Physics Lesson 20.1.1 - The Atom

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Welcome to our Physics lesson on The Atom, this is the first lesson of our suite of physics lessons covering the topic of Atomic Nucleus and Its Structural Properties, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

The Atom

Everybody nowadays knows that matter is made up by microscopic particles called atoms. However, this has not always taken as granted. Despite people have doubted since antiquity about the apparent continuous nature of matter (Democritus was the first who used the term "atom" to describe the tiniest particle of matter in the 1st century), only in 19th century scientists widely accepted (albeit without any clear evidence) that matter is not continuous but rather, discrete. They already had realized that matter is composed by atoms, and that it has a well-determined structure in microscopic level that lies beyond the ability of our senses to detect it. However, they had no idea about the atomic structure. The atomic theory of those years produced a unique framework containing all physics and chemistry related phenomena of that period, but without being able to explain a number of mysteries that began to appear towards the end of 19th century. As a result, the old atomic theory resulted as insufficient to describe matter in all its complexity.

New facts discovered during experiments that required a convincing explanation did arise in that period, where the two most significant were:

1. The discovery of electron in 1887 by Thompson
   In 1887, Thompson discovered the negatively charged electric particles as basic atomic components. These particles were given the name "electrons". This was the first step towards the discovery of the true atomic structure. The fact that electrons contains a negative charge and the entire atom is neutral implied the existence of a balancing positive charge at the same amount inside the atom.
2. The discovery of penetrating EM radiation or "radioactivity" by Becquerel in 1896
   In 1896, Henry Becquerel was observing the properties of X-rays discovered one year earlier by Rontgen, using natural fluorescent minerals. He believed that if using uranium salt, it would radiate X-rays after absorbing the incident sunlight. For some personal reasons, he gave up from this experiment but anyway, a few days later he developed the photographic films planned for the experiment and surprisingly, he noticed that the images in them were strong and clear without having need for any sunlight exposure. Becquerel therefore realized that uranium radiates naturally, without the need for any external source of energy such as the sunlight. In this way, he unintentionally had discovered the phenomenon of natural radioactivity. He then studied further and more in detail this phenomenon and reached the conclusion that it was not the X-radiation already known but another form of an unknown radiation.
   The inability to explain such phenomena derived from the fact that science at that time had reached to study of atom as a whole only, without being able to go more in detail. For scientists of 19th century, atom represented the last station of knowledge. However, in the light of new facts discovered at that period, it was evident the need for a revised atomic structure, which would give answer to the mysteries of micro-world.

You have reached the end of Physics lesson 20.1.1 The Atom. There are 9 lessons in this physics tutorial covering Atomic Nucleus and Its Structural Properties, you can access all the lessons from this tutorial below.

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