Classification of Elements and Periodicity in Properties: Class 11 Chemistry Chapter 3 Hinglish mein

Ek bheed wale medical store mein jao aur cough syrup maango. Pharmacist har shelf nahi dhoondhta: syrups ek saath rakhe hote hain, ek tarah ki tablets ek saath, aur naya brand seedha apni jagah par chala jaata hai. Periodic table chemistry ke liye yahi kaam karti hai. Yeh ek hi chart hai jo batata hai ki har element kahan belong karta hai, kaun se elements ek jaisa behave karte hain, aur jo element tumne kabhi handle nahi kiya woh lagbhag kaise behave karega.

Yeh chapter table ko uske pehle kachche drafts se lekar tumhari book mein chhape long form tak follow karta hai, aur dikhata hai ki iska shape electronic configuration se kyun aata hai. Phir table ki madad se samjhata hai ki size, ionization enthalpy, electron gain enthalpy, electronegativity, valence aur reactivity ek element se doosre element tak kaise badalte hain. Chemistry ke lagbhag har aage ke chapter mein yeh trends kaam aate hain.

3.1 Elements ko classify karne ki zaroorat kyun hai?

Saal 1800 ke aas-paas chemists lagbhag tees elements jaante the. 1860s tak yeh ginti saath se upar chali gayi, aur aaj 118 elements confirm hain, jinmein sabse heavy wale accelerators mein ek baar mein kuch hi atoms banaye jaate hain. Har element kai compounds banata hai. Sabko ek-ek karke seekhna namumkin hota.

Ek achha classification yeh problem do tarah se solve karta hai. Yeh jo pehle se pata hai use organise karta hai: jab tumhe pata hai ki sodium aur potassium ek jaise hain, to ek ke baare mein koi fact doosre ke baare mein bhi bahut kuch bata deta hai. Yeh jo abhi pata nahi hai use predict bhi karta hai: pattern mein khaali jagah ek aise element ki taraf ishaara karti hai jo abhi milna baaki hai, aur uski properties ka hint bhi deti hai. Yahi do faayde unnisvi sadi mein elements ki ek natural arrangement ki khoj ko aage badhate rahe.

3.2 Periodic Classification ki Shuruaat

Periodic table kisi ek insaan ne invent nahi ki. Kai chemists ne, jo apne zamaane ke atomic weights (relative atomic masses) ke saath kaam kar rahe the, pattern ke alag-alag tukde notice kiye. Unki koshishon ko Fig. 3.1 mein aur neeche ki timeline mein follow karo.

Triads (Döbereiner, 1829). German chemist Johann Döbereiner ne teen-teen similar elements ke aise sets dhoondhe jinmein beech wale member ka atomic weight baaki do ke average ke kareeb tha, aur uski properties bhi dono ke beech mein thi. Sulphur, selenium aur tellurium aisa ek triad banate hain. Yeh idea sirf kuch hi sets par kaam karta tha, isliye zyadatar chemists ne ise ittefaq maana.

Cylinder par spiral (de Chancourtois, 1862). French geologist A. E. B. de Chancourtois ne elements ko atomic weight ke order mein ek cylinder par lipti spiral line ke saath likha. Tab similar elements ek ke upar ek aa gaye. Unka paper kam chemists tak pahuncha aur idea par dhyaan nahi gaya.

Octaves (Newlands, 1865). English chemist John Newlands ne elements ko atomic weight ke hisaab se list kiya aur dekha ki har element apne se saat jagah aage wale element jaisa hai, yaani ginti mein aathve jaisa. Unhone ise music se compare kiya: Sa Re Ga Ma Pa Dha Ni ke baad upar wala Sa aata hai, jo pehle jaisa sunai deta hai. Yeh rule sirf calcium tak chala; uske aage iron jaise metals non-metals ke beech pahunch gaye. Saathi chemists ne is idea ka mazaak udaaya, phir bhi Royal Society ne 1887 mein Newlands ko Davy Medal diya.

Curves aur tables (Lothar Meyer aur Mendeleev, 1868–1870). Germany mein Lothar Meyer ne graph wala raasta liya. Unhone atomic volume, yaani ek mole atoms ka gheraa hua volume, ka atomic weight ke against curve banaya. Yeh har alkali metal par ek tez peak tak chadhta hai aur beech mein neeche aata hai, lehron ki ek line ki tarah, aur heavy elements ke liye lehrein chaudi hoti jaati hain, jo Newlands ke fixed octaves pakad nahi sakte the. Meyer ki apni table 1868 ki hai, lekin Russian chemist Dmitri Mendeleev (1834–1907) ki table 1869 mein pehle chhap gayi, isliye periodic law ka credit aam taur par Mendeleev ko milta hai.

Kaha jaata hai ki Mendeleev ne har element, uska atomic weight aur uski properties ek card par likhi, phir cards ko tab tak rearrange kiya jab tak families line mein nahi aa gayi. Unki table rows mein atomic weight ke hisaab se chalti thi, aur chemically similar elements columns, yaani groups, mein ek ke upar ek the. Ise Meyer ki table se mazboot banaya iske peeche ke evidence ne: ek physical property nahi balki kai chemical properties, sabse zyada har element ke oxide aur hydride ke formulas. Do bold faislon ne ise sabse alag bana diya (Fig. 3.2).

