Chapter 5: Elementary Particles and the Forces of Nature

Aristotle thought that matter was made up of earth, water, fire, and air. He believed that the force of gravity could act on the four "elements". He also believed that the other acting on the "elements" was levity and the ability to rise. He also believed that matter was made up of tinnier particles that could be broken down infinitely. He believed that there was no limit in the amount of matter that one could have. In fact, this belief of infinite division remained alive until John Dalton challenged the theory in 1803. He proposed that chemical compounds had set proportions of certain components. Even after his preposition, the main belief remained the Aristotle theory. When the existence of the atom became known in the early 20th century, scientists found that atoms were made up of the smallest particles known to mankind. At first, scientists believed that the nucleus of the atom was made up electrons and protons. In 1932, however, James Chadwick discovered that a particle called a neutron also existed in the nucleus. In fact, the true model of the atom was not known until much later. It was discovered that the nucleus consisted of the protons (positively charged particles) and neutrons (neutral particles) while negatively charged electrons orbit around the nucleus in an electron cloud.