Atomic Structure and Theory

Development of Atomic Structure and Theory

Definition: The atomic theory is a theory that states that all matter consists of tiny indivisible particles called atoms, that atoms of the same element are all identical and cannot be created or destroyed and that compounds are formed by the combination of atoms in simple ratios to give molecules.

Timeline

Democritus: (in 460BC – 370BC)

▪ All matter is made of tiny indivisible particles called atoms
▪ Atoms differ in only size and shape
▪ Atoms are in constant motion
▪ Atoms are able to join with other atoms to form different types of matter

John Dalton: (in 1803)

▪ All matter is made of tiny particles called atoms

▪ Atoms of the same element are alike

▪ In chemical changes, atoms of different elements bond to form compounds

▪ Atoms involved in these chemical changes are not broken down

▪ When atoms combine, they do this in whole number ratios by weight

J.J. Thomson: (in 1896)

▪ When high electrical voltage is placed in vacuum glass vessels electrical discharges occur.

▪ These (known as ‘cathode rays') travel in straight lines except when bent by electric or magnetic fields. This revealed they consisted of electrical charges

In motion and the direction of deflection indicated that these particles had to have a negative charge.

▪ Electrons are found in all atoms.

▪ Suggested ‘The Plum Pudding Model'.

Marie Curie: (in 1898)

▪ Created the theory of radioactivity which helped to change the theory of atomic structure.

Ernest Rutherford: (in 1911)

▪ According to ‘The Plum Pudding Model', shooting a beam of alpha particles at gold foil would result in all the alpha particles passing straight through. This hypothesis was not supported as a few alpha particles were deflected.

▪ Because of this result Rutherford concluded that The ‘Plum Pudding Model' was Incorrect and stated that the atom has a dense core called the nucleus Which consists of positively charged protons, and the atom consists of mostly empty space. The electrons are attracted to the nucleus but remain outside it.

Niels Bohr: (in 1913)

▪ Improved on Rutherford's model of the atom by creating an equation that could predict the “spectral lines” of the hydrogen atom.

▪ This led to Bohr's discovery that electrons orbited the nucleus in “shells” which determined the chemical properties of an element.

James Chadwick: (in 1932)

▪ The theory of the weight of an atom did not work with isotopes and thus the neutron was discovered. It weighed the same as the proton and had no charge.

Ernest Rutherford's Achievements

Cambridge University

Ernest Rutherford's significant achievements began in 1886 when he invented a detector of electromagnetic waves during a research project to gain admittance into the University of Cambridge. Rutherford's apparatus for detecting electromagnetic waves was simpler than Heinrich Hertz's and had commercial potential. In 1896, soon after the detection of uranium radiation by Henri Becquerel, Rutherford diverted his attention towards studying this radiation. He discovered that by placing uranium near thin foils one type of radiation emitted by the uranium was blocked or absorbed by the foil, and he named it ‘alpha'. The second type of radiation passed through the foil, and he gave it the name ‘beta'.

McGill University

Rutherford's research on radiation provided him with a professorship at McGill University in Montreal in 1898. There Rutherford and a colleague created a new branch of physics called ‘Radioactivity'. They started with the study of another radioactive element, Thorium, which emitted a gas which he named “emanation” which left a solid element which still remained radioactive. Rutherford continued to investigate other groups of radioactive elements which led to him in developing the theory of transformation in 1902 which stated that:

“Radioactivity was where the atoms of an element break down into atoms of an entirely different element which also remained radioactive.”

University of Manchester, 1908 Nobel Prize

In 1907 Rutherford accepted the request to study at the University of Manchester which was followed by him earning the Nobel Prize for chemistry in 1908 for his work in Montreal. In 1908 with Hans Geiger, a German physicist, Rutherford developed a counter for ionised particles which became a universal tool for measuring radioactivity. With this tool, known as the Geiger counter, Rutherford and Thomas Royds counted the particles emitted by radium; also measuring the total charge collected and combining the results with the rate of production of helium from radium. With this information and Bertram Borden Boltwood's assistance they calculated the exact value of Avogadro's number.

The Gold Foil Experiment

In 1909 Rutherford made his greatest contribution to science with his gold foil experiment. This experiment was carried out by firing alpha particles through thin gold foil. The results were that most of the alpha particles passed through the gold foil with some being mildly deflected but surprisingly some particles bounced back to the particle source. In 1911 Rutherford came to the conclusion that the atom was mostly empty space with a dense positively charged core called the nucleus. This theory of atomic structure is known as the Planetary or Rutherford Atomic Model.

