Disproving the Plum Pudding model (3)
Alpha particles scattered with thin metallic foil (Au)
Scattered through large angles in 1/8000 cases
This is not possible with the Plum Pudding model
Calculating work done moving an alpha particle towards a gold atom - assumptions (2)
Ignore recoil - gold atom remains stationary throughout
Ignore electrons surrounding gold atom
Potential energy
U(r) = Qq/4πεor
Calculating work done - alpha particle outside atom
Force is the same as that produced by a point charge of the same magnitude - treat atom as point charge
Calculating work done - alpha particle inside atom (2)
Atom treated as hollow sphere of charge so electric field is zero
Alpha particle only experiences the fraction of charge inside the sphere
Force varying with distance of alpha particle from atom (2)
Force increases proportional to distance until r = a
Force proportional to 1/r^2 when r > a
Why would the Plum Pudding model of an atom be unable to backscatter an alpha particle?
Insufficient energy - the positive charge of the atom must be more concentrated
Why does large angle scattering occur? (3)
Large angle scattering only occurs if the particles never penetrate the nucleus
Rutherford calculated the scattering probability as a function of angle assuming point charges (so alpha particle would never enter the atom)
Experimental data matched the model showing that the point charge assumption is correct so the alpha particle doesn’t enter the atom
Why was the Rutherford model flawed?
Accelerating charges radiate energy so atoms should collapse
What is the Bohr model of the atom?
Electrons can only move in certain, non-radiating orbits called stationary states - radiation only emitted/absorbed when electron transitions between orbits so atomic energy levels are quantised
Why is the Bohr model more accurate than previous models? (3)
Permits orbits of hydrogen to be calculated
Accurately predicts ionisation potential of hydrogen
Explains emission spectra of hydrogen
What is a continuous spectrum?
Light of all wavelengths is present (light source is a hot solid or liquid)
What is an emission line spectrum? (2)
Spectrum produced by a heated gas, containing isolated parallel lines
The spectral lines correspond to a definite wavelength and frequency
What is an absorption line spectrum? (2)
The spectrum produced when white light is passed through cool gas
The transmitted light produces a spectrum of dark lines corresponding to the absorbed wavelengths
