UP Board - Class 12 physics - Chapter 12: Atoms Handwritten Notes

An atom is the basic unit of matter that consists of a dense central nucleus surrounded by a cloud of electrons. The study of atoms define the  structure, behavior, and interactions, forming the foundation of modern physics and chemistry.

Key Points:

1. Structure of Atom:

   • An atom consists of a nucleus containing protons and neutrons, with electrons revolving around it in specific orbits.
   • The nucleus is positively charged due to protons, while electrons are negatively charged.

2. Rutherford’s Model of Atom:

   • Proposed that most of an atom's mass is concentrated in a small, dense nucleus.
   • Electrons revolve around the nucleus in circular orbits.
   • This model failed to explain the stability of atoms and the emission spectra.

3. Bohr’s Model of Atom:

   • Suggested that electrons revolve around the nucleus in discrete orbits called energy levels.
   • Electrons do not radiate energy while in these stable orbits.
     

4. Emission and Absorption Spectra:

   • When an electron jumps from a higher to a lower energy level, it emits energy in the form of light, producing an emission spectrum.
   • When an electron absorbs energy, it moves to a higher energy level, producing an absorption spectrum.

5. Hydrogen Atom Spectrum:

   • The hydrogen atom exhibits a series of spectral lines (Lyman, Balmer, Paschen, etc.) in different regions of the electromagnetic spectrum.
   • These lines correspond to electron transitions between specific energy levels.

6. Limitations of Bohr’s Model:

   • Could not explain the spectra of atoms with more than one electron.
   • Did not account for the splitting of spectral lines in magnetic fields (Zeeman effect) or electric fields (Stark effect).

7. Modern Atomic Model:

   • The quantum mechanical model of the atom, based on Schrödinger’s equation, describes electrons as wave-like entities with probabilities of being found in certain regions around the nucleus (orbitals).

8. Applications of Atomic Theory:

   • Understanding atomic structure has led to advancements in spectroscopy, quantum mechanics, and nuclear physics.
     

Conclusion:

The study of atoms has been pivotal in understanding the fundamental structure of matter. From Rutherford’s and Bohr’s models to modern quantum mechanics, atomic theory has evolved significantly, explaining the behavior of elements and their interactions. This understanding forms the basis for advancements in science, technology, and industry.

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