1. Weights se pehle properties. Tellurium (127.6) iodine (126.9) se heavy hai, isliye sakht weight order iodine ko oxygen aur sulphur ke saath aur tellurium ko halogens ke saath rakh deta. Mendeleev ne numbers se zyada chemistry par bharosa karke dono ko swap kar diya, aur shak kiya ki weights galat hain.
2. Unknown ke liye gaps. Jahan koi known element kisi column mein fit nahi hota tha, wahan unhone khaali jagah chhodi aur missing element ko describe kiya. Aluminium ke neeche wale ko unhone eka-aluminium aur silicon ke neeche wale ko eka-silicon kaha (eka Sanskrit mein 'ek' hai). Baad mein gallium (1875) aur germanium (1886) mile, jinki properties unke forecasts ke bahut kareeb thi.

3.3 Modern Periodic Law aur Periodic Table ka Present Form

Mendeleev ne bina yeh jaane kaam kiya ki atom mein kya hota hai. Electrons 1897 mein aur nucleus 1911 mein hi discover hue, isliye woh samjha nahi sakte the ki unka law kyun kaam karta hai, ya tellurium aur iodine jaise kuch pairs weight order kyun nahi maante. Jawab X-rays se aaya.

1913 mein English physicist Henry Moseley ne alag-alag elements se bane targets par tez electrons maare. Har element ne apni characteristic frequencies $\nu$ wali X-rays chhodi. Jab unhone sabse strong line ke liye $\sqrt{\nu }$ ko table mein element ki position ke against plot kiya, to points ek seedhi line par aa gaye; atomic mass ke against nahi aaye (Fig. 3.3). Jo quantity har element se agle tak theek ek se badhti thi, woh thi atomic number $Z$. Yeh nucleus par charge hai, jo protons ki sankhya ke barabar hai aur neutral atom mein electrons ki sankhya ke bhi.

Atomic number ko order banane par purane misfits theek ho jaate hain. Argon ($Z=18$) heavy hone ke bawajood potassium ($Z=19$) se pehle aata hai; cobalt (27) nickel (28) se pehle; tellurium (52) iodine (53) se pehle. Mendeleev ki chemical samajh sahi thi, aur weights kabhi galat nahi the: bas sort karne ke liye woh sahi cheez nahi the.

Agar hum uranium ores mein banne wale neptunium aur plutonium ke traces ko ginein, to nature mein chauraanve elements milte hain; isse heavy har element artificially, reactors ya accelerators mein, banaya gaya hai. Aaj hum periodic law ko kisi gehri cheez ka nateeja maante hain: shells bharne ke saath electronic configurations repeat hote hain, aur configuration chemistry tay karta hai. Section 3.5 is connection ko precise banata hai.

Tumhari book wali table long form hai, jo chemists ke try kiye gaye kai layouts mein, spirals se pyramids tak, sabse zyada use hoti hai. Iske across padho to tum ek period (आवर्त) ke saath chalte ho; neeche padho to tum ek group (वर्ग) ke andar rehte ho, yaani elements ki ek family jinke outer electronic configurations match karte hain. IUPAC ki salah ke hisaab se groups left se right 1 se 18 tak numbered hain; purani books IA, IIB, VIII aur 0 jaise labels use karti hain, isliye dono dekhne ke liye taiyaar raho.

• Saat periods hain. Period number outermost shell ka principal quantum number $n$ hota hai.
• Periods mein 2, 8, 8, 18, 18, 32 aur 32 elements hain. Elements 113 se 118 confirm hone ke baad se period 7 poora hai.
• Period 6 ke chaudah elements (lanthanoids) aur period 7 ke chaudah (actinoids) table ke neeche do alag rows mein chhape jaate hain, taaki main table bahut chaudi na ho.

3.4 Atomic Numbers > 100 wale Elements ka Nomenclature

Naye element ka naam kaun rakhta hai? Paramparik roop se uske discoverers, IUPAC ki approval ke saath. Yeh tab tak chalta hai jab tak do teams ek hi discovery ka daava na karein. 1960s ke aakhir se, USA mein Berkeley aur Soviet Union mein Dubna ki laboratories, dono ne element 105 report kiya. Berkeley ne ise hahnium kaha, Dubna ne nielsbohrium, aur bahas tab tak chalti rahi jab tak 1997 mein IUPAC ne dubnium tay nahi kiya. Aise jhagde aasaani se hote hain, kyunki superheavy elements ek baar mein kuch hi atoms banaye jaate hain, gini-chuni laboratories mein bahut mehnge equipment se, aur kai atoms seconds mein decay ho jaate hain.

IUPAC ka hal ek temporary naam hai jis par koi bahas nahi kar sakta, kyunki yeh bas atomic number ko shabdon mein likhna hai. $Z$ ke har digit ko Table 3.2 se uske root ke roop mein likho (roots Latin aur Greek numbers se aate hain), roots ko jod do aur aakhir mein -ium lagao. Roots ke pehle letters se teen-letter ka symbol banta hai.

• Spelling ke do rules: jab enn nil se pehle aata hai, to ek n hata diya jaata hai (ennil); jab naam bi-ium ya tri-ium par khatam hone wala ho, to i hata diya jaata hai (bium, trium).
• Jaise, element 116 tab tak ununhexium (Uuh) tha jab tak uska naam livermorium (Lv) nahi rakha gaya.

