Discovery of electron (Thomson, cathode rays), proton (Goldstein), neutron (Chadwick)

1.2.2 · Chemistry › Atomic Structure (Classical)

80/20 core: Teen particles, teen experiments. Electron (Thomson, cathode rays, negative), proton (Goldstein, anode/canal rays, positive), neutron (Chadwick, neutral). Thomson ne $e / m$ measure kiya; Millikan ne $e$ measure kiya; dono milake mass dete hain. Yeh ek akeli chain is topic ka 80% hai.

1. The Electron — Cathode Rays (J.J. Thomson, 1897)

HUM UNKI PROPERTIES KAISE JAANTE HAIN (observations):

Observation	Yeh kya prove karta hai
Seedhi lines mein travel karte hain (kisi object ki sharp shadow banate hain)	Yeh particles / rays ki tarah move karte hain
Path mein ek light paddle-wheel spin karta hai	Yeh momentum → mass carry karte hain
Electric field mein + plate ki taraf deflect hote hain	Yeh negatively charged hain
Magnetic field se deflect hote hain	Moving charge confirm hota hai
Kisi bhi cathode metal ya kisi bhi gas ke liye same hain	Electron universal constituent of all matter hai

Figure — Discovery of electron (Thomson, cathode rays), proton (Goldstein), neutron (Chadwick)

$e / m$ ko first principles se derive karna

Fields ko beam ke perpendicular set karo.

Charge $e$ par field $E$ mein Electric force: $F_{E} = e E$

Speed $v$ se move ho rahe charge par Magnetic force (fields crossed): $F_{B} = e v B$

Step 1 — Speed $v$ nikalo. $E$ aur $B$ adjust karo taaki beam undeflected rahe. Forces balance hoti hain: $e E = e v B ⟹ v = \frac{E}{B}$ Yeh step kyun? Jab net deflection nahi hota toh charge/mass cancel ho jaata hai — hum $v$ ko sirf measurable field strengths se isolate karte hain.

Step 2 — Sirf $E$ se deflect karo ( $B$ switch off karo). Particle length $L$ ki plates cross karne mein time $t = L / v$ lagata hai. Vertical acceleration $a = e E / m$ . Vertical deflection: $y = \frac{1}{2} a t^{2} = \frac{1}{2} \frac{e E}{m} \frac{L ^{2}}{v ^{2}}$ Yeh step kyun? Yeh projectile motion hai — constant horizontal $v$ , constant vertical push. Right side par sab kuch measurable hai except $e / m$ .

Step 3 — Solve karo. $v = E / B$ substitute karo: $\frac{e}{m} = \frac{2 y E}{B ^{2} L ^{2}} \cdot \frac{...}{...} \Rightarrow \frac{e}{m} = \frac{2 y v ^{2}}{E L ^{2}}$

2. The Proton — Anode / Canal Rays (Goldstein, 1886)

Key clue ki yeh GAS par depend karti hain:

In positive rays ka $e / m$ sabse bada tab hota hai jab gas hydrogen ho.
Sabse bada $e / m$ ⟺ sabse chhota mass ⟺ sabse halka positive particle.
Yeh sabse halki unit = proton.

3. The Neutron (Chadwick, 1932)

CHADWICK NE KAISE NIKALA: Usne beryllium ko $α$ -particles se bombard kiya. Ek highly penetrating, uncharged radiation nikli (electric/magnetic fields se undeflected). Usne paraffin se protons knock out kiye. Momentum/energy conservation se pata chala ki invisible radiation ka mass $\approx$ ek proton ka tha. $_{4}^{9} Be +_{2}^{4} He \to_{6}^{12} C +_{0}^{1} n$

Summary Table

Particle	Discoverer	Experiment	Charge (C)	Mass (kg)	Rel. mass (amu)
Electron	Thomson	Cathode rays	$- 1.602 \times 1 0^{- 19}$	$9.11 \times 1 0^{- 31}$	$\approx 1/1836$
Proton	Goldstein	Anode/canal rays	$+ 1.602 \times 1 0^{- 19}$	$1.672 \times 1 0^{- 27}$	$\approx 1$
Neutron	Chadwick	Be + $α$	$0$	$1.675 \times 1 0^{- 27}$	$\approx 1$

Forecast-then-Verify

Feynman

Recall Ek 12-saal ke bachche ko explain karo

Ek andheri glass tube imagine karo. Ek badi battery on karo aur ek green glow minus end se plus end ki taraf shoot karta hai. Thomson ne kaha: "Yeh plus plate ki taraf bend karta hai, toh yeh tiny negative balls hain" — electrons. Phir kisine minus plate mein ek hole kiya aur rays doosri taraf jaati hui mili — woh positive bits (protons) hain, jo tab bante hain jab electrons gas atoms ko smash karte hain. Aakhir mein, atoms ko weighing karte hue, scientists ne extra weight paya jiska koi charge nahi tha — jaise ek invisible sandbag — neutron. Teen players: minus, plus, aur zero.

Connections

Millikan Oil Drop Experiment — $e$ provide karta hai taaki $e / m$ se mass nikaala ja sake.
Rutherford Nuclear Model — yeh protons/neutrons kahan rehte hain (nucleus).
Thomson Plum Pudding Model — electron use karne wala pehla model.
Isotopes and Mass Number — neutrons explain karte hain ki isotopes kyun exist karte hain.
Charge to Mass Ratio — general technique.

Flashcards

Cathode rays kis cheez se bane hain?

Negatively charged particles (electrons) cathode se.

Thomson ne actually kya measure kiya — charge, mass, ya ratio?

Ratio

e / m = 1.758 \times 1 0^{11}

C/kg.

Electron ka

e / m

value?

1.758 \times 1 0^{11}

C kg⁻¹.

Cathode-ray properties gas/electrode par depend kyun nahi karti?

Electron universal constituent of all matter hai.

Thomson ne beam speed

v

kaise nikali?

Electric aur magnetic forces balance kiye:

e E = e v B \Rightarrow v = E / B

Electron ka charge kisne measure kiya aur kaise?

Millikan ne, oil-drop experiment se;

e = 1.602 \times 1 0^{- 19}

Canal (anode) rays kya hain aur inhe kisne discover kiya?

Gas se positive ions; Goldstein ne, perforated cathode use karke.

Canal rays hydrogen ke saath sabse bada

e / m

kyun deti hain?

Hydrogen ion (proton) sabse halka hai, toh sabse chhota mass → sabse bada

e / m

Canal-ray

e / m

gas par depend kyun karta hai lekin cathode-ray

e / m

nahi?

Positive rays ionised gas atoms hain (mass vary karta hai); electrons har jagah identical hain.

Neutron ki zaroorat kyun thi?

Atomic mass sirf protons ke mass se zyada thi, jabki charge already balanced tha — extra neutral mass chahiye tha.

Neutrons banane ki Chadwick ki reaction?

_{4}^{9} Be +_{2}^{4} He \to_{6}^{12} C +_{0}^{1} n

Neutron ka charge aur relative mass?

Charge 0; mass ≈ 1 amu (proton se thoda zyada).

Proton mass aur electron mass ka ratio?

Lagbhag 1836.

Discovery ka order (particle aur scientist)?

Electron–Thomson (1897), Proton–Goldstein (canal rays 1886), Neutron–Chadwick (1932).