Models of the atom

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Models of the atom

The word atom finds its origins in the Greek word ‘atoma‘ meaning “unable to break up”. One of the earliest recorded usages of this word was by Leucippus and Democritus (460 – 360 BC). They proposed the idea that if you take a sample of matter and broke it in two, then continued to break each piece down into smaller pieces, there must be a point at which it is impossible to split the sample any further. Whatever this is, they called an atom.

Think: When looking at the history of the atom, we must remember that the older models are wrong, while each respective model may have been accepted at the time, it is the quantum mechanical model that is the currently accepted model. In class, sometimes the Bohr model is used because it is more easily visualised and understood by students. 

John Dalton (Figure 1) proposed the Atomic Theory of Matter in 1810 which was arguably the first new advancements in the development of the atom in over 2000 years. Dalton’s research and experimentation showed that matter is made up of extremely small particles called atoms. Atoms of the same type were identical. Atoms can exist as elements or compounds and that atoms cannot be created nor destroyed in a chemical reaction. Dalton did not have any evidence of any sub-atomic particles.

Figure 1: John Dalton. Image credit: Charles Turner [Public domain], via Wikimedia Commons

The model of the atom from Democritus through to Dalton can be visualised as shown in figure 2. A simple particle that is the smallest unit of matter, with no known understanding of the sub-atomic particles.

Figure 2: The earliest known visualisation of an atom. Image credit: my own work.

The Thomson Model (the Plum Pudding Model)

The next significant milestone in the development of the atom was the Plum Pudding Model. This was first proposed by Joseph John Thomson in 1897.

The Plum Pudding Model (Credit: Tjlafave, 2016, Wikipedia.org)

Thompson describes the atom as being of positive charge containing scattered negatively charged electrons.

The Rutherford Model

The next significant advancement in the atomic model was made by Ernest Rutherford in 1927.  Rutherford’s model improved upon Thompson’s because he described the atom as being made of positive charges in a central nucleus surrounded by electrons. Rutherford’s gold foil experiment is the famous experiment he used to back his theory.

The Bohr Model

The next advancement in the atomic model was by Neils Bohr in 1927.  Bohr suggested small electrons orbit rapidly around the nucleus in shells according to their energy level.

It is the Bohr Model that you a most likely familiar with. A lot of concepts in Chemistry can be described or demonstrated using the Bohr Model. When first learning Chemistry, it is the Bohr model that is learnt. However, it is not the most correct model.

The Quantum Mechanical Model 

The Quantum mechanical model is the latest atomic model. It uses orbital clouds to describe the region where electrons are found.  It is this model that uses the uncertainty principle, which states that we cannot know exact position of electrons, the best we can do its find an area of probability – an area where the electrons are most likely found.

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