Temporary naam sirf tab tak rehta hai jab tak ek joint IUPAC–IUPAP panel discovery claim ko check nahi kar leta. Phir discoverers ek- ya do-letter symbol ke saath ek permanent naam propose karte hain, jo aksar kisi jagah, laboratory ya scientist ko samman deta hai, aur public comment ke ek period ke baad IUPAC use approve karta hai. 118 tak har element ab is process se guzar chuka hai; aakhri chaar naam 2016 mein confirm hue (Table 3.3).

3.5 Elements ke Electronic Configurations aur Periodic Table

Pichhle chapter ka faayda yahan milta hai. Aufbau order lo, 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, aur har naye element ko pichhle se ek electron zyada do. Har baar jab naya shell khule to nayi row shuru karo, aur har element ko us element ke neeche rakho jiske outer subshells match karte hain. Table ka long form bina kisi aur rule ke ban jaata hai.

(a) Periods mein electronic configurations

Har baar jab electrons naya shell shuru karte hain, naya period shuru hota hai, isliye period number valence shell ke $n$ ke barabar hota hai. Period ki length un subshells mein jaa sakne wale electrons ki sankhya ke barabar hai jo us period mein bharte hain, yaani bhare ja rahe orbitals ki sankhya ka double. Fig. 3.5 mein table ko row by row banao.

• Period 1 ($n=1$): sirf 1s orbital, isliye do elements, hydrogen ($1s^1$) aur helium ($1s^2$).
• Periods 2 aur 3: ek $ns$ aur teen $np$ orbitals, isliye har ek mein aath elements, lithium se neon aur sodium se argon. 3d orbitals energy mein 4s se upar hain, isliye period 3 mein woh use nahi hote.
• Period 4: potassium aur calcium 4s bharte hain; phir scandium ($Z=21$, $3d^{1}4s^2$) se zinc ($Z=30$, $3d^{10}4s^2$) tak ke das elements 3d bharte hain, jo pehli transition series hai; gallium se krypton 4p bharte hain. Nau orbitals, athaarah elements.
• Period 5: rubidium se xenon tak usi pattern mein, aur 4d series yttrium ($Z=39$) se shuru hoti hai. Phir se athaarah.
• Period 6: 6s, 4f, 5d aur 6p, solah orbitals, 32 elements. 4f subshell cerium ($Z=58$) se lutetium ($Z=71$) tak bharta hai: yeh lanthanoids hain.
• Period 7: 7s, 5f, 6d aur 7p, phir se 32 elements, jo group 18 mein oganesson ($Z=118$) par khatam hote hain. Actinium ($Z=89$) ke baad 5f subshell thorium se lawrencium tak bharta hai: yeh actinoids hain, jinmein se zyadatar artificially banaye gaye hain.

Lanthanoids aur actinoids ko alag kyun chhapte hain? Periods 6 aur 7 mein 28 aur elements thoosne se table bahut chaudi ho jaati. Usse bhi zaroori baat yeh hai ki har series ke chaudah members itne ek jaise hain ki sab group 3 ke ek hi column ke haqdaar hote. Unhe do rows mein chhapne se har column similar elements ki ek family bana rehta hai.

(b) Group ke hisaab se electronic configurations

Row ke saath padhne ki jagah column mein neeche padho, to outer configuration same rehta hai jabki $n$ badhta hai. Isliye ek group ke elements mein valence electrons ki sankhya same hoti hai aur woh same tarah se arranged hote hain, aur isi wajah se woh ek jaisa behave karte hain. Group 2 ke alkaline earth metals sab $n{s}^2$ par khatam hote hain (Table 3.4).

Dhyaan do ki group mein neeche jaate hue atomic numbers ek fixed amount se nahi badhte: woh 8, 8, 18, 18 aur 32 se jump karte hain, jo beech ke periods ki lengths hain. Configuration, aur isliye chemistry, atomic number par depend karti hai, isi wajah se modern law atomic number use karta hai, atomic mass nahi.

3.6 Electronic Configurations aur Elements ke Types: s-, p-, d-, f-Blocks

Kyunki aakhri electron jis subshell mein jaata hai wahi element ka column tay karta hai, table natural tareeke se chaar blocks mein bant jaati hai, jinke naam usi subshell par hain. Unke shapes subshell capacities se match karte hain: s ke liye do columns, p ke liye chhe, d ke liye das aur f ke liye chaudah.

Do elements simple rule todte hain. Helium $1s^2$ hai, isliye strictly yeh s-block element hai, lekin iska shell bhara hua hai aur yeh baaki noble gases jitna hi unreactive hai, isliye yeh group 18 ke top par hai. Hydrogen ke paas ek akela 1s electron hai. Group 1 metals ki tarah yeh use khokar $\mathrm{H}{\phantom{X}}^{+}$ bana sakta hai; halogens ki tarah ek electron lekar helium configuration tak pahunch sakta hai aur $\mathrm{H}{\phantom{X}}^{-}$ banata hai. Kyunki yeh kisi bhi family mein theek se fit nahi hota, kai tables ise top par alag rakhti hain.

