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Class 9 Structure of Atom (Advanced): cheat sheet
CBSE Science. How the electron, proton and neutron were found, why atoms give line spectra, the series of hydrogen lines, and what Rutherford's and Bohr's models could and could not explain.
Key ideas
- Cathode rays bend towards the positive plate and are the same for every gas: electrons are in all atoms.
- Canal rays are positive ions of the gas, so they change with the gas; from hydrogen the lightest is the proton.
- Chadwick's uncharged particles, from beryllium hit by alpha particles, are neutrons; they explain why atoms weigh more than their protons.
- White light gives a continuous spectrum; a glowing element gives a line spectrum that identifies it.
- Rutherford's model could not explain the atom's stability or the line spectrum of hydrogen.
- Bohr: electrons stay in fixed orbits without radiating and give out light only when they jump down, so only certain lines appear.
- Bohr's model fails for many-electron atoms, the fine structure, and the Zeeman and Stark effects.
Remember
| Hydrogen series (lower level) | Lyman 1, Balmer 2, Paschen 3, Brackett 4, Pfund 5 |
| Rydberg equation | wave number = R(1/n₁² − 1/n₂²), R = 109,677 cm⁻¹ |
| Series limit | n₂ → ∞: wave number = R ÷ n₁² |
| Particles | protons = electrons = Z; neutrons = A − Z; ions: electrons = Z − charge |
| Lines possible among k levels | k(k − 1) ÷ 2 |
Watch out
- Naming a series by the upper level instead of the level the electron falls to.
- Writing 1/n₁ − 1/n₂ without squaring the levels.
- Thinking canal rays, like cathode rays, are the same for every gas.
Worked example
An electron in a hydrogen atom falls from n = 7 to n = 4, and then from n = 4 to n = 3. (a) Name the series of the first line. (b) Name the series of the second line. (c) Using R = 109,677 cm⁻¹, find the wave number of the single jump from n = 7 to n = 3, to the nearest whole number, in cm⁻¹ (per cm). (d) Is it the sum of the two smaller jumps' wave numbers? (Yes or No)
Working: (a) lower level 4; (b) lower level 3; (c) 109,677 × (1/3² − 1/7²); (d) (1/4² − 1/7²) + (1/3² − 1/4²) = 1/3² − 1/7², so energies add
Answer: (a) Brackett; (b) Paschen; (c) 9,948 per cm; (d) Yes
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