3. If you fire an alpha particle at a plum pudding atom why do you expect the al

Question

3. If you fire an alpha particle at a plum pudding atom why do you expect the alpha particle to come blasting right through? (Hint: the answer must include the relative mass of the alpha particle and the electron.)

4. Ernst Rutherford fired alpha particles at metal foils: a) What happened?

b) This experiment disproved the t h e n current model of the atom. Name and describe the model that was disproved.

c) On the basis of the experiment, Rutherford proposed a new model. Name and describe the new model.

d) How does the new model explain the Rutherford experiment?

5. With his oil drop experiment, Millikan determined charge on the electron. How did this lead to the determination of the mass of the electron?

Explanation / Answer

3.  The alpha particle is equivalent of a helium nucleus. It is very small and relatively heavy for its size. It should plow right through the plum pudding model atom like a bullet through a plum pudding. Also, the plum pudding model had the positive and negative charges equally dispersed within the atom, making it neutral overall, so a positively charged alpha particle was not expected to interact with it and to pass straight through.

4. a) i) Most of the alpha particles went straight through undeflected. ii) Some of the alpha particles were deflected back through large angles. iii) A very small number of alpha particles were deflected backwards.

4 b) JJ Thomson's Plum Pudding Model - In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”.

4 c) Rutherford's Planetary Model of the Atom - In this model, all the protons reside in the nucleus and the electrons orbit around the nucleus like planets around the sun.

4 d) i) An atom consists a very small positively charged body located at the center, known as the "nucleus". This nucleus contains protons and neutrons. Since electrons have negligible mass, the entire mass of the nucleus is due to the presence of protons and neutrons which reside in the nucleus.

ii) Since a very less number of alpha particles get deflected, it proves that the volume of the nucleus is very less compared to the whole atom. The radius of a nucleus is 10-13- 10-12 m and for an atom, it is 10-8 cm.

iii) Electrons revolving around the nucleus in a certain path are known as orbits.

iv) The extremely small, negatively charged electrons are distributed around the nucleus and balance the charge of it.

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