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CBSE Class 12 · Physics

Atoms

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Atoms” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom

Rutherford: alpha scatter as taught — most go through, a few bounce — forced a tiny positive nucleus. Nuclear atom is that look, not a plum-pudding leftover. A nucleus-drawn as half the atom is a lying scale.

A soft-ball-with-specks as the leftover this look killed.

Figure. Most alpha paths cross the thin foil. One path hits the tiny massive core and scatters. Empty space is the through paths; the nucleus is the rare scatter. Sizes are schematic, not to scale.

How it works

  1. Name the scatter-lookMost through, few bounce.
  2. Land on a tiny + centreThe nucleus.
  3. Keep the scale tinyHonest.

2Atomic Spectra

Atomic spectra: sharp lines as taught, not a paint-rainbow. A line is a jump-look waiting for Bohr. Spectrum here is hydrogen’s named series if listed.

A continuous bulb-spectrum as this heading is a miss.

Figure. A hot solid fills a continuous band of wavelengths. A gas of atoms emits only a few lines. Discrete lines are the atomic spectrum, not the band.

How it works

  1. Name sharp linesThe spectrum.
  2. Keep a series-name if taughtThe extra.
  3. Refuse a paint-rainbow as the lookLines.

3Bohr Model of the Hydrogen Atom

Bohr for hydrogen: allowed orbits, angular-momentum quantum as taught, E ∝ −1/n^2. A jump emits or absorbs a photon. Bohr is a hydrogen-story, not every atom. n=1 is the ground as framed.

Planet-roads for oxygen as Bohr is a steal.

Figure. Bohr allows only whole-number levels. An electron falling from n=3 to n=2 emits one photon. Level gaps are schematic, not to scale; a circular orbit is not drawn.

How it works

  1. Stay on hydrogenThe gate.
  2. Write allowed n and E tracking −1/n^2The model.
  3. Name a jump as a lineThe use.

4The Line Spectra of the Hydrogen Atom

Hydrogen lines: 1/λ = R (1/n1^2 − 1/n2^2) as taught. Balmer is n1=2 if named. A line is a pair of n, not a paint. One n=3 to n=2 is enough.

Using n=1 to n=1 as a line is a miss.

Figure. Hydrogen’s visible Balmer lines sit at 434 nm, 486 nm and 656 nm on a 400 to 700 nm axis. H-alpha is the long-wavelength line. Positions are to scale on that axis.

How it works

  1. Copy n1 and n2The given.
  2. Form the taught 1/λThe line.
  3. Keep Balmer as n1=2 if askedHonest extra.

3 to 2

A jump n=3 to n=2 (Balmer as taught). What sits in the (1/n1^2 − 1/n2^2) bracket?

  • n12
  • n23
  • Bracket1/4 − 1/9 = 5/36

Pro tip. Lower n is n1 in the usual write.

5DE Broglie’s Explanation of Bohr’s Second Postulate of Quantisation

de Broglie on Bohr: the orbit fits a standing-wave number of λ as taught — 2πr = n λ. This heading is a why-the-quantum-second-postulate, not a new atom. One standing-wave mention is enough.

A sound-standing-wave as the whole Bohr is a miss.

Figure. de Broglie: an allowed path holds a whole number of wavelengths. Here n=2, so two loops fit. Amplitude is signed about the mid axis. A circular orbit is not drawn.

How it works

  1. Write 2πr = n h/p as taughtThe fit.
  2. Keep it a reason for allowed nThe why.
  3. Refuse a university dumpThis class.

6A definition is a test you can run

Atom-word is a test: a scatter-nucleus, a line, Bohr, or a standing-wave fit. If you only say “tiny”, you have a heading.

A glow-sticker is not the test.

Figure. A model is a test you can run. Thomson spreads the plus charge. Rutherford puts mass in a tiny core. Bohr adds discrete levels. Scatter picks Rutherford; line spectra pick Bohr.

How it works

  1. Name nucleus, line, n, or the fitThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.

7Name the given before the unknown

The given is n1, n2 or a scatter-look. The unknown is 1/λ or the model-word. Copy n=3 to n=2 before you treat it as 4 to 2.

Using Bohr orbits for a multi-electron dump is a silent steal.

Figure. Write the two levels first. The Rydberg line then asks for 1/lambda. The wavelength is the unknown, not a third level you invent.

How it works

  1. Copy the n-pair or the lookThe given.
  2. Name the line or the nucleusThe unknown.
  3. Then writeGiven first.

8One worked case is enough at this class

One worked case is enough: 5/36 in the Balmer bracket, or the gold-foil look. Do not stack four series.

A poster of twenty lines is not more science.

Figure. One hydrogen case: n=2 at -3.4 eV falls to n=1 at -13.6 eV. The photon carries 10.2 eV. Levels are schematic in spacing; the energies are the school numbers.

How it works

  1. Take one jump or one scatterThe case.
  2. Run the writeThe teach.
  3. Stop — one caseThis class.
Bohr’s school model is sized for
  1. Hydrogen as taught
  2. Every atom equally
  3. Molecules only

A hydrogen-story.

Notes

  • Mapped to the official NCERT chapter “Atoms”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • E tracks −1/n^2 (H, as taught)
  • 1/λ = R (1/n1^2 − 1/n2^2)

Recap

Hold these pegs from the official chapter “Atoms”. The wording is ExamMaster’s teaching, not a textbook recap.

Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom
Rutherford: alpha scatter as taught — most go through, a few bounce — forced a tiny positive nucleus.
Atomic Spectra
Atomic spectra: sharp lines as taught, not a paint-rainbow.
Bohr Model of the Hydrogen Atom
Bohr for hydrogen: allowed orbits, angular-momentum quantum as taught, E ∝ −1/n^2.
The Line Spectra of the Hydrogen Atom
Hydrogen lines: 1/λ = R (1/n1^2 − 1/n2^2) as taught.
DE Broglie’s Explanation of Bohr’s Second Postulate of Quantisation
de Broglie on Bohr: the orbit fits a standing-wave number of λ as taught — 2πr = n λ.
A definition is a test you can run
Atom-word is a test: a scatter-nucleus, a line, Bohr, or a standing-wave fit.

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