Journey Inside the Atom (Atom ke Andar ka Safar): Class 9 Science Chapter 8 Hinglish mein

Ek pencil chheelo aur uski grey dhool ko ungliyon ke beech ragdo. Woh itni patli layer mein phail jaati hai jitna koi bhi powder tum pees nahi sakte. Isko tum kitni door tak todte ja sakte ho? Class 8 mein tumne chemists ka jawab dekha tha: tumhare aas-paas sab kuch, kamre ki hawa se lekar tumhare haath ke cells tak, atoms se bana hai, jo itne chhote hain ki achhe microscope se bhi nahi dikhte.

Yeh chapter khud atom ko khol kar dekhta hai. Hum un experiments ko follow karenge jinhone dikhaya ki atom ke bhi hisse hote hain, un models ko jo scientists ne har naye result ko samjhane ke liye banaye, aur un rules ko jo batate hain ki atom mein kitne particles hote hain aur woh kahan baithe hain. Chapter ke end tak tum ${\phantom{X}}_{\phantom{11}}^{\phantom{23}}{\phantom{X}}_{\phantom{2}11}^{\phantom{2}23}\mathrm{N}\mathrm{a}$ jaisa label dekh kar bata paoge ki us atom ke andar kya hai aur woh doosron ke saath kaise combine karega.11Atoms ka apna koi rang nahi hota. Hamari drawings ke rang sirf hisson ko alag pehchaanne ka ek code hain.

8.1 Atomic Theory ki Jadon ki Phir se Khoj

Sawaal sab kuch kis cheez se bana hai? kisi bhi laboratory se kahin zyada purana hai. Do hazaar saal se bhi pehle India aur Greece ke thinkers ne sirf soch-vichaar se is par kaam kiya, aur dono ek hairat-angez roop se milte-julte jawab tak pahunche.

Socho ki tum mitti ke ek dhele ko aadha todte ho, phir aadhe ko aadha, aur aise hi aage. Acharya Kanada ne tark diya ki yeh hamesha nahi chal sakta: todna ek aise particle par rukna hi chahiye jiske koi hisse na hon, yaani parmanu (परमाणु). Vaisheshika Sutra mein, jis text mein unki shiksha milti hai, koi aankh ya sparsh ek akele parmanu ko kabhi mehsoos nahi kar sakta. Kanada ne socha ki parmanu pehle jode banate hain aur phir aage groups banate hain, aur isi tarah patthar, nadi aur jeevit shareer sab aise groups se bane hain. Unki scheme ne ek baat khuli chhodi thi, yaani recipe: yeh nahi batati ki kisi substance ko banane mein har tarah ke kitne parmanu lagte hain.

Door Greece mein Leucippus aur unke shishya Democritus bhi milti-julti picture tak pahunche. Unhone matter ke aakhri, na katne wale tukde ko atomos naam diya, jiska Greek mein matlab hai 'jo kaata na ja sake', aur isi se hamara shabd atom aaya. Dono paramparaon ne apni baat sirf logic se rakhi; tab tak kisi ke paas ise test karne ka tareeka nahi tha (Fig. 8.1).

Yeh idea 1808 mein science bana, jab English teacher John Dalton ne isse naape gaye facts samjhaye. Chemists ne paaya tha ki elements hamesha mass ke hisaab se fixed proportions mein combine karte hain. Dalton ne kaha ki har element apne khaas tarah ke atom se bana hai, atoms na toote ja sakte hain, na banaye ja sakte hain, na khatam kiye ja sakte hain, aur compounds tab bante hain jab atoms simple whole-number ratios mein judte hain. Unki theory experimental evidence par bani atoms ki pehli theory thi, aur baad ka har model isi se nikla.

Dalton ki theory ne jitne jawab diye, utne hi naye sawaal bhi khade kiye. Kya atom ka koi andar ka hissa hai? Kya hum kabhi ek atom dekh sakte hain, chahe indirectly hi sahi? Aur agar har element ke apne atoms hain, to gold ka atom oxygen ke atom se theek-theek kaise alag hai? Yeh chapter in sawaalon ko ek-ek karke uthata hai.

