Chapter 12 Atoms
NCERT Class 12 Physics - Chapter 12: Atoms
Topics Covered
- Introduction to Atoms
- Thomson's Model of Atom
- Rutherford's Nuclear Model of Atom
- Atomic Spectra
- Bohr's Model of Hydrogen Atom
- Energy Levels and Spectral Series
- De Broglie's Explanation of Bohr's Second Postulate
- Sommerfeld's Extension of Bohr's Theory
Introduction to Atoms
An atom is the smallest unit of matter that retains the properties of an element. It consists of a nucleus containing protons and neutrons, surrounded by electrons in various energy levels.
Example
A carbon atom has 6 protons, 6 neutrons, and 6 electrons, which determines its chemical properties.
Thomson's Model of Atom
J.J. Thomson proposed the "plum pudding" model of the atom, where electrons were embedded in a positively charged "pudding," like plums in a pudding.
Example
Thomson's model was like a watermelon, where the seeds represent electrons and the red part represents the positive charge.
Rutherford's Nuclear Model of Atom
Ernest Rutherford proposed that the atom has a small, dense, positively charged nucleus surrounded by electrons. This model came from his gold foil experiment, where most alpha particles passed through the foil, but some were deflected at large angles.
Example
Rutherford's model is like a solar system, where the nucleus is the sun and the electrons are planets orbiting around it.
Atomic Spectra
Atomic spectra are the lines of color or wavelengths of light emitted or absorbed by an element. These spectra are unique to each element and arise from electrons transitioning between energy levels.
Example
The hydrogen atom produces a series of spectral lines called the Balmer series when its electrons fall to the second energy level.
Bohr's Model of Hydrogen Atom
Niels Bohr proposed that electrons orbit the nucleus in specific energy levels without radiating energy. When electrons jump between these levels, they emit or absorb photons of specific wavelengths.
Example
Bohr's model explained the discrete spectral lines of hydrogen by assuming electrons move in quantized orbits.
Energy Levels and Spectral Series
Energy levels are the fixed orbits around the nucleus where electrons reside. Spectral series are groups of lines seen in the emission or absorption spectra of atoms, corresponding to electron transitions between energy levels.
Example
The Lyman series involves transitions of electrons to the first energy level of hydrogen, producing ultraviolet light.
De Broglie's Explanation of Bohr's Second Postulate
De Broglie suggested that electrons exhibit wave-like properties and their orbits are standing waves. This explained Bohr's second postulate that only certain discrete orbits are allowed.
Example
According to de Broglie, the circumference of an electron's orbit is an integer multiple of its wavelength.
Sommerfeld's Extension of Bohr's Theory
Sommerfeld extended Bohr's model by introducing elliptical orbits and subshells, accounting for fine spectral lines and relativistic effects in heavier atoms.
Example
Sommerfeld's model explained the fine structure of spectral lines in hydrogen and other elements.
Word Meanings
Atom
The smallest unit of matter that retains the properties of an element.
Nucleus
The central part of an atom containing protons and neutrons.
Electron
A negatively charged subatomic particle orbiting the nucleus of an atom.
Proton
A positively charged subatomic particle found in the nucleus of an atom.
Neutron
A neutral subatomic particle found in the nucleus of an atom.
Plum Pudding Model
Thomson's model of the atom where electrons are embedded in a positively charged "pudding."
Nuclear Model
Rutherford's model of the atom with a small, dense nucleus surrounded by electrons.
Gold Foil Experiment
Rutherford's experiment that led to the discovery of the nucleus by observing the deflection of alpha particles.
Atomic Spectra
Lines of color or wavelengths of light emitted or absorbed by an element due to electron transitions.
Balmer Series
Spectral lines of hydrogen resulting from electron transitions to the second energy level.
Photon
A quantum of light energy emitted or absorbed during electron transitions.
Energy Level
Fixed orbits around the nucleus where electrons reside.
Spectral Series
Groups of lines seen in the emission or absorption spectra of atoms corresponding to electron transitions.
Lyman Series
Spectral lines of hydrogen resulting from electron transitions to the first energy level, producing ultraviolet light.
De Broglie Wavelength
The wavelength associated with a moving particle, as proposed by de Broglie.
Standing Wave
A wave that remains stationary, with nodes and antinodes, as seen in de Broglie's explanation of electron orbits.
Elliptical Orbits
Sommerfeld's concept of orbits that are elliptical rather than circular, accounting for fine spectral lines.
Fine Structure
The splitting of spectral lines into closely spaced components due to relativistic effects and electron spin.
FAQs
1. What is an atom?
An atom is the smallest unit of matter that retains the properties of an element, consisting of a nucleus surrounded by electrons.
2. What is Thomson's model of the atom?
Thomson's model, also known as the "plum pudding" model, suggested that electrons are embedded in a positively charged "pudding."
3. What is Rutherford's nuclear model of the atom?
Rutherford's model proposed that the atom has a small, dense, positively charged nucleus surrounded by electrons.
4. What is the significance of Rutherford's gold foil experiment?
Rutherford's gold foil experiment showed that most alpha particles passed through the foil, but some were deflected, leading to the discovery of the nucleus.
5. What are atomic spectra?
Atomic spectra are the lines of color or wavelengths of light emitted or absorbed by an element due to electron transitions between energy levels.
6. What is Bohr's model of the hydrogen atom?
Bohr's model proposed that electrons orbit the nucleus in specific energy levels and emit or absorb photons when transitioning between these levels.
7. What are energy levels in an atom?
Energy levels are the fixed orbits around the nucleus where electrons reside.
8. What is the Balmer series?
The Balmer series is a set of spectral lines produced by hydrogen when its electrons fall to the second energy level.
9. How did de Broglie explain Bohr's second postulate?
De Broglie explained Bohr's second postulate by suggesting that electrons exhibit wave-like properties and their orbits are standing waves.
10. What is the significance of the de Broglie wavelength?
The de Broglie wavelength is significant because it shows that particles, like electrons, have wave-like properties.
11. What is Sommerfeld's extension of Bohr's theory?
Sommerfeld extended Bohr's theory by introducing elliptical orbits and subshells, which explained the fine structure of spectral lines.
12. What is the Lyman series?
The Lyman series involves spectral lines produced by hydrogen when electrons fall to the first energy level, emitting ultraviolet light.
13. What is the fine structure of spectral lines?
The fine structure is the splitting of spectral lines into closely spaced components due to relativistic effects and electron spin.
14. What is the photoelectric effect?
The photoelectric effect is the emission of electrons from a material when it is exposed to light of sufficient frequency.
15. What is Planck's constant?
Planck's constant is a fundamental constant used in quantum mechanics, representing the proportionality between the energy and frequency of a photon.
16. What is an elliptical orbit?
An elliptical orbit is an orbit that is shaped like an ellipse, rather than a perfect circle.
17. What is a standing wave?
A standing wave is a wave that remains stationary, with nodes and antinodes.
18. What is the significance of the atomic model?
The atomic model helps us understand the structure and behavior of atoms, including how electrons are arranged and how they interact with light.
19. What is the nucleus of an atom?
The nucleus is the central part of an atom, containing protons and neutrons.
20. What is a photon?
A photon is a quantum of light energy emitted or absorbed during electron transitions.