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The History of the Atomic Model: Rutherford and Bohr

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The work of J.J Thomson’s student, Ernest Rutherford, led to the discovery of the Proton. Working with alpha particles fired at a piece of gold foil it was observed that instead of passing straight through it was scattered. Suggesting there was something large in the centre of the atom.

The plum pudding model did not last long however, in 1909 a former pupil of Thomson’s, Ernest Rutherford discovered that the atom itself had a mass of positive charge at the centre, contrary to the plum pudding model. It was through the Geiger Marsden experiment that Rutherford made this conclusion. In this experiment alpha particles were fired at a sheet of gold foil and the scattering of the alpha particles measured on fluorescent paper. The scientists predicted that that as a plum pudding model the alpha particles would go through the gold foil in a straight line, but what they discovered was that it was scattered everywhere. This led the scientists to the conclusion that at the centre of the atom was a large positive mass and Rutherford suggested a planetary model where electrons moved around this central mass like the planets around the sun.


Rutherford further followed this up in 1917 when he proved that a hydrogen nucleus (1 proton) is present in other nuclei of different elements most notably nitrogen gas in the air. Rutherford conducted a number of experiments with hydrogen nuclei and nitrogen in air using alpha particles and after a number of theories concluded that the hydrogen atom made up other atoms. He named this new fundamental particle as a proton. Now the atomic model had a central particle and electrons around it, reversing he plum pudding model of Thomson.


It was not until the earlier 20th Century that the scientific community arrived at the modern day atomic model. Max Planck and Albert Einstein in the field of physics postulated that light energy can be absorbed and emitted as quanta. This theory was adopted by Niels Bohr in 1913 who theorised that electrons could orbit the nucleus in a circular orbits and that the distance of the electron to the nucleus was fixed unless it moved between energy levels with the absorption or emission of light. This conclusion led to the theory that electrons exist in energy levels around the positive nucleus and have their own distinct properties in each of their energy levels.

Terms in section
Plum Pudding Model

The plum pudding model was suggested as the first atomic model by J.J Thomson where he suggested that the atom was a sea of positive charge that surrounded small negative electrons

Ernest Rutherford

Ernest Rutherford was a British physicist who by experimenting with gold foil and alpha particles found that there was a large central mass at the centre of the atom with a positive charge.

Geiger Marsden Experiment

The Geiger Marsden experiment was conducted by two research partners of Ernest Rutherford where alpha particles were fired at a sheet of gold foil and were deflected in all directions.

Alpha

Alpha particles are made of 2 protons and 2 neutrons released from a nucleus when it breaks apart

Max Planck

Max Planck was a German physicist who discovered that energy that is emitted is released in small packets called quanta. He related the amount of energy released to the frequency of the wave.

Albert Einstein

Albert Einstein was a German physicist who was pivotal in many scientific discoveries in his life. He contributed to the field of chemistry through his work on the photoelectric effect and mathematics of the atom.

Quanta

Quanta is the plural term for quantum which means a small packet of energy. For example a photon is defined as a small packet of energy of light.

Niels Bohr

Niels Bohr was a Danish physicist who made many leaps in theoretical chemistry using mathematical modelling. He developed the model of electrons existing in shells or energy levels.

Nucleus

The nucleus is the term given to the centre of the atom comprising of the proton and neutron

Orbit

An orbit is the circular or dumbbell shaped motion that the electrons follow around the nucleus. Much like the planets orbiting the sun

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