Further Work

In 1919 Rutherford examined the collision of alpha particles with gases producing the first artificial breakdown of an element when he found that when an alpha particle collided with a nitrogen atom its protons were removed.

In 1920 Rutherford also predicted the existence of the neutron which was soon discovered with the help of Chadwick in 1932, indicating that the nucleus was composed of protons and neutrons.

From 1933 until his death Rutherford became president of the Academic Assistance Council which was an organisation designed to aid fellow scientists who had fled Nazi Germany.

Rutherford's Contribution to the Structure of the Atom

Rutherford is well known for his “gold foil experiment” conducted in 1909 at the University of Manchester. This experiment led to the discovery that most of an atom's mass is located in a dense core called the nucleus.

The previously accepted model to Rutherford's was the “plum pudding model” by J.J Thompson. His model incorrectly showed that negatively charged electrons of an atom were “floating in a sea of positive charge.”

The gold foil experiment involved a series of tests where positively charged alpha particles were shot at a very thin layer of foil with detection screens of zinc sulphide. If J.J. Thompsons model was correct all the alpha particles would have passed through the foil with no huge deflections. However approximately one in eight thousand alpha particles were deflected at large angles and some even rebounded towards the alpha particle emitter. Rutherford described the result as the most incredible event of his life saying it was:

“as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

These results proved that the alpha particles had collided into dense positively charged concentrations. In 1911 Rutherford proposed his own model of the atom where the entire atom's positive charge was concentrated into a dense core which was ten thousand times smaller than the entire atom with the rest being empty space with electrons orbiting the nucleus like planets orbit the sun.

Rutherford's Model: Discussion

Validity:

Rutherford's Atomic Model was “state of the art” for only three years as it was not entirely correct. The major flaw in his model was explaining how electrons orbited the nucleus.

Rutherford modelFirstly, according to his model which was based on the solar system when the sun is orbited by the planets, the atom's negatively charged electrons orbited at high speeds around the nucleus which was positively charged. The flaw here was that, firstly, the components of an atom are extremely small and the sun and planets were composed of extremely large masses. This meant that Newtonian laws of motion did not apply to the motion of the extremely small atom.

Secondly, the sun and planets are not charged like protons and electrons are. Thirdly, according to Maxwell's electromagnetic theory, when a charged particle (in this case the electron) revolves in a field of force (like the nucleus's) it loses energy as radiation. This loss of energy as radiation would gradually slow down the electron contracting the length of its orbit which would result in the electron eventually collapsing into the nucleus in a calculated 10-8 seconds. However this does not occur to the atom and Rutherford's Atomic Model could not explain how the atom did not collapse accordingly. Another flaw in Rutherford's model was that it did not explain how the theory of the weight of an atom did not apply to isotopes. This was due to Rutherford's model not including the neutron that he had predicted but did not find.

Impact:

Rutherford's research impacted the scientific community in several ways. Firstly, his discovery of the nucleus provided greater understanding of the phenomena of atoms. Rutherford was also one of the key people involved in creating another branch of physics called “Radioactivity”. He was responsible for the discovery of Alpha radiation and helped to create the Geiger counter which greatly benefited the scientific community. Rutherford was also the first to suggest and predict the existence of the neutron and due to his research Chadwick in 1932 verified the existence of the neutron. This helped to explain the changing weight of an atom in isotopes and hence the arrangement of some elements of the periodic table. Rutherford's discovery led to the understanding that the orbiting electrons determined the reactivity of atom.
Another significant impact was its contribution to the development of the first nuclear weapons of the Manhattan Project during the years 1941 to 1946.

Limitations:

The limitation of technology at the time impaired some of Rutherford's final conclusions. Without advanced equipment to detect alpha particles, Rutherford could not accurately state the exact ratio of reflected alpha particles in his gold foil experiment. Rutherford also did not have access to some of today's technologies such as the scanning tunnel microscope which would have made more reliable and valid his experiments by providing further evidence to explain why the alpha particles had been deflected. This could have possibly led him towards the discovery of the sub atomic particle of the neutron. A basic limitation was that in using the sun/ planets analogy Rutherford and all scientists at the time were not aware of the laws of quantum physics and how they differ from Newtonian physics.

Judgement:

Rutherford's model of the atom is one of the most significant developments in the theory of atomic structure. The model explained the presence of a dense positive core which disproved the previous “plum pudding model.” Without this major development the improvement of the model by Niels Bohr (explaining how electrons are arranged in shells and therefore providing some information on the chemical behaviour of some atoms) would obviously not have been discovered. Rutherford's atomic theory and research was also instrumental in the discovery of the neutron by Chadwick two decades later.

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