8.2 Atomic Models ka Ek Chhota Aitihaasik Safar

Ek scientific model ek simple picture hota hai jo samjhata hai ki hum kya dekhte hain aur predict karta hai ki humne abhi kya try nahi kiya. Jab koi naya experiment usse match nahi karta, to model badla jaata hai ya uski jagah naya aata hai. Atom iska badhiya example hai: lagbhag sau saal mein chaar models, aur har ek ne woh theek kiya jo pichhla model samjha nahi paaya.

'Na katne wale' atom mein pehli daraar 1800s ke end mein aayi. Uranium aur radium jaise kuch elements apne aap rays aur particles chhodte paaye gaye, aur is property ko radioactivity kaha gaya. Agar atom se particles bahar aa sakte hain, to atom ke hisse hone hi chahiye.

1890s tak physicists ke paas ek dilchasp device tha: ek band glass tube, jisse lagbhag saari hawa nikaal di gayi thi, aur jiske dono ends par ek metal plate thi. Plates ko kai hazaar volts ki supply se jodo, to tube ka door wala end chamakne lagta hai. Negative plate, yaani cathode, se positive plate, yaani anode, ki taraf kuch na dikhne wala beh raha hota hai; in streams ko cathode rays naam diya gaya. 1897 mein Cambridge mein J. J. Thomson ne pata lagane ki thaani ki yeh kya hain. Unhone charged plates aur magnets se beam ko mod kar naapa ki woh kitna mudta hai (Fig. 8.2).

Beam positive plate ki taraf muda, isliye woh negatively charged particles ki stream hi ho sakti thi. Unke measurements ne dikhaya ki har particle hydrogen atom se bhi kahin halka hai, jo sabse halka atom hai; aaj hum jaante hain ki woh lagbhag 1840 guna halka hai. Aur result is baat par depend nahi karta tha ki cathode kis metal ka hai ya tube mein kaunsi gas bachi hai. Jo particle har material deta hai, woh har atom ka hissa hona chahiye. Yeh particles electrons (इलेक्ट्रॉन) hain.

Ek electron par $-1.602\times {10}^{-19}\mathrm{C}$ ka charge hota hai. Is number ko baar-baar likhna mushkil hai, isliye chemists ise seedha $-1$ charge kehte hain aur doosre charges ko isi unit mein naapte hain.

8.2.1 Thomson ka atom model

Electrons ne ek seedha sawaal khada kiya. Atoms par koi overall charge nahi hota, isliye jo bhi electrons ke negative charge ko balance karta hai, woh bhi andar hi hona chahiye. Koi halka positive particle nahi mila tha, isliye Thomson ne socha ki positive charge poore atom mein barabar phaila hai, positive 'jelly' ki ek ball ki tarah, aur electrons usmein yahan-wahan chipke hain (Fig. 8.3).

Kyunki electrons positive ball mein waise hi baithe hain jaise English Christmas pudding mein sookhe fal, isliye ise plum pudding model kaha gaya. Kishmish lage besan laddoo se bhi yahi picture banti hai: laddoo positive charge hai, kishmish electrons, aur dono charges ek doosre ko bilkul cancel karte hain. Yeh model zyada din nahi chala, lekin isne woh pattern banaya jo baad ke har model ne follow kiya: batao ki har tarah ka charge kahan baitha hai, aur dikhao ki charges kaise balance hote hain.

8.2.2 Thomson ke model ka test: Gold foil experiment

Achhe model ko tests mein tikna padta hai. Lagbhag 1910 mein Manchester mein Ernest Rutherford ki laboratory mein kaam karte hue Hans Geiger aur Ernest Marsden ne Thomson ke model ka ek test kiya. Unki 'goliyaan' thi alpha particles ($\alpha$-particles, अल्फ़ा कण): tez, positively charged particles jo radium jaise radioactive elements se nikalte hain. Har ek electron se lagbhag 7300 guna bhaari hota hai aur uska charge $+2$ hota hai; asal mein yeh bina electrons wala ek helium nucleus hai.

