Discovery of Atoms Short Questions and Answers

What was the need of modifying Dalton's billiard ball model of the atom?

When Dalton presented his billiard ball model, atoms were considered to be the smallest particle of matter. But after the discovery of negatively charged particles in the atom, Thomson presented his plum pudding model of the atom. The finding of the cathode ray experiment showed that cathode rays are part of all kinds of elements which means they are present in every atom.

Why does Thomson's plum pudding model of the atom have a sea of positively charged particles rather than discrete particles?

Atoms were considered to be neutral even at that time. So, to balance the negative particles with the positive ones, Thomson proposed that the rest of the atom is positively charged. 

Why did Rutherford expect that the alpha particles would pass through the thin gold foil?

Rutherford wanted to confirm Thomson's model of the atom according to which electrons are present in the sea of positive charge. As the whole volume is filled by a positive charge,  the density of the atom must be very low. So, when the alpha particles would pass the thin gold foil, they must go straight or face very little deflection.