Explain the efforts made by scientists to explain the struture of atom by developing various atomic models?
In deatail one by one.
Answers
Dalton's model of the atom (ESAAO)
John Dalton proposed that all matter is composed of very small things which he called atoms. This was not a completely new concept as the ancient Greeks (notably Democritus) had proposed that all matter is composed of small, indivisible (cannot be divided) objects. When Dalton proposed his model electrons and the nucleus were unknown.
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Figure 4.2: The atom according to Dalton.
Thomson's model of the atom (ESAAP)
After the electron was discovered by J.J. Thomson in 1897, people realised that atoms were made up of even smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet and so the “plum pudding model” was put forward in 1904. In this model, the atom is made up of negative electrons that float in a “soup” of positive charge, much like plums in a pudding or raisins in a fruit cake (Figure 4.3). In 1906, Thomson was awarded the Nobel Prize for his work in this field. However, even with the Plum Pudding Model, there was still no understanding of how these electrons in the atom were arranged.
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Figure 4.3: The atom according to the Plum Pudding model.
The discovery of radiation was the next step along the path to building an accurate picture of atomic structure. In the early twentieth century, Marie and Pierre Curie, discovered that some elements (the radioactive elements) emit particles, which are able to pass through matter in a similar way to X–rays (read more about this in Grade 11). It was Ernest Rutherford who, in 1911, used this discovery to revise the model of the atom.
Two other models proposed for the atom were the cubic model and the Saturnian model. In the cubic model, the electrons were imagined to lie at the corners of a cube. In the Saturnian model, the electrons were imagined to orbit a very big, heavy nucleus.
Rutherford's model of the atom (ESAAQ)
Rutherford carried out some experiments which led to a change in ideas around the atom. His new model described the atom as a tiny, dense, positively charged core called a nucleus surrounded by lighter, negatively charged electrons. Another way of thinking about this model was that the atom was seen to be like a mini solar system where the electrons orbit the nucleus like planets orbiting around the sun. A simplified picture of this is shown alongside. This model is sometimes known as the planetary model of the atom.
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Figure 4.4: Rutherford's model of the atom.
Bohr's model of the atom (ESAAR)
There were, however, some problems with Rutherford's model: for example it could not explain the very interesting observation that atoms only emit light at certain wavelengths or frequencies. Niels Bohr solved this problem by proposing that the electrons could only orbit the nucleus in certain special orbits at different energy levels around the nucleus.
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Figure 4.5: Bohr's model of the atom.
James Chadwick (ESAAS)
Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! To make sure that the atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass.
The Plum Pudding Model
The so-called plum pudding model was proposed by the scientist J.J. Thomson in 1904. This model was conceived after Thomson's discovery of the electron as a discrete particle, but before it was understood that the atom had a central nucleus. In this model, the atom is a ball of positive charge -- the pudding -- in which the electrons -- the plums -- are located. The electrons rotate in defined circular paths within the positive blob that makes up the majority of the atom.
Planetary Model
This theory was proposed by the Nobel Prize winning chemist Ernest Rutherford in 1911 and is sometimes called the Rutherford model. Based on experiments that showed the atom appeared to contain a small core of positive charge, Rutherford postulated that the atom consisted of a small, dense and positively charged nucleus, around which electrons orbited in circular rings. This model was one of the first to propose the odd idea that atoms are mostly made up of empty space through which the electrons move.
Bohr Model
The Bohr model was devised by Neils Bohr, a physicist from Denmark who received the Nobel prize for his work on the atom. In some ways it is a more sophisticated enhancement of the Rutherford model. Bohr proposed, as did Rutherford, that the atom had a small, positive nucleus where most of its mass resided. He stated that the electrons orbited around this nucleus like planets around the sun. The main improvement of Bohr's model was that the electrons were confined to set orbits around the nucleus, each having a specific energy level, which explained experimental observations such as electromagnetic radiation.
Electron Cloud Model
The electron cloud model is currently the most sophisticated and widely accepted model of the atom. It retains the concept of the nucleus from Bohr and Rutherford's models, but introduces a different definition of the motion of electrons around the nucleus. The movement of electrons around the nucleus in this model is defined by regions where there is a greater probability of finding the electron at any given moment. These regions of probability around the nucleus are associated with specific energy levels and take on a variety of odd shapes as the energy of the electrons increase.