Unhone $\alpha$-particles ka ek patla beam gold ki ek sheet par daala jo itni patli peeti gayi thi ki sirf lagbhag ek hazaar atoms moti thi. Foil ke chaaron taraf zinc sulphide ($\mathrm{Z}\mathrm{n}\mathrm{S}$) se coated ek screen thi, jahan bhi $\alpha$-particle takrata, wahan roshni ki ek chhoti chamak hoti. Ek andhere kamre mein baith kar experimenters ne microscope se alag-alag angles par chamak gini (Fig. 8.4).

Agar Thomson sahi the to kya hona chahiye? Unka positive charge har gold atom mein patla phaila hai, isliye atom ka koi hissa bhaari, tez $\alpha$-particle ko zor se dhakka nahi de sakta. Har particle ko lagbhag seedha nikal jaana chahiye, zyada se zyada ek degree ke aas-paas mudna chahiye.

Zyaadatar particles sach mein seedhe nikal gaye. Lekin kuch bade angles se mud gaye, aur lagbhag 8000 mein se ek source ki taraf waapas aaya. Rutherford ne baad mein kaha ki yeh utna hi hairaani bhara tha jitna tissue paper ki sheet par ek bhaari shell chalana aur use waapas apni taraf uchhalte dekhna. Physicists is tarah mudne ko scattering kehte hain, isliye is kaam ko aksar $\alpha$-particle scattering experiment kaha jaata hai.

Thomson ka model itne bade deflections nahi de sakta tha. Patla, phaila hua positive charge itna kamzor hai ki $\alpha$-particle jaise bhaari aur tez particle ko waapas nahi mod sakta. Atom ke andar kuch kahin zyada concentrated hona hi chahiye tha.

Rutherford ka atom model

1911 mein Rutherford ne har observation se atom ka ek feature nikala:

• Zyaadatar particles seedhe nikal gaye, isliye atom ka zyaadatar hissa khaali jagah hai.
• Kuch bade angles se mud gaye, isliye atom ka positive charge ek bahut chhote region mein bhara hai jo bahut paas aane wale kisi bhi $\alpha$-particle ko zor se dhakelta hai. Unhone ise nucleus (नाभिक) kaha.
• Bahut kam seedhe waapas uchhle, isliye is chhote nucleus mein atom ka lagbhag poora mass bhi hai. Sirf ek compact, bhaari cheez hi bhaari, tez particle ko ulta mod sakti hai.
• Is nucleus ke chaaron taraf door-door electrons ghoomte hain, jaise planets Sun ke chakkar lagate hain. Isliye Rutherford ki picture ko nuclear model ya aam bolchaal mein planetary model kaha jaata hai.

Nucleus kitna chhota hai? Atoms lagbhag ${10}^{-10}\mathrm{m}$ ke hote hain aur nuclei lagbhag ${10}^{-15}\mathrm{m}$ ke, isliye nucleus apne atom se lagbhag ek lakh (${10}^5$) guna patla hai. Socho ki nucleus 2 cm ka ek glass ka kancha hai jo ek bahut bade maidan ke beech mein pada hai. Tab atom ke sabse bahari electrons har taraf lagbhag ek kilometre door honge, yaani atom lagbhag 2 km ki ek ball hoga, jo lagbhag poora khaali hai.

Sim dikhata hai ki sirf bahut paas aana kyun matter karta hai: har baar distance aadha hone par dhakka chaar guna ho jaata hai. Kyunki nucleus itna chhota hai, sirf woh ek-aadh particle jo lagbhag seedha uski taraf aata hai, waapas mudne layak zor ka dhakka mehsoos karta hai.

Rutherford ke model ki kamiyaan

Nuclear model mein ek chhupi kami thi. Radio aerials ke experiments se physicists jaante the ki charged particle jab bhi tez hota hai, dheema hota hai ya mudta hai, to energy radiate karta hai. Nucleus ke chaaron taraf ghoomta electron har pal mud raha hota hai; Chapter 4 ne dikhaya tha ki circle mein motion accelerated motion hota hai. Isliye electron ko light ke roop mein energy khoni chahiye, har chakkar ke saath nucleus ke aur paas aana chahiye aur ek second ke billionth hisse se bhi kam mein usmein takra jaana chahiye (Fig. 8.7).

