo colour absorb nahi hota wahi complementary colour hume dikhta hai. Jitne zyada unpaired electrons aur jitna zyada crystal field splitting, utna intense colour. d0 aur d10 ions (jaise Sc3+, Zn2+) colourless hote hain kyunki d-d transition possible hi nahi.
- Magnetic properties: unpaired d-electrons ki wajah se paramagnetic hote hain. Magnetic moment: μ = √[n(n+2)] BM, jahan n = unpaired electrons ki sankhya. d0 aur d10 species diamagnetic hote hain.
- Catalytic property: variable oxidation states ki wajah se ye reactants ke saath asaani se intermediate compound bana lete hain jisse reaction ka alternate path milta hai jiski activation energy kam hoti hai — jaise V2O5 (Contact process), Fe (Haber process), Ni (hydrogenation).
- Alloy formation: ek series ke transition metals ka atomic size similar hota hai, isliye ek metal ke atoms doosre metal ki crystal lattice me easily replace ho jaate hain — isse alloys (jaise steel, brass) bante hain jo pure metals se zyada hard aur high melting point wale hote hain.
4. Mn2+ vs Fe2+ ki stability
Mn2+ (3d5) oxidation ke against stable hai kyunki iska configuration half-filled hai — half-filled d-subshell symmetrical distribution ki wajah se extra stable hoti hai, isliye Mn2+ ko Mn3+ (3d4) me oxidise karna mushkil hai.
Fe2+ (3d6) easily oxidise ho jaata hai Fe3+ (3d5) me kyunki Fe3+ ka half-filled d5 configuration Fe2+ ke d6 se zyada stable hai. Isliye Fe2+ salts (jaise FeSO4) air me dheere-dheere Fe3+ me oxidise ho jaate hain.
5. KMnO4 (Potassium permanganate)
Preparation (pyrolusite MnO2 se):
2MnO2 + 4KOH + O2 → 2K2MnO4 (green, potassium manganate) + 2H2O
3MnO42− + 4H+ → 2MnO4− (purple) + MnO2 + 2H2O (disproportionation — neutral/acidic medium)
Alternatively K2MnO4 ko chlorine se bhi oxidise kar sakte hain:
2K2MnO4 + Cl2 → 2KMnO4 + 2KCl
Properties: KMnO4 strong oxidising agent hai, medium ke hisaab se alag-alag products deta hai:
Acidic: MnO4− + 8H+ + 5e− → Mn2+ + 4H2O (E° = +1.51 V)
Neutral / faintly alkaline: MnO4− + 2H2O + 3e− → MnO2 + 4OH−
Strongly alkaline: MnO4− + e− → MnO42−
Acidic medium me use: I− → I2, Fe2+ → Fe3+, oxalate → CO2, SO2 → SO42−, H2S → S ko oxidise karne me hota hai. Example:
2MnO4− + 5C2O42− + 16H+ → 2Mn2+ + 10CO2 + 8H2O
6. K2Cr2O7 (Potassium dichromate)
Preparation (chromite ore FeCr2O4 se):
4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 (yellow) + 2Fe2O3 + 8CO2
2Na2CrO4 + 2H+ → Na2Cr2O7 (orange) + 2Na+ + H2O
Na2Cr2O7 + 2KCl → K2Cr2O7 (crystallises, less soluble) + 2NaCl
Chromate–dichromate equilibrium (pH-dependent):
2CrO42− (yellow) + 2H+ ⇌ Cr2O72− (orange) + H2O
Yellow CrO42− alkaline medium me stable hai, orange Cr2O72− acidic medium me.
Oxidising action (acidic medium):
Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O (E° = +1.33 V)
Iodide ko iodine me, Fe2+ ko Fe3+ me, aur H2S ko S me oxidise karta hai.
7. f-Block elements: Lanthanoids aur Actinoids
- Lanthanoids: La (57) ke baad 4f orbital fill hone wale 14 elements (Ce–Lu, Z=58–71). General configuration: [Xe] 4f1-14 5d0-1 6s2.
- Actinoids: Ac (89) ke baad 5f orbital fill hone wale 14 elements (Th–Lr, Z=90–103). General configuration: [Rn] 5f1-14 6d0-1 7s2.
- Dono series me sabse common oxidation state +3 hai, lekin actinoids +3 se +7 tak zyada wide range of oxidation states dikhate hain kyunki 5f, 6d, aur 7s orbitals ki energies bahut close hoti hain.
8. Lanthanoid contraction
Lanthanoid contraction matlab La se Lu tak atomic aur ionic radii me steady, gradual decrease. Iska karan hai 4f electrons ka imperfect (poor) shielding effect — jaise-jaise atomic number badhta hai, ek extra proton nucleus me add hota hai aur ek extra electron 4f subshell me jaata hai, lekin 4f electrons apas me ek-doosre ko poorly shield karte hain (diffuse shape ki wajah se), isliye effective nuclear charge outer electrons pe steadily badhta jaata hai aur radius ghatta jaata hai.
