Physics · Atoms · NEET
One: most alpha particles passed straight through the gold foil with no deflection. Two: a small number were deflected through small angles. Three: a very few - about 1 in 8000 - were deflected by more than 90 degrees, and some even bounced almost straight back. NCERT notes only about 0.14 percent scattered by more than 1 degree.
An alpha particle is fast and heavy. If the atom were solid throughout (like Thomson's model), every particle would feel a force and bend. Instead most felt almost nothing, so they must have travelled through empty space. This means the atom is mostly empty.
To turn a fast, heavy alpha particle right around needs a very large repulsive force. Such a strong force is only possible if all the positive charge and most of the mass are squeezed into a very small region. That small region is the nucleus. A backward bounce happens only in a near head-on hit, which is rare - so the nucleus must be tiny.
An observation is what was seen in the experiment (most pass, few bend, rare bounce back). A conclusion is what we infer from it (atom is mostly empty, has a tiny dense positive nucleus, electrons are outside). NEET often asks you to match each observation to the correct conclusion.
No. Electrons are about 7000 times lighter than an alpha particle, so they cannot appreciably change its path. The large deflections come only from the heavy, concentrated positive nucleus, not from the light electrons.
The number N is very large for small scattering angles and falls very steeply as the angle increases. It follows N proportional to 1/sin^4(theta/2). This confirms that most particles scatter only a little and only a few scatter through large angles.
In the Geiger-Marsden experiment, the number of scattered alpha-particles N(theta) is plotted as a function of scattering angle theta. Which option represents the correct plot?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
He used a very thin gold foil, about 2.1 x 10^-7 m thick. Gold is very malleable, so it can be beaten into an extremely thin sheet only a few atoms thick. This ensures each alpha particle is scattered by at most one nucleus, making the result clean to analyse.
Alpha particles were emitted by a radioactive bismuth-214 source and collimated into a narrow beam using lead bricks. The scattered particles were detected as tiny flashes (scintillations) on a zinc sulphide screen viewed through a rotatable microscope.
Very rare. Only about 0.14 percent of alpha particles scattered by more than 1 degree, and only about 1 in 8000 deflected by more than 90 degrees. This rareness is exactly why Rutherford concluded the nucleus is extremely small.
They disproved Thomson's plum pudding model, in which positive charge was spread evenly through the atom. If that were true, no alpha particle could be deflected by a large angle. The results led to Rutherford's nuclear (planetary) model.
The rare large-angle bounce-back is the most important. It directly proves the existence of a tiny, dense, positively charged nucleus. The straight-through majority proves the atom is mostly empty space.