Tab to universe ka har atom bahut pehle hi collapse ho chuka hota. Phir bhi sau saal purane pul ka iron aur ek purane sikke ka gold ab bhi wahin hain, bina badle. Rutherford ki picture mein kuch na kuch badalna hi tha.

Proton ki khoj

Isi beech Rutherford khud nucleus ko jaanch rahe the. 1919 mein unhone nitrogen gas mein $\alpha$-particles chhode aur paaya ki woh aise positively charged particles bahar nikaalte hain jo hydrogen atom ke nucleus jaise hi hain. Unhone nateeja nikala ki har nucleus mein aise particles hote hain, aur 1920 mein unhe protons (प्रोटॉन) naam diya.

Proton par $+1$ charge hota hai, jo electron ke charge ke bilkul barabar aur ulta hai, lekin proton lagbhag 1836 guna bhaari hota hai. Poora atom neutral hota hai, isliye atom mein protons ki sankhya electrons ki sankhya ke barabar hoti hai. Lithium atom mein 3 protons aur 3 electrons hote hain; chlorine atom mein dono 17 hain. Dono mein positive aur negative charges bilkul cancel ho jaate hain.

8.2.3 Bohr ka atom model

1913 mein Danish physicist Niels Bohr ne Rutherford ka nucleus rakha lekin electrons ke rules badal diye. Unhone apne naye rules prove nahi kiye. Unhone unhe postulates ke roop mein rakha, yaani aise statements jo isliye maane jaate hain kyunki unke predictions experiments se match karte hain.

• Sirf kuch orbits allowed hain. Electron nucleus ke chaaron taraf sirf khaas paths par ghoom sakta hai, jinhe shells (कोश) kehte hain, inhe orbits ya stationary states bhi kehte hain, aur unke beech ki jagah mein kabhi nahi.
• Har shell ki ek fixed energy hoti hai, isliye shells ko energy levels bhi kehte hain. Bahar ki taraf jaate hue unke naam K, L, M, N, … hain ya number $n=1,2,3,4,\dots$; K ki energy sabse kam hai aur har aage wale shell ki zyada.
• Shell ke andar koi energy nahi khoti. Apne shell mein ghoomta electron bilkul utni hi energy rakhta hai, chahe woh kitni bhi der ghoome.
• Shell badalne mein ek exact amount of energy lagti hai. Upar wale shell mein chadhne ke liye electron ko dono ke beech ka energy gap bilkul utna hi absorb karna padta hai; waapas girte hue woh utni hi energy chhodta hai.
• Shells mein jagah limited hai. Har shell mein sirf kuch hi electrons ki jagah hoti hai.

Ek seedhi ke baare mein socho. Ball kisi bhi step par ruk sakti hai lekin do steps ke beech hawa mein kabhi nahi, aur use upar ya neeche le jaane mein hamesha poore ek step jitni energy lagti hai. Bohr ke shells in steps ki tarah kaam karte hain. Apne shell mein electron ki energy fixed rehti hai jo kam nahi hoti, isliye woh andar spiral nahi karta, aur atom stable rehta hai. Bohr ke rules ne yeh bhi samjhaya ki chamakti hydrogen sirf kuch saaf rangon ki light kyun deti hai, aur isi success ki wajah se physicists ne unke ajeeb rules ko seriously liya.

8.3 Atom ke Mass mein Kaun se Components Yogdaan Dete Hain?

Electrons itne halke hote hain ki sab milkar bhi atom ke mass ka ek percent ke dasven hisse se bhi kam banate hain, isliye zyaadatar kaamon ke liye hum unhe ignore kar sakte hain. Mass nucleus mein hi hona chahiye. Agar nucleus mein sirf protons hote, to atom ka mass seedha protons ki sankhya guna ek proton ka mass hota. Hydrogen fit baithta hai: ek proton, mass 1 unit. Lekin helium, jismein 2 protons hain, lagbhag 4 units ka hota hai, 2 ka nahi (Fig. 8.9). Bhaari atoms mein farak aur bhi zyada hai: gold nucleus mein 79 protons hain phir bhi woh lagbhag 197 units ka hai. Nucleus mein kuch bhaari lekin bina charge wala bhi hona hi chahiye tha.

