STRUCTURE OF ATOM in One Shot: All Concepts & PYQs Covered | JEE Main & Advanced

Structure of Atom - Lecture Transcript

Key Concepts: Subatomic particles, electron discovery, cathode ray tube, cathode rays, ionization, properties of cathode rays, Millikan's oil drop experiment, charge quantization, anode rays, proton discovery.

1. Introduction

The lecture introduces the chapter "Structure of Atom," highlighting its straightforward nature, especially at the JEE Main level. While questions are often formula-based, careful calculations are crucial to avoid errors and maintain confidence. The speaker emphasizes the importance of practicing specific, frequently tested topics within the chapter. The lecture will cover subatomic particles, their discovery, and related experiments.

2. Discovery of Electron: Cathode Ray Experiment

2.1. Experimental Setup:

• A cathode ray tube (CRT) is used, consisting of a glass tube with two electrodes: a cathode (negative terminal) and an anode (positive terminal).
• The tube is filled with a gas at very low pressure (10^-4 mm Hg).
• A high voltage (approximately 10,000 volts) is applied across the electrodes.

2.2. Process of Cathode Ray Generation:

1. Cathode Ionization: The high voltage causes the cathode material to ionize, releasing electrons. The origin of cathode rays is the cathode material itself.
2. Gas Ionization: These emitted electrons collide with the gas atoms in the tube, ionizing them. For example, if the gas is A2, the electron collides with A, forming A+ and releasing more electrons.
3. Cathode Ray Formation: The electrons from both the cathode and the ionized gas atoms accelerate towards the anode, forming a beam of electrons known as cathode rays.
4. Detection: When these electrons strike a phosphor material (e.g., zinc sulfide) coated on the back of the anode, they cause it to glow, indicating the presence of the electron beam.

2.3. Key Statement: "कैथोड रे की जो शुरुआत है या कैथोड रेज जो बन रही है उसका जो ओरिजिन है वो एक्चुअली हमारा कैथोड मटेरियल कैसे सर? जब आप इतने हाई वोल्टेज पर इस इस किसी गैस को लो प्रेशर पर इस कैथोड रे ट्यूब के अंदर डालते हैं तो ये जो आपका कैथोड मटेरियल है ये कैथोड मटेरियल जो है ये सबसे पहले आयनाइज होता है।" (The origin of cathode rays is actually our cathode material. When you put a gas at low pressure in this cathode ray tube at such a high voltage, then this cathode material, this cathode material is ionized first.)

3. Properties of Cathode Rays

3.1. Straight Line Travel:

• Cathode rays travel in straight lines. This is evidenced by the formation of shadows of objects placed in their path.
• Conclusion: "हमारे जो कैथोड रेज हैं वो स्ट्रेट लाइन में ट्रेवल करते हैं।" (Our cathode rays travel in a straight line.)

3.2. Negative Charge:

• Cathode rays are deflected towards the positive terminal when an electric field is applied.
• Conclusion: "दे आर made of negatively charged particle" (They are made of negatively charged particles.)

3.3. X-ray Production:

• When cathode rays strike heavy metals (e.g., tungsten, copper, molybdenum), they produce X-rays.

3.4. Mass and Momentum:

• Cathode rays can rotate a light paddle wheel placed in their path, indicating that they possess mass and momentum.
• Conclusion: "They have mass also" (They have mass also.)

3.5. Magnetic Field Deflection:

• Cathode rays are deflected by magnetic fields, further confirming their charged nature.

4. Independence of Cathode Ray Properties on Gas and Cathode Material

• The nature of the gas used in the CRT and the material of the cathode do not affect the properties of cathode rays.
• Regardless of the gas (A2, B2, etc.), the cathode rays are always composed of electrons.
• The e/m (charge-to-mass) ratio of cathode rays is constant, irrespective of the gas or cathode material used.

4.1. Specific Charge (e/m Ratio):

• The specific charge of cathode rays is calculated as: e/m = (-1.6 x 10^-19 C) / (9.1 x 10^-31 kg) ≈ -1.76 x 10^11 C/kg.
• This constant e/m ratio suggests that cathode rays are composed of a fundamental, negatively charged particle present in all atoms, which was later named the electron.

