Rutherford Model of the Atom Ernest Rutherford (1871-1937)

Rutherford Model of the Atom

Ernest Rutherford (1871-1937)

• Learned physics in J.J. Thomson’ lab.

• Noticed that ‘alpha’ particles were sometime deflected by something in the air.

• Gold-foil experiment

Rutherford ‘Scattering’

• In 1909 Rutherford undertook a series of experiments

• He fired  (alpha) particles at a very thin sample of gold foil

• According to the Thomson model the  particles would only

• be slightly deflected

• Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source

Rutherford’s Apparatus

Rutherford’s Apparatus

Geiger-Muller Counter

Geiger Counter

What He Expected

• The alpha particles to pass through without changing direction (very much)

• Because

• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

Interpreting the Observed Deflections

Density and the Atom

• Since most of the particles went through, the atom was mostly empty.

• Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

• Small volume and big mass = big density

• This small dense positive area is the nucleus

Rutherford Scattering (cont.)

• Rutherford interpreted this result by suggesting that the  particles interacted with very small and heavy particles

Table: hypothetical description of alpha particles

Explanation of Alpha-Scattering Results

Results of foil experiment if plum-pudding had been correct.

Interpreting the Observed Deflections

Rutherford’s Gold-Leaf Experiment Conclusions: Atom is mostly empty space Nucleus has (+) charge Electrons float around nucleus

Rutherford’s Experiment

Actual Results of Gold-Leaf Experiment

The Rutherford Atom

Scale of the atom.

Discovery of the electron

• 1807 Davy suggested that electrical forces held compound together.

• 1833 Faraday related atomic mass and the electricity needed to free an element during electrolysis experiments.

• 1891 Stoney proposed that electricity exists in units he called electrons.

• 1897 Thomson first quantitatively measured the properties of electrons.

Coulomb’s Law

• Why don’t electrons collide while moving around the outside of atom?

Hit moth driving car – no change in car direction

• Hit moth driving car – no change in car direction

• Hit deer – car changes direction

Hit moth driving car – no change in car direction

• Hit moth driving car – no change in car direction

• Hit deer – car changes direction

Force