3.6.1 s-Block Elements

s-block sirf do columns chauda hai: group 1, alkali metals ($n{s}^1$), aur group 2, alkaline earth metals ($n{s}^2$). Sodium ko kerosene mein rakhna padta hai, aur inmein se koi bhi metal rocks ya seawater mein free form mein nahi milta. Inki ionization enthalpies kam hain, isliye yeh apne ek ya do outer electrons aasaani se de dete hain aur $\mathrm{M}{\phantom{X}}^{+}$ ya $\mathrm{M}{\phantom{X}}^{2+}$ ions ban jaate hain. Kisi bhi column mein neeche jaane par atoms bade hote hain, outer electrons aur aasaani se nikalte hain aur metals zyada reactive hote jaate hain; lithium paani mein dheere se fizz karta hai, caesium phat jaata hai. Zyadatar s-block compounds, jinmein common salt bhi hai, ionic hote hain. Lithium aur beryllium, jo chhote pehle members hain, exceptions hain: unke compounds kaafi had tak covalent hote hain.

3.6.2 p-Block Elements

Chhe columns, groups 13 se 18, p-block banate hain, jahan outer configuration $n{s}^{2}n{p}^1$ se $n{s}^{2}n{p}^6$ tak jaata hai. Ise right se padho. Group 18 mein noble gases hain: har valence orbital bhara hua hai, isliye yeh na aasaani se electrons dete hain na lete hain aur mushkil se hi react karte hain. Inke padosi ulte hain. Group 16 ke chalcogens aur group 17 ke halogens full shell se bas thoda peeche hain, do aur ek electron se. Inki electron gain enthalpies strongly negative hain, isliye yeh aasaani se electrons pakad lete hain; yeh sabse important non-metals mein se hain. Block ke andar left jaane par, ya iske kisi bhi column mein neeche jaane par, metallic character badhta hai. s- aur p-blocks milkar main group banate hain, aur inke members representative elements kehlate hain.

3.6.3 d-Block Elements (Transition Elements)

Iron, copper, silver, gold aur platinum sab table ke beech ke das-column wale d-block se aate hain, groups 3 se 12. Purana naam transition elements (संक्रमण तत्व) inki position se aata hai, left ke bahut reactive s-block metals aur right ke groups 13 aur 14 ke kam active metals ke beech. Electronically inhe alag banata hai yeh ki aakhri electron kahan jaata hai: outermost shell mein nahi balki uske neeche ke $(n-1)d$ subshell mein, jisse pattern banta hai $(n-1)d^{1-10}n{s}^{0-2}\mathrm{.}$ Palladium, $4d^{10}5s^0$, mein koi outer s electron hai hi nahi. Har d-block element metal hai. Kai elements kai oxidation states dikhate hain, coloured ions dete hain (blue copper sulphate solution yaad karo), paramagnetic hote hain aur catalysts ka kaam karte hain. Zinc, cadmium aur mercury, jinka $(n-1)d^{10}n{s}^2$ arrangement poora hai, inmein se kam features dikhate hain. Class 12 chemistry mein d-block ka apna alag chapter hai.

3.6.4 f-Block Elements (Inner-Transition Elements)

Table ke neeche chhapi do rows f-block banati hain. Yahan aane wala electron bahar se do shells andar, 4f ya 5f mein, jaata hai, isliye outer pattern $(n-2)f^{1-14}(n-1)d^{0-1}n{s}^2$ hota hai aur in elements ko inner-transition elements kehte hain. Pehli row, cerium ($Z=58$) se lutetium ($Z=71$), lanthanoids hai; doosri, thorium ($Z=90$) se lawrencium ($Z=103$), actinoids hai. Kyunki jode ja rahe electrons itne andar dabe hain, har series ke andar padosi kamaal ke ek jaise hain, aur sab metals hain. Har actinoid radioactive hai. Shuruaati actinoids kai oxidation states dikhate hain, jisse unki chemistry lanthanoids se zyada rich hai. Kai baad wale sirf nanogram ya usse bhi kam amounts mein banaye gaye hain, isliye unki chemistry ke baare mein kam pata hai. Uranium ($Z=92$) se aage ke elements transuranium elements kehlate hain.

3.6.5 Metals, Non-metals aur Metalloids

Electrons ke baare mein pata chalne se bahut pehle chemists elements ko metals aur non-metals mein baantte the, aur yeh division aaj bhi important hai. Saare elements mein se teen-chauthai se zyada metals hain, jo table ke left side aur beech mein bhare hain; non-metals iske upar wale right corner mein simte hue hain.

Dono tarah ke elements ko chhookar aur use karke pehchana ja sakta hai. Metal chamakdaar hota hai, heat aur electricity achhe se conduct karta hai, aur peet kar sheets banaya ja sakta hai (malleable, jaise mithai par chaandi ka varq) ya kheench kar wire (ductile, jaise copper). Lagbhag saare metals high melting points wale solids hain; mercury liquid hai, aur gallium (lagbhag 30 °C) aur caesium (lagbhag 28 °C) garam din mein pighal jaate hain. Non-metals room temperature par solids ya gases hain, sirf bromine liquid hai. Yeh aam taur par kam temperatures par melt aur boil hote hain, kharab conduct karte hain, aur solid hone par mudne ki jagah toot jaate hain; boron aur carbon, jinke melting points bahut high hain, exceptions hain.

