Atoms make up everything around us. They are the smallest unit of matter. Atoms have two major parts – the central nucleus containing protons (the positively charged particles) and neutrons (no charge particles), and the electron shells (outermost region) containing electrons (negatively charged particles). 

Meaning of Rutherford’s Structure of Atom

Physicist Ernest Rutherford developed this particular structure of atoms. He described an atom as a dense, small, positively charged core known as a nucleus. He studied that all the mass was centred around the nucleus. Negatively charged particles known as electrons revolve around the nucleus just as the planets revolve around the sun. 

The Rutherford atomic model was preceded by the ‘plum pudding’ atomic model of physicist Sir J.J. Thomson. In the model, negatively charged electrons were stuck with a positively charged atom, just like plums in a pudding. 

Alpha Ray Scattering Experiment/Gold Foil Experiment

• To explain the atomic structure of elements and define a precise model of an atom, Rutherford conducted the alpha ray scattering experiment, also known as the gold foil experiment.
• In the experiment, he blasted the alpha particles into a thin sheet of gold foil. Then, he observed the interaction between the alpha particles and the gold foil and the path followed by the particles.
• Rutherford used very high rays of alpha particles in the experiment. A radioactive source, known as an alpha-particle, emitter produced them.
• The alpha particles were blasted into a thin sheet of gold at 100 mm thickness. To study the deflection created by the alpha particles, he placed a screen of fluorescent zinc sulphide around the gold foil.
• The gold foil was selected because he wanted as thin a layer as possible.
• Doubly charged helium ions are known as alpha particles. The particles have a mass of 4 u, are fast-moving and have considerable energy.
• Rutherford’s observations opposed Thomson’s atomic model.

Observations of Alpha Ray Scattering Experiment 

• Rutherford’s first and foremost observation was that most of the alpha particles passed through the gold sheet without any change in the path, showing that most of the space was empty.
• The second observation was that the gold sheet deflected some particles at minimal angles. This showed that the positively charged particles were unevenly distributed in an atom and that the positive charge was concentrated across a significantly less volume of an atom.
• About 1-2% of the alpha particles were deflected back to the extent of 180 degrees. This meant that the volume of positively charged particles was much less than the total volume.

Rutherford’s Structure of an Atom

According to Rutherford’s structure of atoms, the positively charged particles were unevenly distributed in an atom, and the positive charge was concentrated across a significantly less volume of an atom. Most of the mass was also in that volume. He called this part nucleus.

• Rutherford suggested a negative charge around the nucleus in the form of electrons. He also proposed that the electrons revolved at a very high speed around the nucleus. He called these paths orbits.
• The positively charged nucleus and negatively charged electrons are held together by a force of attraction known as the electrostatic force of attraction.

Limitations of the Alpha Ray Scattering Experiment 

• According to Maxwell, an electron revolving around the nucleus should release electromagnetic radiation. Still, Rutherford’s structure of atoms says that the electrons revolve around the nucleus in fixed paths known as orbits. The radiation would carry energy from the motion, leading to rotation shrinking, and eventually, electrons would collapse inside the nucleus.
• There were significant differences between Maxwell’s and Rutherford’s structure of the atom, and later, Rutherford could not explain the stability of the atom.
• Rutherford was also unable to describe the positions of electrons in his model.

Conclusion  

From Rutherford’s alpha ray scattering experiment, we can draw the following conclusions regarding Rutherford’s structure of the atom:

• Most of the atom is made up of nothing but empty spaces.
• Most of the mass present inside the atom was concentrated in a small area at the centre, known as the nucleus.
• Paths taken by the electrons while revolving around the nucleus are known as orbits.
• The atom is stable because of the strong electrostatic force of attraction between the positively and negatively charged particles.