Physics · Atoms · NEET
Because the atom is mostly empty space. The whole positive charge and almost all the mass are packed into a tiny nucleus at the centre. The nucleus is about 10,000 times smaller than the atom, so most of the space in the atom has nothing that can push the alpha particle. An alpha particle travelling through this empty region feels no strong force, so it keeps going straight and comes out undeflected.
A small number of alpha particles happen to travel very close to a nucleus. The nucleus is positive and the alpha particle is also positive, so they repel each other (Coulomb repulsion, force = k(2e)(Ze)/r squared). The closer the alpha particle comes to the nucleus, the stronger this push, so it bends by a large angle. A very few, on an almost head-on path, are pushed straight back by nearly 180 degrees. These close approaches are rare, so large deflections are rare.
No, not in any real way. An electron is about 7,000 times lighter than an alpha particle. A heavy alpha particle passing a light electron is like a fast truck passing a small ball - the truck barely changes direction. So electrons cannot bend the alpha particle. Only the heavy, concentrated nucleus can cause a large deflection.
In the empty region of the atom there is no concentrated charge close to the alpha particle. The positive charge is not spread out (that was Thomson's wrong idea); it is squeezed into the tiny nucleus. So unless the alpha particle passes near that tiny nucleus, the electric force on it is almost zero, and it moves in a straight line.
The atom has a radius of about 10 to the power minus 10 metre. The nucleus has a radius of about 10 to the power minus 15 metre. So the nucleus is about 100,000 times smaller in size than the atom. By volume the nucleus takes up an extremely small fraction of the atom. That is why almost every alpha particle finds only empty space on its path and passes straight through.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Almost all of them. In the Geiger-Marsden experiment the very large majority of alpha particles passed straight through with little or no deflection. Only about 1 in 8,000 was deflected by more than 90 degrees, showing how rare a close encounter with the nucleus is.
It proves the atom is mostly empty and that the positive charge is not spread out. If Thomson's model were right (positive charge spread over the whole atom), no alpha particle would be pushed back strongly. The straight-through majority plus the rare large deflections together point to a tiny, dense, positive nucleus.
Yes, it is the other side of the same fact. Because the nucleus is tiny, most alpha particles miss it (pass straight), but the few that hit it almost head-on meet the full concentrated positive charge at very short distance and are repelled through large angles, even close to 180 degrees.
It is not a true vacuum - light electrons move in that region. But for the heavy alpha particle those electrons are far too light to matter, so effectively the alpha particle sees empty space with a tiny hard nucleus in the middle.
