Published by Patrick Mutisya · 14 days ago
The experiment involved directing a narrow beam of positively‑charged alpha (α) particles at an extremely thin sheet of gold foil.
A fluorescent screen surrounding the foil recorded the deflection angles of the particles.
Very small nucleus surrounded by mostly empty space
Most α‑particles passed straight through the foil with little or no deflection, indicating that the majority of the atom is empty space. Only a very small fraction were scattered at large angles, implying that a tiny, dense region caused the strong repulsive force.
Nucleus contains most of the mass of the atom
The large-angle scattering required a very massive target to change the momentum of the fast α‑particles. Since the α‑particle mass is comparable to that of a helium nucleus, the scattering centre must be much heavier than the surrounding electrons, showing that the mass is concentrated in the nucleus.
Nucleus is positively charged
α‑particles are positively charged (+2e). The observed repulsion (deflection away from the centre) can only occur if the scattering centre also carries a positive charge. The magnitude of the deflection matched calculations assuming a charge of approximately +Ze, where Z is the atomic number.
| Observation | Interpretation | Supporting Equation |
|---|---|---|
| \overline{96}% of α‑particles pass through undeflected | Atom is mostly empty space | \$\displaystyle P{\text{undeflected}} \approx 1 - \frac{2\pi Z e^2}{mv^2 b{\text{min}}}\$ |
| \overline{4}% scattered at small angles | Weak electric field from electrons | \$\displaystyle \theta \approx \frac{Ze^2}{2\pi\varepsilon_0 mv^2 b}\$ |
| \overline{0}.5% scattered at angles > 90° | Direct hit on a dense, massive, positively charged nucleus | \$\displaystyle \theta = \pi - 2\arctan\!\left(\frac{b}{a}\right)\$ where \$a = \frac{Ze^2}{4\pi\varepsilon_0 mv^2}\$ |