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The History of the Atom  

What makes up our universe? What are the building blocks that make up reality? How does science and knowledge change as we discover more about the world? Today, we usually talk about atoms (somewhat) as the building blocks of the universe. This worksheet will look at the history of atomic theory and how it has developed and changed drastically to the modern era. We will travel from the Ancient Greeks through to modern discoveries of the electron, proton and neutron.  

The first development of atomic theory began in ancient Greece with the philosophers Leucippus and Democritus. In their view the world was made up of two things: a vacuum and atoms. Atoms comes from the Greek word: ‘atomos’ which means ‘uncut’ or ‘undivided’. Democritus imagined this – take this table:

One finds that this table is made of wood. Then Democritus supposes, what happens when we divide this table into two, we now have two things made of wood. If we continue to divide and divide and divide, Democritus claims that we will eventually reach a point where we cannot divide anymore. All we have left will be the atoms that made up that table, assuming that the table was made completely out of wood – we might call these atoms ‘wood atoms’.

 
These atoms are not visible to the human eye because they are so small, they have an infinite number of varieties and they have existed from the beginning of time and are indestructible. What we sense in the world are merely the combinations of the various atoms that make up the tables, plants and everything you see. Anything that isn’t an atom is a void where nothing is.  

This theory was not very popular during Democritus’ time. Philosophers such as Aristotle disputed it and provided alternative explanations for what makes up the world. Modern atomism sees its history as beginning with Democritus’ view. Atomism would not regain popularity until the 19^th century.  

Dalton developed the beginnings of the modern atomic theory. By this point, chemistry had developed to understand what elements were – substances which could not be broken down by chemical reactions. Chemical reactions are what happens when two chemicals interact. Dalton then why elements were divided in such a way and developed this theory: 

1. Elements are made of extremely small particles called atoms. 

2. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties. 

3. Atoms cannot be subdivided, created or destroyed. 

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 

In chemical reactions, atoms are combined, separated or rearranged. 

This model explained why elements could not be further divided into parts since elements were made of a specific atom. Yet, there were more discoveries to be made.  

In 1897, J.J. Thomson discovered the electron. Which was found to be a smaller form of matter than the atom. This had the property of being negatively charged. Atoms themselves were made of these electrons with a negative charge and positive charge spread throughout the atom to balance it out. This led to the Plum Pudding Model of the Atom proposed in 1905: 

Yet, Thomson’s model would not survive a decade as in 1911, there was now an explanation for the positive charge in the atom – the nucleus. Rutherford, Geiger and Marsden ran experiments on gold foil and found something rather surprising. What they would do is shoot positively charged particles at the atom. Now, if Thomson was right, then the particles would just pass through the atom and would not really scatter.  But what they discovered was that the particles would bounce off the foil and hit in many different directions. This suggested that there was some concentrated area of positive charge.

The nucleus then was proposed as the positively charged centre of the atom, and the electron would surround it and that would make up the atom. In 1913, Niels Bohr would then develop a model which explains how the electron moves and interacts with the nucleus. He conceived the interaction to be like how the planets interact with the Sun, the planets orbit the sun and so too do the electrons orbit the nucleus. Instead of gravity being the one to cause the electrons to move up and down, it would be the charges (or electrostatic forces), so if an electron gained more energy, it would go further from the nucleus as it can resist the positive energy of the nucleus. 

But science moves on quickly! It would not be long for Bohr’s Model was replaced. By 1925, Werner Heisenberg developed his own orbital model. This is the model which is presently the standard model for understanding the atom. It involves a lot of complex quantum mechanics but effectively the way that electrons move is not according to planetary orbits and more zones around the nucleus. Electrons are too fast to put to a single orbit, they could be anywhere around a nucleus – this model accounts for where the electron is most likely to be than to give it a concrete area where the electron will be.