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ATOMIC THEORYPart 2: Rutherford and the Gold Foil Experiment
The Story So Far…
The Discovery of the Nucleus Ernest Rutherford was a student under J.J.
Thomson
Moved to Montreal to study at McGill!
Holla - Canada!
Rutherford’s Work Radioactive elements were just recently
understoodThey give off rays of energy as they break down
○ These rays are known as “radioactive decay”
Rutherford’s Work Rutherford studied alpha particles
Emitted from decaying radioactive elements (ex. Radium, Polonium)
Alpha particles are large and have a positive charge
Rutherford’s Work To further study the atom, he decided to fire alpha
particles at a thin sheet of gold
He expected the particles being shot at the gold would pass through in a straight line
Rutherford’s Work But he was surprised!
Most of the particles went straight throughSome were deflected off course (weird)Some bounced right back at him (weirder!)
How in the world would he explain this??
Gold foil atoms
Rutherford’s Work
Since alpha particles are positive and they bounced right back at him…
Rutherford proposed that a small area of positive charge existed in the atom
The alpha particles were repelled by the like-charged centre of the gold atoms
Rutherford’s Work
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Rutherford’s Work
Rutherford named the centre of the atom “nucleus”
Proposed the nucleus had a positive charge
Proposed electrons orbited the nucleus, like planets around the Sun
Rutherford’s Work Rutherford’s observations can be
summarized as follows:
OBSERVATIONS CONCLUSIONS
1.Most alpha particles went straight through the foil
Atoms are mostly empty space
2.Some alpha particles were deflected a little
The nucleus is positive and repelled the alpha particles
3.A few alpha particles were deflected straight back
The nucleus is very tiny compared to the size of the whole atom
Thomson’s model vs Rutherford’s model
Major advancement in the understanding of the atom!
The next find…
A Danish scientist (Neils Bohr) traveled to England to work with now-famous Rutherford
Famous Curious
Bohr’s Contribution Bohr figured Rutherford was mostly correct…
He passed electricity through hydrogen gas and bands of light appearedLight bands corresponded with quantity of energy
applied to the gas
Unique bands of light were given off by ALL elements
Bohr’s Contribution Bohr reasoned that electrons moved in fixed
regions or energy levels
For an electron to move up a level, it needed outside energy to “bump it up”
The amount of energy required to excite an electron was called a “quantum”
The discipline of quantum mechanics was born!
Where can electrons be found?
In concentric “shells” around the nucleus…
… but in 3 dimensions
Bohr-Rutherford Model of the Atom
Think of a person on a ladder:You can’t stand between the rungs!
Bohr-Rutherford Model of the Atom
Electrons can be “bumped up” after absorbing energy or they can “fall down” if energy is not sustained
