The Development of the Atomic Structure and Theory

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The Development of the Atomic Structure and Theory

The Development of the Atomic Structure and Theory

The shape of a water molecule was thought to be spherical as they slipped past each other easily

Origins of the structure

In 465 BC, Greek atomists such as Leucippus and Democritus hypothesised that all objects are made up of atoms that are indivisible and that their physical properties are determined by the shape of the atom.They were simple shapes. For example, fire was thought to be made up of sharp atoms as it injured people while solids were made of small and pointy atoms. Hence, the atomic structure at that time was based on the physical propeties of the object rather than the chemical propeties of the atom.

Today, many people think that after Democritus' theory, there was not much development of the theory until Dalton's time. This is incorrect as there were many important milestones during that timeframe. One of this was that, the atomic theory became widely accepted by scientists. This was largely due to Wilhelm Ostwald's textbook Outline of General Chemistry.In the 19th Century, John Dalton improved on the atomic theory, introducing new concepts. This included the concept that atoms of the same element had the same mass while atoms of different element had different masses.

Plum Pudding Model

In the 19th Century, J.J. Thompson found out that cathode rays were unique particles and they were renamed as electrons. As the atomic nucleus was not discovered yet, the model only consisted of electrons in a cloud of positve charge. The electrons could move around freely in the cloud of positive charge. His model tells us that the propeties of an element is not determined by its phyiscal propeties but rather its number of electrons.

Plum pudding model: free electrons in a cloud of positive charge

Rutherford Model

In 1911, Ernest Rutherford proved that an atom has a nucleus, based on results of his Gold Foil Experiment. This meant that J.J. Thompson's model was incorrect. The model added a nucleus in the sphere of positive charge with free electrons

Rutherford's Model

In 1913, J.J. Thomas discovered stable istotopes in an experiment which he concluded that some of the atoms in an element had a larger mass than the rest. In 1921, Ernest Rutherford hypothesised that there were neutrons. This was proven by James Chadwick in 1932.

Bohr Model

The Bohr Model (made by Ernest Rutherford and Neils Bohr) is the most widely accepted model as it explained many other concepts. It was an improvement on Rutherford's Model.It includes the positively charged nucleus in the centre with negatively charged electrons orbiting around it in shells. Also, Electron configuration originated from the Bohr model of the atom. This model is simple yet it encapsulates Chemistry.

Nucleus(blue) is positively charged. Negatively charged electrons orbit around it


Zacchaeus Chok (5) 2-2

Discovery of stable isotopes

John Dalton's Atomic Theory and the discovery of electrons

Plum Pudding Model(1904)

Bohr Model(1913)

Discovery of positively charged nucleus

Democritus Model (465 BC)

Rutherford Model(1911)

Discovery of Neutron(1932)

In 1904,Hantaro Nagaoka introduced the Saturnian model(below). It has electons revolving around a positively charged particle. Hantaro Nagaoka made 2 accurate predictions with his model. He predicted that there was a massive nucleus in the atom and electrons revolved around it.

Saturnian Model(1904)


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