The plum pudding model

Before Geiger and Marsden did the important gold leaf experiment for Ernest Rutherford, the best idea about the inside of an atom was called the "Plum Pudding" model.

• They had just discovered the electron so they knew that there had to be negative particles inside the atom.
• If there were negatives, there had to be positive as well.
• They imagined the positive charge as being thinly smeared all over the atom. There were no definite positive particles in this model.
• The positive is supposed to be like the dough in a Christmas Pudding, and the electrons are like the raisins.

Any alpha particles fired at it should go straight through because

• alpha particles are much bigger and heavier than electrons so they would knock them out of the way.
• the positive charge is so thinly spread that there is no chance of repelling the doubly positive alpha particle.

Then the gold leaf experiment was done and Rutherford invented the Solar System model with the positive nucleus and orbitting electrons. The Plum Pudding model was consigned to the dustbin of history.