8.3.1 Neutron ki Khoj

Gum particle 1932 mein mila. Germany aur France ke experimenters ne ek ajeeb, bahut penetrating radiation dekha tha jo paraffin wax se protons bahar nikaal sakta tha. Cambridge mein Rutherford ke purane saathi James Chadwick ne dikhaya ki yeh particles ki ek stream thi jo lagbhag protons jitne bhaari the lekin unpar koi charge nahi tha: neutrons (न्यूट्रॉन), symbol $\mathrm{n}$. Charge na hone ki wajah se woh electric forces par tike har pichhle test se bach nikle the.

Nucleus mein neutrons hone se helium ka mass samajh aata hai: 2 protons aur 2 neutrons milkar lagbhag 4 units. Aam hydrogen ek akela exception hai, bina neutron ka ek akela proton. Har atom ka lagbhag poora mass nucleus mein hota hai. Table 8.1 teeno particles ka saar deta hai.

Kuch nuclei mein particles gino to ek pattern dikhta hai. Nitrogen-14 mein 7 protons aur 7 neutrons hain; magnesium-24 mein 12 aur 12. Lekin silver-107 mein 47 protons aur 60 neutrons hain, aur gold-197 mein 79 protons aur 118 neutrons. Nucleus jitna bhaari, neutrons protons se utne hi zyada aage nikal jaate hain; neeche ka box batata hai kyun.

8.4 Elements ke Symbols

Jab Dmitri Mendeleev ne 1869 mein elements ki apni mashhoor table banayi, tab unke paas lagaane ke liye saath se thode zyada elements the. Aaj 118 jaane jaate hain, jinmein se zyaadatar nature mein milte hain aur kuch sirf laboratories mein banaye gaye hain. Unke poore naam likhna chemistry ko dheema bana dega, aur naam har bhasha mein alag hote hain. Isliye har element ka ek chhota symbol hai jise har jagah ke chemists pehchaante hain.

1800s ki shuruaat mein Dalton ne sabse pehle elements ko symbols diye: chhote circles jinke andar dots, lines ya letters the. Inhe print karna aur yaad rakhna mushkil tha. 1813 mein Swedish chemist Jöns Jacob Berzelius ne suggest kiya ki iske bajaye har element ke naam ke ek ya do letters use kiye jaayein, aksar uske Latin naam se. Aaj ek international body, IUPAC (International Union of Pure and Applied Chemistry), har naye naam aur symbol ko approve karti hai. Symbols kuch simple rules follow karte hain (Fig. 8.10).

• Kai symbols English naam ka pehla letter hain: hydrogen H, carbon C, nitrogen N.
• Jab pehle letters takrate hain, to doosra letter joda jaata hai: helium He, calcium Ca, aluminium Al.
• Kabhi doosra letter naam mein baad ka koi letter hota hai: magnesium Mg, manganese Mn, chromium Cr.
• Kai Latin naamon se aate hain (Table 8.2), aur tungsten ka symbol W uske German naam wolfram se aata hai.
• Pehla letter hamesha capital hota hai aur doosra hamesha chhota. Co element cobalt hai, lekin $\mathrm{C}\mathrm{O}$ carbon monoxide hai, jo carbon aur oxygen ka compound hai.

8.5 Atomic Number

Gold atom ko gold kya banata hai, oxygen kyun nahi? Electrons nahi, kyunki atom unhe le ya kho sakta hai. Neutrons bhi nahi, jaisa tum Section 8.9.1 mein dekhoge. Yeh nucleus mein protons ki sankhya hai. Yeh sankhya element ka atomic number (परमाणु संख्या) hai, jise $Z$ likhte hain.

