Development in the Study of Atomic Structure
Bohr’s and Rutherford’s Contributions
Mihret Gelan
17th September 2014
Word Count (title page & references excluded): 1459

Atomic Theory & Structure
"By convention sweet, by convention bitter, by convention hot, by convention cold, by convention color: but in reality atoms and void." Those are the words of Democritus, one of the founding fathers of the ancient atomic theory, on his philosophical viewpoint on the nature of matter. It began as a philosophical concept in ancient Greece and India, but today, the atomic theory is not mere philosophy, but a scientific concept or model of the fundamental nature of matter. How it went from a philosophical belief called atomism to an empirically-proven, well-studied, scientific concept is what I’m going to explain throughout this essay; focusing on two renowned scientists of the early 20th century, Ernest Rutherford and Niels Bohr, who made significant contributions to this growing theory of the atom and its structure (the composition of the atom).

The Atom before Bohr and Rutherford
The modern model of the atom was built upon the many vital discoveries of the major scientists throughout history since the birth of the theory in ancient Greece. Bohr and Rutherford were able to make the discoveries and scientific conclusions they made because they were “standing on the shoulders of giants.” To begin with, Democritus, although not entirely correct, laid a philosophical foundation for the scientists-to-come on the fundamental nature of matter.
Fast forward to two thousand years later, in 1803, John Dalton publishes the first evidence-based theory of the atom in a set of five principles about the atom, coining it as the atomic theory. Although having a few critical mistakes, he further developed Democritus’ plain philosophy by evidently stating that mass is conserved throughout a chemical reaction, as opposed to it being created or destroyed. Dalton’s principle of the discretion and indivisibility of an atom lasted less than a century. Towards the end of the 19th century, JJ Thompson, an English physicist discovers the corpuscle, an elementary particle which we today refer to as the electron. A few years later, in 1904, Thomson, based on his experiments with cathode ray tubes, proposes the Plum Pudding Model of the atom, which suggests that the atom is made up of a positively-charged sphere or diffuse with negatively-charged electrons scattered all around the diffuse, similar to the way raisins are scatted in a pudding. Around the same time, science partners and spouses, Pierre and Marie Curie discover the radioactivity phenomenon which, along with Thomson’s discoveries will play a significant role in the findings of both Rutherford and Bohr.

The father of Nuclear Physics and his contributions
Sir Ernest Rutherford, a New-Zealand born English physicist and one of the preeminent scientists of the early 20th century, once said, “Gentlemen, now you will see that now you see nothing. And why you see nothing you will see presently.” Although not confirmed, the quote may be regarding man’s poor understanding of the elementary and basic nature of the matter on which he lives upon, that is to say, the atom. That all changed once Rutherford and the scientists who followed his footsteps published their findings about the atom to the public world.
Classifying the various forms of Radiation
To begin with, Rutherford, after many experiments devised the names alpha (), beta (), and gamma () rays or particles in order to classify the various forms of radiation, a concept that was poorly understood at the time. He concluded that on the one hand, -particles were positively charged, and on the other hand, that -particles were in contrast negatively charged while the third ray, the -particle was a neutral particle.
The Gold Foil Experiment and the Nuclear Model
In his most famous experiment, Rutherford discharged -particles into a thin piece of gold foil surrounded by a zinc sulfide screen through a particle emitter in order to see the direction the particles will take after contacting the foil. The zinc sulfide screen was to be used as a detecting device for the radioactive particles. Given the high mass and momentum of -particles, the small mass of electrons and the low concentration of positive charges in the atom, in accordance to the Plum Pudding Model of Thomson, Rutherford assumed that all the particles would go straight through with no deflection. However, although many did, a small amount of the emitted particles went either off to the sides or backwards to the stream of alpha atoms being emitted. This was at odds with the principles of the Plum Pudding Model.
After a close analysis of his results, Rutherford proposed a revised model of the atom in 1911 which he dubbed as being the Nuclear Model. Specifically, the nuclear model states that all the positively-charged particles of the atom and a large part of its atomic mass are concentrated at the center of the atom, the nucleus, while the negatively-charged particles, the electrons, orbit the nucleus as planets orbit the sun at considerable distance. According to classical physics, such a system would be unstable due to the fact that any charged particle moving on a curved path emits electromagnetic radiation, thus the electrons would lose their energy and spiral into the nucleus. To give a more sophisticated and proper explanation of the atomic system, Rutherford would soon later require the help of another giant in the field of atomic structure.
The Problem with the Atomic Mass In 1918, Rutherford provided empirical evidence and identification of the positively-charged subatomic particles found in the nucleus to which he designated the name protons. Although he was able to explain the charge of the atomic nuclei with the idea of protons, positively-charged particles, being concentrated in the nuclei of atoms, the atomic mass always seemed to exceed the sum of its protons. In order to explain this, he postulated the existence of a neutral particle in the nucleus with a mass near to that of the proton which would together collectively give the atom its overall mass. He dubbed this hypothetical subatomic particle as the neutron. 12 years later, English physicist James Chadwick was able to prove the existence of Rutherford’s neutron through experimentation.
The other giant and his Quantum viewpoint
A year after the proposal of the nuclear model, Danish physicist, Niels Bohr began his postdoctoral research in England with Ernest Rutherford at the University of Manchester. Regarding Rutherford’s incomplete model and explanation of the atom and its system of remaining stable, Bohr felt compelled to postulate an explanation. Influenced by the emerging theory of quantum mechanics being developed by scientists such as Max Planck and Albert Einstein, Bohr incorporated this modern concept to the more classical and predating study of atomic structure. Bohr suggested that electrons inhabited orbits situated at a fixed distance away from the nucleus as opposed to randomly buzzing around the nucleus to explain how they remained stable. In this model, each orbit is associated with a particular energy level (shell), and the electron is able to change energy levels or orbits through the emission or absorption of energy in specified and discrete amounts (called quanta). Bohr referred to the energy of electrons as being quantized, meaning that electrons could have one energy level or another, but nothing in between. Normally, the shell which an electron occupies is called its ground state. But then again, it can move to a higher-energy, less-stable level through absorbing energy; this is called the electrons excited state. When it is done being excited, the electron is able to return to its ground state by releasing the energy it has absorbed.
Bohr made several conclusions concerning the energy of electrons, the different energy levels in an atom, and the distance at which an electron is found away from its nucleus. First, he discovered that the closer an electron was to its nucleus, the less energy it needed, but as the electron got farther away from the nucleus, the more energy that was required. Although Bohr’s theory was much more developed and sophisticated than that of his professors and although it paved the way for many scientific advances, it still had its flaws. The model was misleading in several ways. The maturing field of quantum physics revealed that knowing an electrons position and velocity was impossible. Thus, Bohr’s orbits were replaced with electron probability “clouds” as a way of predicting where an electron was likely to be.

The Impact of Bohr and Rutherford
The majority of the transition from mere philosophy to an empirically-proven scientific concept took place between a student and his teacher; respectively Bohr and Rutherford. All that came before laid a significant foundation with either mere philosophy such as Democritus or erroneous theories such as John Dalton. All that came after such as Werner Heisenberg built upon the vital knowledge and understanding of the atom and its structure gained through Rutherford and Bohr’s reseach.

References
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