Ek tarah se doosri tarah ka badlav dheere-dheere hota hai. Period ke across metallic character kam hota hai aur non-metallic character badhta hai; group mein neeche jaane par elements zyada metallic hote hain. p-block mein ek seedhi-daar diagonal line ke saath mixed properties wale elements hain: boron, silicon, germanium, arsenic, antimony aur tellurium (Fig. 3.7). Inhe metalloids (उपधातु) ya semi-metals kehte hain. Jaise silicon metal ki tarah chamakdaar dikhta hai lekin brittle hai aur electricity sirf thodi conduct karta hai, aur yahi cheez ise computer chips aur solar cells mein useful banati hai.

3.7 Elements ki Properties mein Periodic Trends

Period 3 ke saath dekho to reactivity U-shape follow karti hai: sodium zor se react karta hai, silicon mushkil se, aur chlorine phir se zor se. Group mein neeche dekho to direction family par depend karti hai: alkali metals lithium se caesium tak zyada reactive hote jaate hain, halogens fluorine se iodine tak kam reactive. Aise patterns ko atoms ke electrons aur energy levels ke basis par samjhana zaroori hai. Yeh section wahi samjhata hai, pehle isolated atoms ki paanch measure ki ja sakne wali ya estimate ki gayi properties se shuru karke aur phir chemical behaviour par aakar.

3.7.1 Physical Properties mein Trends

Lagbhag kisi bhi physical property ko atomic number ke against plot karo, chahe woh temperature ho jis par element melt hota hai ya use boil ya atomise karne ke liye zaroori energy, aur ek repeat hone wala pattern dikhta hai. Paanch atomic properties zyadatar chemistry samjha deti hain, isliye hum unhe padhte hain. Yeh hain atoms aur ions ke size, ek electron nikalne ke liye zaroori energy, ek electron jodne par energy change, aur shared electrons par atom ka khinchaav.

(a) Atomic radius

Atom khatam kahan hota hai? Uske electron cloud ki koi surface nahi hoti: nucleus se doori ke saath electron milne ka chance bas dheere-dheere kam hota jaata hai (Fig. 3.8 ka pehla step). Atoms lagbhag ${10}^{-10}\mathrm{m}$ ke hote hain, kisi bhi scale ke saamne rakhne ke liye bahut chhote. Isliye chemists kabhi akela atom measure nahi karte. Woh kuch aisa measure karte hain jo sharp hai, yaani molecule ya crystal mein saath bandhe do nuclei ke beech ki doori, jo X-ray diffraction aur spectroscopy bahut precisely dete hain, aur uska aadha radius maante hain.

• Covalent radius, non-metals ke liye: do identical atoms ke beech single bond ki length ka aadha. Iodine mein I–I bond 267 pm lamba hai, isliye iodine ka covalent radius lagbhag 133 pm hai.
• Metallic radius, metals ke liye: metal crystal mein padosi nuclei ke beech ki doori ka aadha. Aluminium mein padosi 286 pm door hain, jisse 143 pm milta hai.
• van der Waals radius: bina bond kiye bas chhoone wale atoms ke beech ki doori ka aadha, jaise solid argon mein. Yeh kaafi bada hota hai, isliye noble gas radii ko covalent radii se compare nahi karna chahiye.

Numbers ek saaf pattern dikhate hain. Period ke across radius chhota hota hai: sodium ke lagbhag 190 pm se chlorine ke lagbhag 100 pm tak. Group mein neeche yeh badhta hai: alkali metal atom lithium ke lagbhag 150 pm se caesium ke lagbhag 265 pm tak jaata hai. Kyun, yeh samajhne ke liye humein effective nuclear charge (प्रभावी नाभिकीय आवेश) ka idea chahiye.

Outer electron nucleus ki taraf attract hota hai lekin baaki har electron se repel hota hai. Inner electrons uske aur nucleus ke beech hote hain aur nuclear charge ke ek hisse ko shield (ya screen) karte hain, isliye outer electron $Z$ se chhota net charge $Z_{\text{eff}}$ feel karta hai. Same shell ke electrons ek doosre ko sirf thoda shield karte hain. Period 2 ke across, lithium se fluorine tak, har step par $Z$ ek badhta hai lekin core wahi do 1s electrons rehta hai, isliye outer electrons lagaatar zyada strong khinchaav feel karte hain aur paas aa jaate hain. Iske ulat lithium se sodium jaane par ek poora naya shell judta hai aur ab das core electrons nucleus ko shield karte hain. Outer electron lagbhag utna hi net charge feel karta hai lekin $n=3$ mein baitha hai, kaafi door (Fig. 3.9).

(b) Ionic radius

Potassium atom (227 pm) ko $\mathrm{K}{\phantom{X}}^{+}$ banao to uska radius 138 pm tak gir jaata hai; chlorine atom (99 pm) ko $\mathrm{C}\mathrm{l}{\phantom{X}}^{-}$ banao to woh 181 pm tak badh jaata hai (Fig. 3.10). Dono badlav ek doosre ke ulte hain. Cation ne ek electron khoya lekin saare protons rakhe, aur yahan woh ek 4s electron hi poora outer shell tha, isliye jo bachta hai woh ek chhota, kas ke pakda hua core hai. Anion ne ek electron liya lekin koi proton nahi, isliye electrons ke beech repulsion badhta hai, har electron nucleus ka thoda kam asar feel karta hai, aur cloud phool jaata hai. Ionic radii khud ionic crystals mein cations aur anions ki spacing se aate hain, aur yeh usual period aur group trends follow karte hain.

