The History of Atomic Theory

Author: Jessica Zhang

Editor: Jasleen Matharu, Yanxi Chen

Artist: Daisy Zheng

We now know that our world is made of atoms, molecules, and ions. In ancient times, people knew little about the microscopic world. However, generations of scientists proposed numerous theories, and these theories gradually developed into our cognition of the world. In this passage, we will examine the history of atomic theory.

Around the fifth century BC, a Greek philosopher, Democritus, suggested that “all matter consists of microscopic, indivisible particles.” Although it was not evidence-based and was doubted by many, it is often considered the pioneer of the atomic theory. Later in 1808, chemist John Dalton proposed a more precise and detailed definition of invisible matter. His hypotheses can be summarized by several points: elements are composed of atoms; different elements have different kinds of atoms; atoms of more than one element form compounds; chemical reactions are the process of separating and combining different atoms. Dalton’s hypotheses correspond with our current understanding of atoms, so he pushed atomic theory to a new stage.

As scientists continued to explore the depths of the atomic theory, they discovered that atoms were not the smallest units of life. An English physicist, J.J. Thomson, invented some clever devices and did a series of experiments to prove this. During the process, he discovered that the cathode ray is attracted by a positively charged anode and a negatively charged cathode on the other side. He concluded that there must be negatively charged particles and called them electrons. The physicist then realized that since atoms are neutral, there must be an equal number of positive charges. Therefore, Thomson proposed the “plum-pudding” model, in which the electrons were randomly placed on the surface of a positive sphere, like raisins on the surface of a cake. This theory was later refuted by Ernest Rutherford, who discovered the nucleus.

In 1910, Rutherford shot a thin layer of gold with alpha particles (a kind of positively charged particle) and surrounded the gold with a detecting screen. He observed that most of the alpha particles had little or no deflection, but there were a few particles deflected at a large scale. This was contradictory to Thomson’s model because the particles should have passed the gold atom with very little deflection. Rutherford then explained that maybe most of the atom is empty so that most of the particles pass the gold with little deflection. However, if the particle approaches the dense and positive core, which is the nucleus in the atom, it will gain a repulsion force and bounce back at a bigger angle. This is because the mass of the nucleus is relatively large, so the particles which come directly towards it will go in a reverse path. He named the positively charged particles in the atom as protons, and in 1932, another scientist, James Chadwick found a third neutral particle called neutrons.

Electrons, protons, neutrons. These are the high school content that we mainly learn about. Since then, many other theories have flourished, including Niels Bohr’s seven different quantum levels, Erwin Schrodinger’s electron cloud, and so on. It is just a pleasure to see people continue to make discoveries in this field, enriching humankind's understanding of the nature of atoms.

 

Citations:

Chang, Raymond. Chemistry. McGraw-Hill, 2010.

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