Consequences:
- Second (4d) aur third (5d) transition series ke corresponding elements (jaise Zr–Hf, Nb–Ta) ka atomic radius almost same ho jaata hai — inki chemistry bahut similar ho jaati hai aur inhe separate karna mushkil hota hai.
- Lanthanoids ko ek-doosre se separate karna bhi mushkil hai kyunki radii bahut close hoti hain — inke properties bahut similar ho jaate hain.
- Basicity of lanthanoid hydroxides La(OH)3 se Lu(OH)3 tak steadily ghatti hai (size chota, covalent character badhta hai).
Actinoid contraction lanthanoid contraction se per-element zyada hoti hai kyunki 5f orbitals 4f se bhi zyada poorly shield karte hain.
Poore Class 12 Chemistry ke handwritten colour notes
IITian & district toppers ke banaye short notes — revision-ready, diagram ke saath. Board se pehle poora syllabus 3 din me revise.
Exercise Questions — Solutions (Q1–Q18)
Q1. Silver ka atomic number 47 hai. Isse transition element kyu maana jaata hai jabki iska d-orbital ground state me poora bhara hua hai (4d10 5s1)?
Silver ko transition element isliye maana jaata hai kyunki iska common oxidation state Ag2+ (jo laboratory conditions me bhi bana sakta hai) me d-orbital partially filled (4d9) ho jaata hai. Definition me "atom ya ions me se koi bhi" partially filled d-subshell rakhe to element transition element hai — sirf ground-state atom dekhna zaroori nahi.
Q2. In-shell configurations likho: Cr3+, Mn2+, Fe2+, Ni2+.
Cr3+ (Z=24, 3 electrons removed) = [Ar] 3d3
Mn2+ (Z=25) = [Ar] 3d5
Fe2+ (Z=26) = [Ar] 3d6
Ni2+ (Z=28) = [Ar] 3d8
Note: transition metal ions banate waqt pehle ns electrons remove hote hain, phir (n-1)d.
Q3. E° (M2+/M) values 3d series me negative se positive ki taraf general trend dikhate hain (Cr aur Cu ko chhodkar). Kyu?
E°(M2+/M) atomic number badhne ke saath kam negative (yaani zyada positive) hota jaata hai kyunki sum of enthalpy of sublimation + ionisation enthalpy + hydration enthalpy ka combined effect badhta hai. Cu ka value sabse positive hai (Cu high sublimation enthalpy aur low hydration enthalpy ki wajah se easily oxidise nahi hota — isiliye Cu dilute acids se H2 gas release nahi karta).
Q4. Zn, Cd, Hg ko transition elements kyu nahi maana jaata, jabki ye d-block me hote hain?
Zn, Cd, Hg ka ground state configuration (n-1)d10 ns2 hai aur inke common ions (Zn2+, Cd2+, Hg2+) me bhi d10 fully-filled hi rehta hai — kabhi partially filled nahi hota. Transition element ki definition (partially filled d-orbital atom ya ion me) inpar apply nahi hoti, isliye ye d-block ka hissa hote hue bhi transition elements nahi hain.
Q5. Cr (Z=24) aur Cu (Z=29) ki electronic configuration me exception kyu hai?
Cr: [Ar] 3d5 4s1 (3d44s2 ke bajaye) — kyunki half-filled d5 aur half-filled s1 combined symmetrical arrangement extra stability deta hai (exchange energy zyada hoti hai).
Cu: [Ar] 3d10 4s1 (3d94s2 ke bajaye) — kyunki fully-filled d10 configuration extra stable hai.
Q6. Mn2+ aur Fe2+ me se kaun oxidation ke against zyada stable hai aur kyun?
Mn2+ (3d5) zyada stable hai kyunki iska half-filled configuration hai. Iske contrast me, Fe2+ (3d6) easily oxidise hokar Fe3+ (3d5, half-filled) ban jaata hai kyunki Fe3+ zyada stable configuration hai.
Q7. d-block elements coloured compounds kyu banate hain? Sc3+ aur Zn2+ colourless kyu hote hain?
Colour d-d transitions se aata hai — unpaired d-electron ek lower-energy d-orbital se higher-energy d-orbital me visible light photon absorb karke jump karta hai, aur jo wavelength absorb nahi hoti wo transmitted/reflected hokar hume dikhti hai (complementary colour).
Sc3+ (d0) aur Zn2+ (d10) colourless hain kyunki inme d-d transition possible hi nahi — d0 me electron hi nahi, d10 me har orbital already fully occupied hai.
Q8. Magnetic moment calculate karo Fe3+ ion ke liye (3d5, 5 unpaired electrons).
