Chapter 12: Atoms

 

Atoms: Class 12 Physics Chapter 12 Notes in Hindi

Pichle chapter mein humne Electrons ke baare mein padha, ab hum jaanenge ki ye electrons atom ke andar kaise rehte hain. Thomson se lekar Bohr tak, atoms ke model kaise badle aur "Atomic Spectra" kya hota hai, ye sab is post mein cover kiya gaya hai.

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1. Alpha-Particle Scattering (Rutherford’s Experiment)

Rutherford ne gold foil par alpha particles ki bochhaar ki aur dekha ki:

• Zyada tar particles sidhe nikal gaye (Atom ka zyada hissa khali hai).

• Kuch particles thode angle par mude.

• 8000 mein se 1 particle wapas laut aaya (Iska matlab center mein ek bahut chota aur bhari Nucleus hai).

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2. Impact Parameter aur Distance of Closest Approach

• Distance of Closest Approach ($r_0$): Wo minimum doori jahan tak alpha particle nucleus ke paas ja sakta hai. Isse nucleus ke size ka andaza milta hai.

• Impact Parameter ($b$): Alpha particle ki velocity vector aur nucleus ke center ke beech ki perpendicular distance.

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3. Bohr’s Model of Hydrogen Atom (Most Important)

Rutherford ke model ki kamiyon ko dur karne ke liye Neils Bohr ne teen main postulates diye:

1. Stable Orbits: Electrons sirf unhi orbits mein ghumte hain jahan wo energy radiate nahi karte.

2. Angular Momentum Quantization: Electron sirf unhi orbits mein reh sakta hai jahan uska angular momentum $L = \frac{nh}{2\pi}$ ho.

3. Energy Transition: Jab electron ek orbit se dusri orbit mein jata hai, toh wo photon emit ya absorb karta hai.

   $$E_2 - E_1 = h\nu$$

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4. Energy of Electron in nth Orbit

Hydrogen atom mein electron ki energy ka formula boards ke liye bahut zaroori hai:

$$E_n = - \frac{13.6}{n^2} eV$$

• Negative Sign: Yeh batata hai ki electron nucleus se "bound" hai (bandha hua hai).

• Jab $n$ badhta hai, energy badhti hai (zero ki taraf jati hai).

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5. Spectral Series of Hydrogen Atom

Jab electron higher orbit se lower orbit mein girta hai, toh alag-alag wavelengths ki lines banti hain:

Series	Lower Orbit (n1​)	Region
Lyman	1	Ultraviolet (UV)
Balmer	2	Visible
Paschen	3	Infrared
Brackett	4	Infrared
Pfund	5	Infrared

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6. Top Important Questions (Chapter 12)

Short Questions (1-2 Marks):

1. Ground state energy kitni hoti hai? Hydrogen ke liye $-13.6 eV$.

2. Ionization Energy kya hai? Electron ko atom se puri tarah bahar nikalne ke liye chahiye energy ($+13.6 eV$).

3. Bohr model ki ek limit batayein: Yeh sirf "Hydrogen-like" (single electron) atoms par hi kaam karta hai.

Long Derivations (3-5 Marks):

4. Derive the expression for the radius of nth Bohr orbit.

5. Prove that the speed of electron in nth orbit is inversely proportional to $n$.

6. Draw the energy level diagram of Hydrogen atom and show Lyman and Balmer series.

7. State Bohr’s postulates for the model of an atom.

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