Neutral atom mein jitne protons, utne hi electrons hote hain, isliye $Z$ uske electrons bhi ginta hai: carbon ke chhe, oxygen ke aath, calcium ke bees. Carbon nucleus mein ek aur proton jodo to woh carbon rehta hi nahi: woh nitrogen ban jaata hai, $Z=7$ ke saath (Fig. 8.11). Atomic number element ke roll number ki tarah kaam karta hai. Yeh batata hai ki tumhare paas kaunsa element hai, aur jo electrons yeh tay karta hai unke zariye yeh decide karta hai ki element reactions mein kaise behave karega.

8.6 Mass Number

Protons aur neutrons ko milakar nucleons kehte hain, kyunki dono nucleus mein rehte hain. Har ek ka mass lagbhag 1 unit hai aur electrons ka mass na ke barabar hai, isliye nucleons ki total sankhya mote taur par batati hai ki atom kitna bhaari hai. Yeh total atom ka mass number (द्रव्यमान संख्या) hai, jise $A$ likhte hain.

Ise ulta karo to neutrons ki sankhya milti hai: $N=A-Z$. Table 8.3 ise teen atoms par lagata hai.

Chemists yeh sab ek chhote label mein bhar dete hain. Mass number symbol ke upar left mein jaata hai aur atomic number neeche left mein. Aluminium, jismein 13 protons aur 14 neutrons hain, ${\phantom{X}}_{\phantom{13}}^{\phantom{27}}{\phantom{X}}_{\phantom{2}13}^{\phantom{2}27}\mathrm{A}\mathrm{l}$ likha jaata hai (Fig. 8.12). Aksar atomic number chhod diya jaata hai, kyunki symbol pehle se use tay kar deta hai, aur hum ${\phantom{X}}_{\phantom{}}^{\phantom{27}}{\phantom{X}}_{\phantom{2}}^{\phantom{2}27}\mathrm{A}\mathrm{l}$ ya sirf aluminium-27 likhte hain.

8.7 Alag-alag Energy Levels mein Electrons Kaise Baante Jaate Hain?

Bohr ka model electrons ko shells mein rakhta hai, lekin har shell mein kitne jaate hain? 1920s ki shuruaat mein Bohr aur chemist Charles Bury ne teen simple rules diye jo pehle bees elements ke liye achhe se kaam karte hain:

1. Number $n$ wale shell mein zyada se zyada $2n^2$ electrons aa sakte hain.
2. Atom ke sabse bahari shell mein kabhi 8 se zyada electrons nahi hote. (Jab K shell hi akela shell ho, to usmein zyada se zyada 2 hote hain.)
3. Shells andar se bahar ke order mein bharte hain: K, phir L, phir M. Bahari shell mein electrons tabhi aate hain jab uske andar ke shells utne le chuke hon jitne woh le sakte hain.

Pehle rules ko hydrogen par try karo. Uska akela electron K shell mein jaata hai. Helium ke do electrons K shell ko bilkul bhar dete hain. Lithium ka teesra electron kahan jaana chahiye?

8.7.1 Atoms banana

Ab elements ko atomic number ke order mein banao, jaise eenten jamate hain. Ek element se agle par jaane mein nucleus mein ek proton (aur ek-do neutrons) aur ek electron judta hai, jo sabse andar ke us shell mein jaata hai jismein jagah khaali ho. Shells ke beech electrons ka jo bantwara banta hai, woh atom ka electronic configuration (इलेक्ट्रॉनिक विन्यास) hai. Hum ise K se bahar ki taraf har shell ke electrons ki sankhya ke roop mein likhte hain: lithium 2, 1 hai aur sodium 2, 8, 1 (Fig. 8.13).

Table 8.5 mein pehle bees elements ki configurations hain. Dekho ki har naya shell ek akele electron se shuru hota hai (lithium, sodium, potassium), aur helium, neon aur argon ka sabse bahari shell poora bhara hota hai.