Same number of electrons wale atoms aur ions ko isoelectronic species (समइलेक्ट्रॉनी) kehte hain. $\mathrm{S}{\phantom{X}}^{2-}$, $\mathrm{C}\mathrm{l}{\phantom{X}}^{-}$, $\mathrm{K}{\phantom{X}}^{+}$ aur $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ sab mein argon ki tarah 18 electrons hain. Electron count fixed ho to size sirf nuclear charge par depend karta hai: jitne zyada protons, utna tight cloud. Isliye size order hai $\mathrm{S}{\phantom{X}}^{2-}>\mathrm{C}\mathrm{l}{\phantom{X}}^{-}>\mathrm{K}{\phantom{X}}^{+}>\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}:$ zyada negative charge wala anion sabse bada hai, zyada positive charge wala cation sabse chhota.

(c) Ionization enthalpy

Atom apne electrons ko kitna kas ke pakadta hai? Iska measure hai ionization enthalpy (आयनन एन्थैल्पी), ${\mathrm{\Delta }}_{i}H$. Yeh woh energy hai jo ground state mein ek isolated gaseous atom se sabse dheele bandhe electron ko nikalne ke liye chahiye. 'Isolated' aur 'gaseous' ka matlab hai ki koi padosi atom ko nahi kheenchta; 'ground state' ka matlab hai ki har atom same lowest-energy arrangement se shuru karta hai. Tabhi alag elements ki values ko theek se compare kiya ja sakta hai. First ionization enthalpy is process ka enthalpy change hai

aur ise is unit mein likhte hain: $\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\mathrm{.}$ Ion se doosra electron nikalne par second ionization enthalpy milti hai:

Negative electron ko positive nucleus se door kheenchne mein hamesha energy lagti hai, isliye har ionization enthalpy positive hoti hai. Har agli enthalpy pichhli se badi hoti hai: doosre electron ko aise ion se nikalna padta hai jis par pehle se +1 charge hai, teesre ko +2 ion se, aur aise hi aage. Jab tak valence electrons nikal rahe hain, badhat dheere-dheere hoti hai aur phir jab agla electron noble-gas core se nikalna pade to bahut bada jump aata hai (Fig. 3.11). Jab chemists bina kuch aur kahe 'ionization enthalpy' bolte hain, to unka matlab first se hota hai.

Kyunki chhota atom apne electrons ko paas rakhta hai, ionization enthalpy atomic radius ka ulta trend follow karti hai, aur wahi do trends dikhte hain (Fig. 3.12).

• Period ke across ${\mathrm{\Delta }}_{i}H$ aam taur par badhti hai. Naye electrons same shell mein jaate hain, ek doosre ko kam shield karte hain, aur badhta $Z_{\text{eff}}$ feel karte hain, isliye zyada kas ke pakde jaate hain.
• Group mein neeche ${\mathrm{\Delta }}_{i}H$ ghatti hai. Outer electron upar wale shell mein, zyada door, zyada inner electrons ke peeche hota hai, aur yeh effects milkar badhte nuclear charge par bhaari padte hain.

Table ke lambe hisse mein nateeja ek saw-tooth hota hai. Noble gases, apne closed shells ke saath, peaks par hain; alkali metals, jinmein se har ek ke paas noble-gas core ke bahar ek akela $ns$ electron hai, troughs mein hain, isi wajah se woh itne reactive hain. Neeche ki activity mein tum yeh pattern khud bana sakte ho.

In dips ko samajhna zaroori hai. Magnesium se aluminium: ek hi shell mein s electron p electron se zyada nucleus ke paas penetrate karta hai, isliye 3p electron inner electrons se zyada shield hota hai. Isliye aluminium ka 3p electron magnesium ke 3s electron se nikalna aasaan hai, bhale hi aluminium mein ek proton zyada hai. Phosphorus se sulphur: phosphorus ke teen 3p electrons teen alag orbitals mein hain (Hund's rule), jabki sulphur mein ek 3p orbital mein pair hai. Us pair ke andar ka repulsion unmein se ek ko nikalna aasaan bana deta hai. Yahi do dips period 2 mein bhi dikhte hain, beryllium aur boron ke beech aur nitrogen aur oxygen ke beech.

(d) Electron gain enthalpy

Ionization enthalpy measure karti hai ki electron nikalna kitna mushkil hai. Ulta sawaal, ki atom ek electron kitni aasaani se leta hai, ka jawab electron gain enthalpy (इलेक्ट्रॉन लब्धि एन्थैल्पी), ${\mathrm{\Delta }}_{eg}H$, deti hai, yaani woh enthalpy change jab ek isolated gaseous atom ek electron lekar anion banta hai:

Ise aise socho ki atom ek extra electron ka kaisa swaagat karta hai. Halogen atom, jo noble-gas shell se ek electron peeche hai, iska garam-joshi se swaagat karta hai: bahut energy release hoti hai aur ${\mathrm{\Delta }}_{eg}H$ bada aur negative hota hai. Noble gas atom ke valence shell mein jagah nahi bachi, isliye naye electron ko agle shell mein akele baithna padega, door aur kamzori se pakda hua; energy deni padti hai aur ${\mathrm{\Delta }}_{eg}H$ positive hota hai. Zyadatar elements beech mein aate hain, chhoti negative values ke saath. Pehle se negative ion mein doosra electron jodne mein hamesha energy lagti hai, repulsion ki wajah se.