μ = √[n(n+2)] BM, n = 5
μ = √[5(5+2)] = √35 ≈ 5.92 BM
Q9. Transition metals achhe catalysts kyu hote hain?
Variable oxidation states aur unfilled d-orbitals ki wajah se transition metals reactants ke saath asaani se surface bonds ya intermediate compounds bana lete hain, jisse reaction ka ek alternate lower-activation-energy path mil jaata hai — jaise V2O5 Contact process me, finely divided Fe Haber process me, Ni hydrogenation reactions me.
Q10. Alloys transition metals hi zyada kyu banate hain?
Ek hi series ke transition metals ka atomic size lagbhag similar hota hai, isliye ek metal ke atoms doosre metal ki crystal lattice me atoms replace kar sakte hain bina structure disturb kiye. Result alloy pure metal se zyada hard aur high melting point wala hota hai — jaise steel (Fe+C+other metals), brass (Cu+Zn).
Q11. KMnO4 ki industrial preparation likho, pyrolusite ore se shuru karke.
Step 1 — MnO2 ko KOH ke saath fuse karke air/O2 present me green K2MnO4 banate hain:
2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O
Step 2 — K2MnO4 ko neutral ya acidic medium me electrolytically ya Cl2 se oxidise karke purple KMnO4 milta hai:
3MnO42− + 4H+ → 2MnO4− + MnO2 + 2H2O
Q12. KMnO4 acidic medium me oxidising agent ke roop me kaam karta hai — half-reaction likho aur ek example do.
MnO4− + 8H+ + 5e− → Mn2+ + 4H2O (E° = +1.51 V)
Example — oxalate ions ka oxidation:
2MnO4− + 5C2O42− + 16H+ → 2Mn2+ + 10CO2 + 8H2O
Q13. K2Cr2O7 ki preparation chromite ore se likho.
Step 1 — chromite ore Na2CO3 ke saath air presence me fuse karte hain:
4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2
Step 2 — yellow filtrate ko acidify karke orange dichromate me convert karte hain:
2Na2CrO4 + 2H+ → Na2Cr2O7 + 2Na+ + H2O
Step 3 — sodium dichromate ko KCl ke saath treat karte hain, kam soluble K2Cr2O7 crystallise ho jaata hai:
Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
Q14. Chromate aur dichromate ions ke beech equilibrium likho aur batao ye pH pe kaise depend karta hai.
2CrO42− (yellow) + 2H+ ⇌ Cr2O72− (orange) + H2O
Alkaline medium me chromate (yellow) stable hai; acidic medium me equilibrium dichromate (orange) ki taraf shift ho jaata hai.
Q15. Lanthanoid contraction kya hai aur iska cause kya hai?
Lanthanoid contraction matlab La se Lu tak atomic aur ionic radii me steady, gradual decrease. Cause hai 4f electrons ka imperfect shielding — 4f orbitals ka diffuse shape hone ki wajah se ek-doosre ko poorly shield karte hain, isliye har naye proton ke saath effective nuclear charge outer electrons par steadily badhta jaata hai.
Q16. Lanthanoid contraction ke do consequences batao.
- Second aur third transition series ke corresponding elements (jaise Zr aur Hf) ka radius almost same ho jaata hai, isliye inki chemistry bahut similar hoti hai aur separation mushkil hota hai.
- Lanthanoids ka radius apas me itna close hota hai ki inhe ek-doosre se separate karna mushkil hai.
Q17. Actinoid contraction, lanthanoid contraction se zyada kyu hoti hai?
5f orbitals 4f orbitals se bhi zyada diffuse hote hain, isliye unka shielding effect aur bhi poor hota hai — is wajah se actinoids me per-element radius decrease (actinoid contraction) lanthanoid contraction se zyada hota hai.
Q18. Actinoids lanthanoids ke comparison me zyada oxidation states kyu dikhate hain?
Actinoids me 5f, 6d aur 7s orbitals ki energies bahut close (comparable) hoti hain, isliye electrons in teeno se bonding me participate kar sakte hain — is wajah se actinoids +3 se lekar +7 tak (jaise U me +6, Np/Pu me +7 tak) wide range of oxidation states dikhate hain, jabki lanthanoids mostly +3 tak hi simit rehte hain.