8.8 Atom ki Combining Capacity: Valency

Atoms judkar molecules banate hain, lekin kisi bhi ginti mein nahi. Methane ($\mathrm{C}\mathrm{H}{\phantom{X}}_4$) mein ek carbon atom chaar hydrogen atoms ko pakadta hai, hydrogen sulphide ($\mathrm{H}{\phantom{X}}_2\mathrm{S}$) mein ek sulphur atom do ko, aur aluminium chloride ($\mathrm{A}\mathrm{l}\mathrm{C}\mathrm{l}{\phantom{X}}_3$) mein ek aluminium atom teen chlorine atoms ko. Hydrogen aur chlorine dono sirf ek partner se judte hain, isliye woh ek kaam ka paimaana bante hain: hum kisi element ki combining capacity yeh gin kar naapte hain ki uska ek atom kitne hydrogen ya chlorine atoms ko pakad kar rakhta hai. Carbon ke liye yeh 4 hai, sulphur ke liye 2 aur aluminium ke liye 3.

Yahi numbers kyun? Chemistry ke akelon se shuru karo. Helium, neon aur argon, yaani noble gases, lagbhag koi compounds nahi banate, isiliye argon ko electric bulbs mein safely bhara ja sakta hai aur neon signboards ke andar chamak sakta hai. Unki configurations hain 2; 2, 8; aur 2, 8, 8. Har ek mein sabse bahari bhara hua shell, yaani valence shell, poora bhara hai: 8 electrons, jise octet kehte hain, ya helium ke liye 2. Atom ke valence shell ke electrons uske valence electrons hain, aur yahi reactions mein hissa lete hain.

Baaki har atom ke valence shell mein khaali jagah hoti hai, aur woh us tareeke se react karta hai jo use noble gas jaisa bhara shell de: electrons de kar, le kar, ya kisi partner ke saath share karke. Iske liye use jitne electrons dene, lene ya share karne padte hain, woh uski valency (संयोजकता) hai, jo uski combining capacity ke barabar hi number hai. (Hydrogen ke liye bhare K shell ka matlab hai 2 electrons.)

Ek simple rule configuration se valency predict karta hai:

• 1, 2 ya 3 valence electrons: atom unhe kho deta hai, isliye valency valence electrons ki sankhya ke barabar hoti hai. Magnesium, 2, 8, 2, 2 khota hai: valency 2.
• 5, 6 ya 7 valence electrons: atom 8 tak pahunchne layak electrons le leta hai, isliye valency 8 minus valence electrons ki sankhya hoti hai. Fluorine, 2, 7, 1 leta hai: valency 1.
• 4 valence electrons: chaar khona ya lena bahut mushkil hai, isliye atom unhe share karta hai. Carbon, 2, 4, 4 share karta hai: valency 4.
• Poora bhara sabse bahari shell (helium, neon, argon): khone ya lene ko kuch nahi, isliye valency 0 hai.

Kuch compounds in simple rules ko todte dikhte hain; unse tum aage ki classes mein miloge.

8.9 Atomic Structure ko Gehraai se Dekhna

Dalton ka ek idea yeh tha ki kisi element ka har atom doosre ki bilkul copy hota hai, mass bhi. Ab jab tum jaante ho ki atom ke andar kya hai, tum ise test kar sakte ho. Protons ki ginti element tay karti hai, isliye kisi element ke saare atoms ka $Z$ same hota hai. Kya unmein neutrons ki sankhya bhi same honi zaroori hai?

8.9.1 Isotopes

Pata chalta hai ki zaroori nahi. Ek hi element ke atoms mein neutrons ki sankhya alag ho sakti hai, aur isliye unke mass numbers alag hote hain. Aise atoms, jinka atomic number same hai lekin mass number nahi, element ke isotopes (समस्थानिक) kehlaate hain.

