Atoms
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Rutherford Scattering and Nuclear Atom
Part of: Rutherford Atom and Spectra
Alpha particles fired at gold foil mostly sail through, but rare ones bounce back, proving positive charge sits in a nucleus 10,000 times smaller than the atom. Head-on particles turn at the distance of closest approach, where kinetic energy fully becomes electric potential energy.
Why it matters. This experiment moved the nucleus to centre stage for all of nuclear physics. Reading large deflection as proof of diffuse charge reverses its lesson.
Formulas
- Coulomb energy = kinetic at closest approach
- Nucleus size about 10^-14 m, atom about 10^-10 m
- Larger impact parameter gives smaller scattering
Key words
- Alpha scattering
- : Deflection of helium nuclei by the nuclear charge, mapping the atom.
- Nucleus
- : Tiny central core holding nearly all atomic mass and positive charge.
- Impact parameter
- : Sideways miss distance setting the scattering angle.
- Closest approach
- : Nearest reach of a head-on particle, fixed by energy balance.
Try it in two minutes
- Count how few particles scatter wide.
- Infer a tiny, heavy target from rare rebounds.
- Equate kinetic and potential energy for closest approach.
Try at home. Roll marbles at a hidden heavy ball and map hits to find it.
Atomic Spectra
Part of: Rutherford Atom and Spectra
Hot gases glow only at fixed wavelengths while cool gases absorb the same lines from white light, giving each element a unique barcode. Hydrogen's visible Balmer lines end on level 2, while the Lyman series ending on level 1 lies in the ultraviolet.
Why it matters. Stars, lamps and lasers are identified by their lines. Expecting continuous rainbows from atoms misses quantisation entirely.
Formulas
- Photon from a jump: E = Ehigh - Elow
- Same gas absorbs what it emits
- Balmer ends at n = 2; Lyman at n = 1
Key words
- Emission spectrum
- : Bright lines from a glowing gas at its fixed wavelengths.
- Absorption spectrum
- : Dark gaps where cool gas removed its fixed wavelengths.
- Balmer series
- : Hydrogen lines ending on level 2, partly visible.
- Lyman series
- : Hydrogen lines ending on level 1, in the ultraviolet.
Try it in two minutes
- Burn salts and note each colour line.
- Match dark absorption lines with bright emission lines.
- Group hydrogen lines by their lower level.
Try at home. View street lamps through a grating and list the lines.
Bohr Model and Quantised Orbits
Part of: Bohr Model and Transitions
Bohr let electrons circle only where angular momentum is a whole multiple of h over 2 pi, freezing radiation on those tracks. Radii grow as n squared times 0.53 angstrom, speeds fall as 1 over n, and energies sit at minus 13.6 over n squared eV for hydrogen.
Why it matters. The model predicts every hydrogen line from first principles. Letting radius grow linearly, or forgetting the minus sign, breaks all level answers.
Formulas
- Angular momentum: m v r = n x h/(2 pi)
- Radius: rn = n^2 x 0.53 angstrom
- Energy: En = -13.6/n^2 eV
Key words
- Bohr radius
- : Smallest hydrogen orbit, 0.53 angstrom.
- Ground state
- : Lowest energy level, n = 1 at -13.6 eV.
- Excited state
- : Any level above the ground state.
- Ionisation energy
- : Energy to free the electron, 13.6 eV from ground.
Try it in two minutes
- Square n and multiply by 0.53 angstrom for radius.
- Divide 13.6 eV by n squared and negate for energy.
- Fit whole de Broglie waves around the circle.
Try at home. List radii and energies for the first four levels in a table.
Energy Levels and Transitions
Part of: Bohr Model and Transitions
Jumping down emits a photon carrying the exact gap energy, and jumping up absorbs the same. The red 3 to 2 line carries 1.89 eV at 656 nm, while the 2 to 1 jump gives 10.2 eV at 122 nm, and series limits mark ionisation energies.
Why it matters. Lasers, discharge lamps and stellar spectra are all gap arithmetic. Adding instead of subtracting level energies flips every photon answer.
Formulas
- Photon: E = Eupper - Elower
- Wavelength: lambda(nm) = 1240 / E(eV)
- Ionisation from n needs 13.6/n^2 eV
Key words
- Energy level
- : Allowed electron energy, negative for bound states.
- Transition
- : Electron jump between levels with photon emission or absorption.
Try it in two minutes
- Write both level energies with signs.
- Subtract lower from upper for the photon.
- Divide 1240 eV nm by the gap for wavelength.
Try at home. Compute the photon for a 4 to 2 jump and name its colour.
What your child will practise
- Atomic Spectra: Candy Quick Fire (Quick Fire)
- Atomic Spectra: Space Puzzle (Clue Puzzle)
- Bohr Model and Quantised Orbits: Ocean Puzzle (Clue Puzzle)
- Energy Levels and Transitions: Ocean Quest (Flip and Match)
- Rutherford Scattering and Nuclear Atom: Ocean Quick Fire (Quick Fire)
- Quiz: Atomic Spectra (8 questions)
- Quiz: Bohr Model and Quantised Orbits (8 questions)
- Quiz: Energy Levels and Transitions (8 questions)
- Quiz: Rutherford Scattering and Nuclear Atom (8 questions)