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The plum pudding question

  1. Jan 28, 2012 #1
    Guys, i need some explanations please. I am in a gen chem class and have some How and why questions. Here goes..So this guy named JJ thomson had a theory he called the plum puddign theory. He says that all the electrons of an atom are surround by a positively charged sphere. Then Rutherford came along and aimed some alpha particles at a piece of gold foil, How? Some of the particles were completly deflected and some were deflected at small angles. From this info he concluded that the atom must have a dense core based on the behavior of the alpha particles being deflected.

    This is where i am confused. How does the deflection show that atoms have a core? How does it realate or not relate to the plum pudding theroy?

    If Nitrogen has two isotopes, N14 with 99.63% abudance, and a mass of 14.003074, and N15 with .037% abundance and a mass of 15.000108. then how and who counted all the atoms to figure that N14 does indeed have an abudance of 99.63%?

    Avogados number. How did he count to 6.206x10^-34? thats a big number.

    Thank you for the help. Just trying to piece it all together.

  2. jcsd
  3. Jan 28, 2012 #2


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    The key observation that disproved the plum pudding model was that most of the alpha particles passed through the gold foil unaltered in their trajectory. This result implies that the atom is mostly empty space, in direct opposition to the view of an atom as a pudding of positive charge with negatively charged "plums" floating around in the pudding.

    Such measurements can be done on instruments called mass spectrometers that can very precisely measure the mass of atoms and molecules. This instrument can very rapidly sort atoms by mass and count their abundance.

    6.206x10^-34 is actually a very tiny number. But, I think you are referring to 6.206x10^24 which is quite a large number. Nobody counted out Avogadro's number to determine it. Rather, it was first calculated. In the early 1900s, scientists had figured out the charge carried by a mole of electrons. They had also figured out the charge of a single electron. Dividing the charge of a mole of electrons by the charge of a single electron allowed scientists to calculate the number of electrons in a mole.
  4. Jan 30, 2012 #3


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    Not only did it disprove the plum pudding model, but it left only the model where all the positive charge was concentrated at the center of the atom. Is there a particular reason why the only model considered is all the positive charge at the center of the atom rather then other models that would still match his results? Specifically I am interested in why an empty very thin "sphere" of charge (like a bubble or a beach ball) would be excluded from consideration.
  5. Jan 31, 2012 #4
    sorry for the late reply. just studying for a chem test. My wife has an interesting question for you as do i. If the density of AL is 2.70g, how can they be a constant. it is based off of mass over volume. But if you compress the object, the mass stays the same but the volume has changed so how is the density of AL or gold always 2.7 and 19.x? I tried an experiment with my wife. i took a scale, a very accurate one, and used two ball bearings. both the exact same size but different masses. however the displacment of water was exactly the same. thats good news cuz they are the same size but what if you compressed the steel ball bearing into a smaller size, lets say half the size, or even chopped it up into testing pellet sizes(like in my chem lab class) The mass is still the same but the volume has changed so when i put it into the cylinder with water, the displacment of water will be differnt. That is bad news since the density is mass of volume. the volume would have to stay the same so that the density comes out the same.

    a aluminum marble 1 inch diameter is compressed down to a aluminum marble .5 diameter. the mass will be the same but diff volume now, but wont the density be differnt.

    Last edited: Jan 31, 2012
  6. Jan 31, 2012 #5


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    Correct. That's why density is not constant, but is a function of pressure. Google compressibility. However, you need insane pressures to compress most solids, so it can be safely assumed volume is constant in the reasonable range of pressures.

    Do I understand you correctly that you assume chopped BB has lower volume than the original BB? That's typical for Piaget's preoperational stage, I suppose you are too old for that :devil:
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