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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(+)(+)

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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