Hydrogen yeh sabse achhe se dikhata hai. Uska nucleus ek akela proton ho sakta hai (protium, ${\phantom{X}}_{\phantom{1}}^{\phantom{1}}{\phantom{X}}_{\phantom{2}1}^{\phantom{2}1}\mathrm{H}$), ek neutron ke saath proton (deuterium, ${\phantom{X}}_{\phantom{1}}^{\phantom{2}}{\phantom{X}}_{\phantom{2}1}^{\phantom{2}2}\mathrm{H}$) ya do neutrons ke saath proton (tritium, ${\phantom{X}}_{\phantom{1}}^{\phantom{3}}{\phantom{X}}_{\phantom{2}1}^{\phantom{2}3}\mathrm{H}$, jo radioactive hai). Har ek mein ek hi electron hai (Fig. 8.16). Nature mein protium aasaani se aage hai, lagbhag 99.98 %: lagbhag 6400 hydrogen atoms mein se sirf ek deuterium hota hai, aur tritium sirf bahut thodi maatra mein milta hai.

Deuterium se bana paani, jise heavy water ($\mathrm{D}{\phantom{X}}_2\mathrm{O}$) kehte hain, bilkul aam paani jaisa dikhta hai. Phir bhi yeh lagbhag 11 % zyada dense hai, 3.8 °C par jamta hai aur 101.4 °C par ubalta hai, kyunki iske molecules bhaari hain. Lekin reactions mein deuterium aam hydrogen jaisa hi behave karta hai: chemistry electrons tay karte hain, aur isotopes ki electron arrangements bilkul same hoti hain. Isliye isotopes ki chemical properties same hoti hain lekin density aur boiling point jaisi physical properties alag. India ke kai nuclear power plants apne reactors mein neutrons ko dheema karne ke liye heavy water use karte hain.

Carbon ek aur example hai: isme lagbhag 99 % carbon-12 hai, lagbhag 1 % carbon-13, aur radioactive carbon-14 sirf bahut thoda. Teeno mein 6 protons hain; unke neutrons 6, 7 aur 8 hain.

Average atomic mass

Nature mein zyaadatar elements isotopes ke mixtures hote hain. Jaise boron lagbhag 20 % boron-10 aur 80 % boron-11 hai. To boron atom ke mass ke roop mein hum kaunsa ek number dein?

Ek idea hai dono masses ka simple average: $(10+11)÷2=10.5\mathrm{u}$. Lekin yeh dono isotopes ko barabar aam maanta hai, aur woh barabar nahi hain. Natural ratio mein 10 boron atoms ka ek jar socho, 2 ka mass 10 u aur 8 ka mass 11 u (Fig. 8.17). Unka total mass $2\times 10+8\times 11=108\mathrm{u}$ hai. 10 atoms mein baantne par yeh har atom ka 10.8 u hai. Yeh weighted average, jismein har mass utna hi gina jaata hai jitna woh aam hai, element ka average atomic mass hai.

Kisi bhi ek boron atom ka mass 10.8 u nahi hota: har ek ya to 10 u ka hai ya 11 u ka. 10.8 u ki value boron ke kisi bhi aam sample ke average atom ko describe karti hai, isiliye periodic table mein yahi number chhapa hota hai.

8.9.2 Isobars

Ab idea ko ulta karo. Kya do alag elements ke atoms ke mass numbers barabar ho sakte hain? Carbon-14 mein 6 protons aur 8 neutrons hain; nitrogen-14 mein 7 protons aur 7 neutrons. Dono mein 14 nucleons hain, isliye dono ka $A=14$ hai, phir bhi woh alag elements hain kyunki unke atomic numbers alag hain (Fig. 8.18). Aise pairs, jo alag elements ke hain lekin jinke mass numbers barabar hain, isobars (समभारिक) kehlaate hain. Iron-58 aur nickel-58 aisa hi ek aur pair hai.

Bohr ke saaf-suthre circles bhi ek simplification hain. Modern physics kehti hai ki electron ka koi fixed path hota hi nahi; hum sirf yeh nikaal sakte hain ki nucleus ke aas-paas har region mein uske milne ki kitni sambhavna hai. Picture mein yeh ek dhundhla electron cloud ban jaata hai, jo wahan sabse ghana hota hai jahan electron ke milne ki sambhavna sabse zyada hai. Yeh model 1920s ke kaam se nikla, aur tum ise aage ki classes mein padhoge.