Rutherfords Gold Foil Rutherfords Gold Foil ExperimentExperiment

The History of Atomic TheoryThe History of Atomic Theory

Intro QuestionsIntro Questions

In the ‘Plum pudding’ model, what part of the atom holds In the ‘Plum pudding’ model, what part of the atom holds the positive charge that balances the negative charge of the positive charge that balances the negative charge of the electrons? the electrons?

If the idea was that the ‘plums’ were electrons, what was If the idea was that the ‘plums’ were electrons, what was the rest of the ‘pudding’ made of?the rest of the ‘pudding’ made of?

Is the ‘Plum pudding’ model the best way of representing Is the ‘Plum pudding’ model the best way of representing the atom?the atom?

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was hard Rutherford was hard at work on an at work on an experiment that experiment that seemed to have little seemed to have little to do with unraveling to do with unraveling the mysteries of the the mysteries of the atomic structure.atomic structure.

Rutherford’s experiment Involved firing Rutherford’s experiment Involved firing a stream of tiny a stream of tiny positively chargedpositively charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 atoms thick)(2000 atoms thick)

MostMost of the positively of the positively charged “bullets” passed charged “bullets” passed right through the gold right through the gold atoms in the sheet of atoms in the sheet of gold gold foilfoil without changing without changing course at all.course at all.

SomeSome of the positively of the positively charged “bullets,” however, charged “bullets,” however, did bounce away from the did bounce away from the gold sheet as if they had hit gold sheet as if they had hit something something solidsolid. He knew . He knew that positive charges that positive charges repelrepel positive charges.positive charges.

http://chemmovies.unl.edu/ChemAnime/http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.htmlRUTHERFD/RUTHERFD.html

http://chemmovies.unl.edu/ChemAnime/http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.htmlRUTHERFD/RUTHERFD.html

This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly sheet were mostly open spaceopen space. Atoms were . Atoms were notnot a pudding filled with a positively charged a pudding filled with a positively charged material.material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

RutherfordRutherford

Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atom’s atom’s edgeedge..

Focus Questions:Focus Questions:

What development in technology allowed Rutherford to direct What development in technology allowed Rutherford to direct a stream of alpha particles?a stream of alpha particles?

Why was gold foil used? Why was gold foil used?

Why did the foil need to be very thin?Why did the foil need to be very thin?

How did the alpha particle emitter allow for nuclei to be How did the alpha particle emitter allow for nuclei to be identified as subatomic particles?identified as subatomic particles?

What is a nucleus?What is a nucleus?

Focus QuestionsFocus Questions How did Rutherford’s scattering experiment results change the accepted How did Rutherford’s scattering experiment results change the accepted

model of how the atom is structured?model of how the atom is structured?

How might Rutherford have determined that the nucleus was much larger How might Rutherford have determined that the nucleus was much larger than the electrons?than the electrons?

Once the nucleus had been discovered, what questions might scientists Once the nucleus had been discovered, what questions might scientists have asked about this nucleus?have asked about this nucleus?

How does the nucleus compare with electrons?How does the nucleus compare with electrons?

Where in the atom is the nucleus located?Where in the atom is the nucleus located?

Why is the term ‘planetary model’ used for the atom?Why is the term ‘planetary model’ used for the atom?