Size aur effective nuclear charge broad pattern tay karte hain, haalaanki yeh ionization enthalpy se zyada ubad-khabad hai. High $Z_{\text{eff}}$ wale chhote atoms, period ke right ki taraf, jode gaye electron ko kas ke pakadte hain; group mein neeche ke bade atoms use kam kas ke pakadte hain. Isliye sabse negative values top right corner mein, groups 16 aur 17 mein, hain.

Phir bhi in groups ka sabse upar wala hissa rule todta hai. Chlorine ki value (−349 kJ mol⁻¹) fluorine ki (−328 kJ mol⁻¹) se zyada negative hai, aur sulphur ki (−200 kJ mol⁻¹) oxygen ki (−141 kJ mol⁻¹) se zyada negative. Fluorine aur oxygen itne chhote hain ki unka $n=2$ shell pehle se bheed bhara hai, aur wahan extra electron baaki electrons se zor se repel hota hai. Chlorine aur sulphur electron ko bade $n=3$ shell mein jodte hain, jahan use zyada jagah milti hai (Fig. 3.13).

(e) Electronegativity

Hydrogen chloride molecule mein bonding pair apna zyada time hydrogen ki jagah chlorine ke paas bitata hai. Molecule mein kisi atom ki shared pair ko apni taraf kheenchne ki tendency uski electronegativity (विद्युत्-ऋणात्मकता) hai. Kyunki yeh isolated atom ki nahi balki bonded atom ki baat karti hai, koi ek experiment ise measure nahi karta; iski jagah ise banaye gaye scales par express kiya jaata hai. Linus Pauling ne 1932 mein bond energies se pehla aur sabse popular scale banaya, jismein fluorine, sabse strong electron kheenchne wala, 4 ke paas rakha gaya (aaj ke revised scale par 3.98). Mulliken–Jaffé aur Allred–Rochow scales doosra data use karte hain lekin milta-julta order dete hain.

Electronegativity period ke across badhti hai (lithium 0.98, fluorine 3.98) aur group mein neeche ghatti hai (fluorine 3.98, iodine 2.66), ionization enthalpy ki tarah, kyunki high $Z_{\text{eff}}$ wala chhota atom bonding electrons ko paas rakhta hai. Numbers ko constants nahi, guide samjho: kisi element ki electronegativity uske oxidation state aur bonding partners ke saath thodi badalti hai. Phir bhi do atoms ke beech electronegativity ka difference achha bata deta hai ki woh kis tarah ka bond banayenge, jaise agla chapter dikhata hai.

High electronegativity non-metal ki pehchaan hai, jo aasaani se electrons leta hai; low electronegativity metal ki pehchaan hai, jo unhe khota hai. Isliye electronegativity ka map character ka map bhi hai: period ke across electronegativity badhne par metallic character girta hai, aur group mein neeche iske girne par metallic character badhta hai. Teeno atomic properties ko Fig. 3.14 mein ek saath explore karo, phir Table 3.6 ko ek nazar wale summary ki tarah use karo.

3.7.2 Chemical Properties mein Periodic Trends

Kuch chemical trends, jaise inert pair effect aur lanthanoid contraction, group by group sabse achhe se samajh aate hain aur aage ke chapters mein aate hain. Do trends configuration se itne seedhe nikalte hain ki hum unhe abhi lete hain: periods ke saath valence kaise repeat hoti hai, aur second-period elements sabse alag kyun hain.

(a) Valence ya oxidation states ki periodicity

Kisi element ke outer electrons gino aur tum aam taur par predict kar sakte ho ki woh kitne bonds banayega. Representative element ke liye valence normally ya to woh ginti hoti hai ya aath minus woh ginti. Sodium, ek outer electron ke saath, valence 1 rakhta hai; chlorine, saat ke saath, hydrogen ki taraf valence 1 (aath minus saat) aur oxygen ki taraf 7 tak dikhata hai. Period 3 ke across hydrogen ki taraf valence 1 se 4 tak badhti hai aur waapas 1 par aa jaati hai, jabki oxygen ki taraf sabse badi valence 7 tak chadhti jaati hai (Table 3.7).

Aaj chemists aam taur par valence ki jagah oxidation state (ऑक्सीकरण अवस्था) ki baat karte hain. Ise nikalne ke liye har shared pair zyada electronegative atom ko de do aur har atom par bacha charge gino. $\mathrm{H}\mathrm{C}\mathrm{l}$ mein chlorine hydrogen se zyada electronegative hai, isliye chlorine $-1$ hai aur hydrogen $+1$. $\mathrm{C}\mathrm{l}{\phantom{X}}_2\mathrm{O}$ mein oxygen chlorine se zyada electronegative hai, isliye oxygen $-2$ hai aur har chlorine $+1$. Isliye ek hi element apne partner ke hisaab se alag oxidation states dikha sakta hai.

Valence hamesha ek hi number nahi hoti. Phosphorus $\mathrm{P}\mathrm{C}\mathrm{l}{\phantom{X}}_3$ aur $\mathrm{P}\mathrm{C}\mathrm{l}{\phantom{X}}_5$ dono banata hai, aur transition metals aur actinoids mein ek element ke kai oxidation states hona exception nahi, rule hai.

