Question

Describe in brief, the \(\alpha\)-scattering experiment. Write down about the atomic-structure from the observation obtained from the experiment.

Hint:

Rutherford famously said of the large-angle scattering, "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." This highlights how surprising and significant the discovery of the nucleus was.

Answer 1

Step 1: Description of the \(\alpha\)-Scattering Experiment (Geiger-Marsden Experiment): 
The \(\alpha\)-scattering experiment was performed by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford around 1909.
Setup: A narrow beam of high-energy alpha particles (which are doubly-ionized helium nuclei, He\(^{2+}\)) from a radioactive source (like Radium) was directed at a very thin gold foil (about \(10^{-7}\) m thick).
Detection: The scattered alpha particles were detected using a movable detector consisting of a rotatable zinc sulfide (ZnS) screen and a microscope. When an alpha particle strikes the screen, it produces a tiny flash of light (scintillation) which can be observed.
Objective: The experiment aimed to study the distribution of mass and charge within an atom by observing the deflection (scattering) of alpha particles as they passed through the gold foil.
Step 2: Observations from the Experiment: 
The experiment yielded three key observations, which were contrary to the predictions of the then-prevalent Thomson's "plum pudding" model:
Most particles passed undeviated: The vast majority of the alpha particles passed straight through the gold foil without any deflection.
Small deflections: A small fraction of the alpha particles were deflected from their original path by small angles.
Large deflections: A very small number of alpha particles (about 1 in 8000) were deflected by large angles (greater than 90\(^{\circ}\)), with some even bouncing back along their incident path (a deflection of nearly 180\(^{\circ}\)).
Step 3: Conclusions about Atomic Structure: 
Based on these observations, Rutherford proposed his nuclear model of the atom with the following conclusions:
Most of the atom is empty space: Since most alpha particles passed through undeflected, Rutherford concluded that the atom must be mostly empty. 
Existence of a Nucleus: The fact that some positively charged alpha particles were deflected means there must be a region of concentrated positive charge within the atom that repels them. Rutherford called this central region the nucleus. 
The nucleus is small and dense: The observation that only a very few alpha particles were deflected by large angles indicated that the nucleus must be extremely small in size compared to the atom, and that almost the entire mass of the atom is concentrated in this tiny nucleus. 
Electrons orbit the nucleus: To account for the overall neutrality of the atom, he concluded that negatively charged electrons must be revolving around the positively charged nucleus, much like planets orbiting the sun. 
This experiment was monumental as it disproved the Thomson model and established the modern concept of a nuclear atom.