Class 10 > Unit # 19:Atomic Structure > Structure of Atom & Geiger And Marsden α- Scattering Experiment

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Atom and Its Structure: Geiger-Marsden Alpha Scattering Experiment

Atom and Its Structure

"An atom is the smallest unit into which matter can be divided without releasing electrically charged particles."

Atoms are too small to be seen with any ordinary microscope. However, by shooting tiny atomic particles through atoms, scientists have developed precise structural models. Every atom is composed of two primary parts:

1. The Nucleus: The central hard-core of an atom is the nucleus—a small, dense region consisting of closely packed protons and neutrons.
2. Orbiting Electrons: Electrons revolve around the nucleus at high speeds. The number of particles (electrons and protons) depends on the specific type of atom.

Most of the atom consists of empty space. The remaining structure comprises a positively charged, dense nucleus surrounded by negatively charged orbiting electrons bound by electrostatic forces.

Geiger and Marsden Alpha ($\alpha$) Scattering Experiment

Scientists Geiger and Marsden investigated atomic structure by using a beam of positively charged $\alpha$-particles to bombard a thin gold foil placed in a vacuum, surrounded by a ring-shaped fluorescent screen. When $\alpha$-particles hit the screen, flashes of light were observed and detected using a rotating detector.

Observations

• Observation 1: Most of the $\alpha$-particles passed straight through the foil undeflected or experienced only minor deflections through small angles.
• Observation 2: A small number of $\alpha$-particles were deflected through surprisingly large angles exceeding $90^\circ$.
• Observation 3: A very few $\alpha$-particles ($\approx 1$ in $8000$) were deflected backward through nearly $180^\circ$.

Conclusions & Rutherford's Atomic Model

To explain these experimental observations, Rutherford postulated his nuclear atomic model:

• The fact that a large number of $\alpha$-particles passed through the foil undeflected indicates that there exist large empty spaces within an atom.
• Those positively charged $\alpha$-particles that deflected through large angles came very close to a massive, concentrated positive charge (the nucleus), experiencing strong electrostatic repulsion which caused them to scatter or bounce backward.

Fig: Geiger-Marsden $\alpha$-scattering experimental setup and deflection trajectories around the atomic nucleus.

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