It is common knowledge today that an atom consists of electrons, protons, and neutrons. Originally, an atom was thought to be the smallest unit (atomic meaning unbreakable) in existence, but that theory has since been proved redundant by the discovery of subatomic particles and research is still underway to find out if these subatomic particles still can be still split further. The discovery and research of quarks is a step in that direction.
There have been many scientists who have proposed different models of the atom, right from the ancient Greek and Indians. The Atom was then considered to be indivisible, but when JJ Thompson discovered negatively charged particles in the atom, the norm was challenged, and the scientific community set out on another expedition.
JJ Thomson proposed the Plum Pudding Model shortly after he observed the presence of negatively charged particles that he named electrons. So, if there was a negative charge and the atom was electrically neutral, there had to be a positive charge. He postulated that the atom was like a Pudding with the mass and positive charge distributed across its surface and electrons embedded in the pudding like plums.
Thompson’s Plum Pudding model fell short of explaining many practical observations and after a revolutionary experiment now known to us as the Alpha Scattering Gold Foil experiment, the classic model of the atom that was the accepted norm for a good part of recent history was the one proposed by Ernest Rutherford. This too has now been proved to be inaccurate falling short of explaining many observed phenomena. Let us look at the model and its different salient features and some of its limitations.
Rutherford’s Alpha Scattering Experiment
In the experiment, Rutherford bombarded high energy Alpha Particles directed from a radioactive source to a very thin gold foil of 100 nm thickness. This was aimed at observing and studying the deflection caused to the Alpha particles. A fluorescent Zinc Sulphide Screen was placed around the gold foil. The observations that came out of the experiment contradicted the established norm and gave rise to the new atomic model.
Observations and Conclusions from Rutherford’s Experiment
• This contradicted Thompson’s model as most of the atom was observed to be virtually made of empty space.
• Some of the alpha particles were deflected by the gold foil by very small angles giving rise to the conclusion that the positive charge is not uniformly distributed in the atom unlike what Thompson proposed but in reality, concentrated at a very small space most likely in the centre of the atom.
• Very few of the alpha particles were deflected back almost retracting the same path they were bombarded in, at an angle of 180 degrees which insinuated that the volume occupied by the positive charge is very minute compared to the entire atomic structure.
Rutherford’s Postulates and their Salient features based on the Experimental Observations
• Based on the experimental observations, Rutherford most of the mass of the atom is due to positively charged particles (later called protons) concentrated at the centre of the atom. This volume was called the nucleus of the atom and the rest of the atomic space was mostly empty. It was later theorized and proved that the nucleus also consisted of uncharged particles called neutrons which had mass but bore no charge.
• The atomic nucleus is surrounded by negatively charged particles called electrons that revolved around the nucleus at very high speeds on fixed circular paths. These fixed paths were called ‘orbits’
• The atom has no net charge and is ‘electrically neutral’ as there are equal numbers of positively and negatively charged particles in an atom. The electrons revolving around the nucleus are negatively charged and the dense nucleus consisting of protons are positively charged. A strong electrostatic force of attraction held these two entities together bound inside the atom.
• The size of the nucleus was very small compared to the total size of the atom making most of the atom largely empty.
Limitations of Rutherford's Model of Atom
Although Rutherford's Model was a huge step up from previous models, it still couldn't explain quite a few physical phenomena.
• To start, Rutherford gave no explanation to support the stability of the atom. According to him, the electrons revolved at high speed around the nucleus in fixed paths. This was in contradiction with Maxwell’s findings that said accelerated charged particles always released electromagnetic radiations when in motion. Therefore, the revolving electrons had to emit radiations when in their orbit.
• This would have caused the orbits to gradually shrink as a result of the outflow of electromagnetic energy when the electron is in motion. Finally, the electron would collapse into the nucleus. If calculated, the life of the atom would be 10-8seconds resulting out of shrinking orbits. Since this was not the case, the model failed to explain atomic stability.
• Another major drawback was the non-explanation of electrons in their orbits around the nucleus. This made the theory virtually incomplete.
• Additionally, Rutherford’s atomic model failed to adequately explain the atomic emission spectra of hydrogen. Why did Hydrogen absorb some specific wavelengths of light and not others?
These drawbacks were further built on by Neil’s Bohr, James Chadwick and many scientists that came after him. Although these didn't sufficiently explain all observed phenomena, the revolutionary experiments, and observations that they conducted changed man's understanding of the atomic theory and formed the basis of today's modern physics which has seen many applications throughout an array of industries and utilities.