11TH GRADE
CHEMISTRY
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Notes
INTRODUCTION:
RUTHERFORD ALPHA PARCTICLES SCATTERING EXPERIMENT (1911).
Rutherford bombarded very thin gold foil with alpha paricles ( gold foil had a circular
flouroscent Zns screen). And observed that:
1. Most of alpha particles passes through gold foil in straight direction.
2. Gold foil is highly malleable
3. A small fraction deflected through very small angle.
4. A very few particles about (1 in 20,000) bounce back i.e; deflected by 180°.
Note: source of alpha particles was Radium, a radio active substance.
●Following conclusions were drown from Rutherford's experiment:
○Most of the space is empty in atom as most of the alpha particles passed
undeflected.
○A few alpha particles were deflected, it proves that whole +ve charge is
comcentrated and space occupied by +ve charge is very small.
○Shape of the nucleus is very small as compared to atom
○Radius of atom 10-¹⁰meter and radius of nucleus 10-¹⁵meter.
○A very few particles returned their path indicating the nucleus is very rigid and
alpha particles recoil due to firect collisions.
on the above observation Rutherford proposed the nuclear model of atom.
According to this model:
1. Mass of atom is comcentrated in bery small region that is called 'nucleus' by Rutherford.
2. Nucleus is surrounded by electrons and electron are moving around the nucleus on
circular orbit.(resembles like solar system)
3. Electron and nucleus are held together by electrostatic force.
Draw backs of Rutherford's atomic model.
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