Wednesday, September 20, 2006

Rutherford’s Experiment

Lesson Title: Rutherford’s Experiment

Learning Outcome:
At the end of the sessions, you must be able to explain Rutherford’s experiment to prove the existence of the nucleus.

Learning Presentation:
Rutherford as you might remember contributed to the idea of what an atom might have looked like click here to find out how he did it

If you want additional references to help you know him better check out your textbook its on pages 61 and 62. :)

In 1899, J. J. Thomson proposed a model of the atom. The atom according to him is a positive ball of electricity wherein negatively charged particles are embedded. For this time, this so-called plum pudding model was accepted.

In 1908-1909, Ernest Rutherford and his co-workers performed the well-known gold foil experiment. They hammered a gold foil into a thin sheet that was a few atoms thick. They fired alpha particles at the thin gold foil. An alpha particle is positively charged (charged 2+) with a mass four times (mass 4 u) that of a H-atom.

Rutherford tested Thomson’s model of an atom. H e concluded that if his model were correct, most of the alpha particles would pass through a thin gold foil with very slight deflections, because the positive charge of the atom was diffused.

play this video to have a clearer and better understanding of the atom as proposed by Rutherford

Rutherford was surprised by the result of his experiments. His observations were the following:
Most of the alpha particles passed through the gold foil undeflected.

A few alpha particles passed through the gold foil with large angles of deflection.
Still fewer alpha particles bounced back in the direction from which they came.

The bouncing back and scattering of the alpha particles could not be satisfactorily explained using Thomson’s model of the atom.

After several years of study, Rutherford interpreted his observations. His conclusions were:
The atom has a central core called the nucleus. It is here where the mass of the atom is concentrated.

The charge of the nucleus is positive.

The nucleus is very small compared to the volume of the atom. The atom is mostly empty space.

The electrons are found outside the nucleus.

Later measurements showed that diameter of a nucleus are very small (about 10-12 cm) compared to the diameter of an atom (1 to 5 x 10-8 cm). This is about the size of a marble compared to the size of a softball field (atom).

To measure if you have gained sufficient knowledge about the atom try this activity
Atomic Hangman

How did he discovered the nucleus? how did the deflection of the alpha particles led him to say that there is a nucleus? Place your answer by clicking here

A score of more than 70% is excellent and an indicator that you are ready for your next lesson
Take this quiz and test your knowledge


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