(b) Second period elements ki anomalous properties

Lithium ek alkali metal hai, phir bhi kai tarah se yeh sodium se zyada magnesium jaisa behave karta hai; isi tarah beryllium magnesium se zyada aluminium jaisa hai. Dono saaf covalent character wale compounds banate hain, jabki groups 1 aur 2 ke heavy members zyadatar ionic compounds banate hain. Isi tarah lithium se fluorine tak har second-period element apne group ke baaki members se alag hai. Kisi element ka apne lower-right padosi jaisa hona diagonal relationship (विकर्ण संबंध) kehlata hai; Fig. 3.15 dekho.

Second-period elements itne alag kyun hain? Inke atoms bahut chhote hain, isliye inke ions apne size ke hisaab se kaafi charge rakhte hain aur electrons ko zor se attract karte hain. Iske upar, second-period atom ke paas sirf chaar valence orbitals hain, 2s aur teen 2p, isliye yeh chaar se zyada covalent bonds nahi bana sakta. Period 3 se aage valence shell mein d orbitals bhi milte hain (3s, 3p aur 3d milkar nau), isliye heavy members chaar se zyada electron pairs le sakte hain. Nitrogen $\mathrm{N}\mathrm{C}\mathrm{l}{\phantom{X}}_3$ par ruk jaata hai jabki phosphorus $\mathrm{P}\mathrm{C}\mathrm{l}{\phantom{X}}_5$ tak jaata hai, aur oxygen $\mathrm{O}\mathrm{F}{\phantom{X}}_2$ par ruk jaata hai jabki sulphur $\mathrm{S}\mathrm{F}{\phantom{X}}_6$ banata hai.

Chhote second-period atoms itne paas bhi aa sakte hain ki unke p orbitals side se overlap karein, aur strong $p\pi$–$p\pi$ multiple bonds banayein. Nitrogen triple-bonded $\mathrm{N}{\phantom{X}}_2$ hai aur oxygen double-bonded $\mathrm{O}{\phantom{X}}_2$, jabki phosphorus $\mathrm{P}{\phantom{X}}_4$ molecules aur sulphur $\mathrm{S}{\phantom{X}}_8$ rings pasand karte hain, dono single bonds se jude. Carbon dioxide do C=O double bonds wala chhota molecule hai; silicon dioxide Si–O single bonds ka ek giant network hai.

3.7.3 Periodic Trends aur Chemical Reactivity

Trends ko saath rakho to reactivity samajh aa jaati hai. Period mein left se right jaate hue atoms chhote hote hain, ionization enthalpy badhti hai aur electron gain enthalpy zyada negative hoti hai. Isliye bilkul left wala element kisi bhi doosre se aasaani se electron deta hai, aur bilkul right wala element (noble gas ko chhodkar) sabse utsukta se electron leta hai. Dono ends bahut reactive hain, ulte tareekon se: ek cations banata hai aur metal ki tarah behave karta hai (aur, jaise aage ke chapters dikhate hain, reducing agent ki tarah); doosra anions banata hai aur non-metal ki tarah behave karta hai (oxidising agent). Period ke beech mein koi bhi process aasaan nahi hai, isliye wahan reactivity sabse kam hoti hai.

Oxides yeh saaf dikhate hain. Sodium ka oxide, $\mathrm{N}\mathrm{a}{\phantom{X}}_2\mathrm{O}$, paani mein ghulkar strong alkali deta hai, isliye yeh basic hai; chlorine ka highest oxide, $\mathrm{C}\mathrm{l}{\phantom{X}}_2\mathrm{O}{\phantom{X}}_7$, strong acid deta hai, isliye yeh acidic hai. Dono ke beech character step by step badalta hai (Fig. 3.16). Aluminium oxide amphoteric (उभयधर्मी) hai, jo acids aur alkalis dono mein ghulta hai, aur non-metals ke kuch oxides, jaise carbon monoxide aur dinitrogen oxide, neutral hain aur kisi se react nahi karte.

Transition metals mein badlav zyada dheema hota hai. Inke jode gaye electrons andar ke d subshell mein jaate hain, isliye 3d series ke across atomic radii sirf thoda chhote hote hain, aur 4f series ke across badlav aur bhi kam hai. Inki ionization enthalpies s-block metals se upar aur zyadatar p-block elements se neeche hain, isliye yeh groups 1 aur 2 ke metals se kam electropositive hain. Iron dheere-dheere jang khaata hai, jabki sodium ka taaza kata tukda seconds mein dhundhla pad jaata hai.

Main group mein neeche bade atoms aasaani se electrons khote hain aur extra electrons ko kam zor se attract karte hain (part (d) ke period-2 exceptions ko chhodkar). Isliye group mein neeche metallic character badhta hai, jisse caesium sabse reactive alkali metal hai, jabki non-metallic character kam hota hai, jisse fluorine sabse reactive halogen hai. Transition metals is trend ko todte hain: gold apne se do jagah upar wale copper se kaafi kam reactive hai. Iske atoms copper se sirf thode bade hain (144 vs 128 pm) jabki iska nuclear charge kaafi zyada hai (+79 vs +29). Isliye gold apne electrons ko zyada kas ke pakadta hai: iski first ionization enthalpy 890 kJ mol⁻¹ hai, copper ki 745 kJ mol⁻¹. Reactivity yeh bhi tay karti hai ki elements nature mein kaise milte hain. Sodium, potassium aur fluorine hamesha compounds mein jude hue milte hain, jabki unreactive gold aur platinum free metals ke roop mein khod kar nikale ja sakte hain.