Important Equations — Ek Nazar Me
| Concept | Detail |
|---|---|
| General configuration (d-block) | (n-1)d1-10 ns0-2 |
| General configuration (Lanthanoids) | [Xe] 4f1-14 5d0-1 6s2 |
| General configuration (Actinoids) | [Rn] 5f1-14 6d0-1 7s2 |
| Cr configuration (exception) | [Ar] 3d5 4s1 |
| Cu configuration (exception) | [Ar] 3d10 4s1 |
| Magnetic moment | μ = √[n(n+2)] BM, n = unpaired electrons |
| Highest oxidation state trend | Group number tak badhti hai (Mn, group 7, me max +7), phir ghatti hai |
| Most stable +2 state | Mn2+ (d5, half-filled) sabse stable hai 3d series me |
| KMnO4 E° (acidic) | MnO4− + 8H+ + 5e− → Mn2+ + 4H2O, E° = +1.51 V |
| K2Cr2O7 E° (acidic) | Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O, E° = +1.33 V |
| Lanthanoid contraction magnitude | La (103 pm) se Lu (~86 pm) tak steady decrease (ionic radii, approx values) |
| Lanthanoid vs actinoid contraction | Actinoid contraction (5f) per element lanthanoid contraction (4f) se zyada hai — 5f shielding zyada poor hai |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Zn, Cd, Hg ko transition elements bol dena. In teeno ka ground state aur common ions dono me d-subshell fully filled (d10) rehta hai — kabhi partially filled nahi hota, isliye d-block me hote hue bhi transition elements NAHI hain.
- Cr ki electronic configuration [Ar] 3d4 4s2 likh dena. Sahi configuration [Ar] 3d5 4s1 hai — half-filled d aur s dono ki extra stability ki wajah se ek electron s se d me shift ho jaata hai.
- Cu ki electronic configuration [Ar] 3d9 4s2 likh dena. Sahi configuration [Ar] 3d10 4s1 hai — fully-filled d10 extra stable hai.
- Mn2+ ko unstable aur Fe2+ ko stable bol dena (ulta). Sahi ye hai: Mn2+ (d5, half-filled) stable hai; Fe2+ (d6) easily oxidise hokar zyada stable Fe3+ (d5) ban jaata hai.
- Lanthanoid contraction aur actinoid contraction ko same cause bata dena bina fark bataye. Dono hi f-electrons ke imperfect shielding se hote hain, lekin actinoid contraction (5f shielding poorer) lanthanoid contraction (4f) se magnitude me zyada hoti hai — ye difference likhna zaroori hai.
- d-block compounds ka colour 'transition metal hone' se explain kar dena, d-d transition ka mechanism na likhna. Answer me specifically likhna chahiye ki unpaired d-electrons visible light absorb karke lower se higher energy d-orbital me transition karte hain — sirf 'transition metal hai isliye coloured hai' likhna incomplete answer maana jaata hai.
Board-Style Important Questions
- 1 mark: Zn ko transition element kyu nahi maana jaata?
- 1 mark: Cr ki electronic configuration likho aur exception ka reason batao.
- 2 marks: Mn2+ compounds Fe2+ compounds se zyada stable kyu hote hain oxidation ke against?
- 2 marks: Transition metal compounds coloured kyu hote hain? d0 aur d10 ions ka example dete hue explain karo.
- 3 marks: KMnO4 ki preparation pyrolusite se likho aur iska ek oxidising reaction acidic medium me likho.
- 3 marks: Lanthanoid contraction kya hai? Iske do consequences likho.
Aksar Poochhe Jaane Wale Sawaal
d aur f-block elements me kya fark hai?
d-block elements me differentiating electron (n-1)d orbital me jaata hai (groups 3-12), jabki f-block elements me (n-2)f orbital me jaata hai (lanthanoids aur actinoids, alag se periodic table ke neeche dikhaye jaate hain).
Zn transition element hai ya nahi?
Nahi. Zn d-block me hai lekin transition element nahi hai kyunki iska ground state aur Zn2+ ion dono me d-subshell fully filled (d10) rehta hai, kabhi partially filled nahi hota.
KMnO4 aur K2Cr2O7 dono strong oxidising agents kyu hain?
Dono me Mn (+7) aur Cr (+6) apni highest possible oxidation states me hain — isliye ye readily electrons accept karke lower oxidation state me reduce ho jaate hain, jo inhe strong oxidising agent banata hai.
Colour sirf transition metals me hi kyu dikhta hai?
Colour ke liye unpaired d-electrons chahiye jo d-d transition kar sakein. s-block aur zyadatar p-block elements me ya to d-orbitals hote hi nahi ya fully filled/empty hote hain, isliye typically colourless hote hain.
Lanthanoid contraction ka periodic table pe practical effect kya hai?
Iski wajah se second aur third transition series ke elements (jaise Zr-Hf, Nb-Ta) ka size almost same ho jaata hai, jisse unki chemistry bahut similar ho jaati hai aur unhe alag karna mushkil ho jaata hai.
Actinoids lanthanoids se zyada radioactive kyu hote hain?
Actinoids me heavy nuclei hote hain jinka proton-neutron ratio unstable hota hai — ye is chapter ka core topic nahi hai, lekin general awareness ke liye: sabhi actinoids radioactive hote hain jabki lanthanoids me sirf Promethium (Pm) radioactive hai.
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