4.2. Key Statement: "कैथोड रेज गैस के नेचर और कैथोड मटेरियल के नेचर पर बिल्कुल डिपेंड नहीं करती इसका कोई लेना देना नहीं है कि आपने कैथोड किस मेटल का लिया है और इसका कोई लेना देना नहीं है कि आपने गैस कौन सी ली है।" (Cathode rays do not depend on the nature of the gas and the nature of the cathode material. It has nothing to do with which metal you have taken the cathode from and it has nothing to do with which gas you have taken.)

5. Millikan's Oil Drop Experiment

5.1. Experimental Setup:

• Oil drops are sprayed into a chamber using an atomizer, creating tiny oil droplets.
• The chamber contains two parallel metal plates: an upper positive plate with a small hole and a lower negative plate.
• A beam of X-rays or electrons is used to ionize the gas between the plates.

5.2. Process:

1. Ionization: The X-rays or electrons ionize the gas, releasing electrons.
2. Oil Drop Charging: These electrons attach to the oil droplets, giving them a negative charge.
3. Balancing Forces: By adjusting the voltage between the plates, the electric force on the charged oil drop can be balanced against the gravitational force, causing the drop to suspend in mid-air.

5.3. Calculation of Charge:

• The charge (q) on the oil drop is calculated using the equation: q = mg/E, where m is the mass of the oil drop, g is the acceleration due to gravity, and E is the electric field strength.

5.4. Charge Quantization:

• Millikan found that the charge on each oil drop was always a multiple of a fundamental unit of charge, approximately 1.6 x 10^-19 Coulombs.
• This demonstrated that electric charge is quantized, meaning it exists only in discrete units.
• Equation: q = n * e, where n is an integer and e is the elementary charge (1.6 x 10^-19 C).

5.5. Key Statement: "किसी भी बॉडी पर जो चार्ज होगा वो इस मिनिमम चार्ज का इंटीग्रल मल्टीपल होगा और इसी वजह से नेगेटिवली चार्ज पार्टिकल का जो मिनिमम चार्ज कंसीडर किया जाता है जिसको एक इलेक्ट्रॉन का चार्ज कहा जाता है इट इज - 1.6 * 10 -19" (The charge on any body will be an integral multiple of this minimum charge and that is why the minimum charge of a negatively charged particle which is considered to be the charge of an electron is -1.6 * 10^-19.)

5.6. Example Problem:

• The lecture presents a problem where different charges are observed on oil drops: 3 x 10^-18 C, 6 x 10^-18 C, 4.2 x 10^-18 C, 15 x 10^-18 C, and 9 x 10^-18 C.
• The task is to determine the minimum charge (elementary charge).
• The correct answer is 1.5 x 10^-18 C, because all the observed charges are integral multiples of this value. 4.2 x 10^-18 C would not be possible if 3 x 10^-18 C was the minimum charge.

6. Discovery of Proton: Anode Ray Experiment

6.1. Experimental Setup:

• Similar to the cathode ray experiment, but with a perforated cathode (cathode with holes).
• When a voltage is applied, positive rays are observed traveling in the opposite direction to cathode rays, i.e., towards the cathode.

6.2. Process:

1. Ionization: Electrons emitted from the cathode ionize the gas atoms in the tube (e.g., A2 gas).
2. Positive Ion Formation: This creates positive ions (A+) and more electrons.
3. Anode Ray Formation: The positive ions are attracted towards the cathode. If the cathode is perforated, some of these positive ions pass through the holes and strike a fluorescent screen behind the cathode, causing it to glow.

6.3. Dependence on Gas:

• Unlike cathode rays, the e/m ratio of anode rays depends on the nature of the gas used in the tube.
• This is because the positive ions formed are different for different gases (A+ for A2 gas, B+ for B2 gas, etc.).

7. Conclusion

The lecture provides a detailed explanation of the discovery of electrons and protons, emphasizing the experimental setups, observations, and conclusions drawn from the cathode ray, Millikan's oil drop, and anode ray experiments. It highlights the fundamental properties of electrons and protons and the concept of charge quantization. The lecture also stresses the importance of understanding the underlying principles and applying them to solve problems.