http://www.ck12.org Chapter 4. Atomic Structure
they have recognized that some matter is not visible, such as the air that we breathe, but it is still there. Early
philosophers believed that matter was comprised of four fundamental substances: earth, air, fire, and water. These
became known as elements. Later, philosophers postulated that all matter was comprised of a fundamental particle,
the atom, which was indivisible. We now know that the atom is comprised of even smaller subatomic particles that
display unique behaviors. In this chapter, we will look at how our understanding of the atom has evolved over time.
Discontinuous Theory of Matter
FIGURE 4.1
Aristotle and Plato.Our understanding of matter today is much different than it was long ago. In 440 BC, Aristotle and Plato (Figure
4.1) proposed that matter was continuous, infinite, present in every form, and always all around us. It was thought
that matter could be divided and subdivided into smaller and smaller pieces without limit. This concept was called the
continuous theory of matter. One debate of the time revolved around how far a grain of sand could be divided. Most
philosophers of the time believed that the sand could be sub-divided indefinitely. These were logical interpretations
of their observations about the natural world.
In 400 BC, Democritus (Figure4.2) proposed an alternate view, referred to as thediscontinuous theory of matter.
He expanded upon the work of Leucippus, a mentor of his, who believed matter was actually finite and not limitless.
Democritus held that a grain of sand could only be divided a finite number of times. However, this idea was not
well-received at the time. Aristotle, who was considered a greater "authority," taught against it and influenced other
philosophers to reject the ideas of Democritus. It would be thousands of years before his ideas were revisited and
found to be consistent with more recently available scientific evidence.
Democritus proposed that all matter is composed of fundamental, indivisible particles that he calledatoms. The
essential ideas behind his theory are the following:
- Everything is composed of “atoms,” which are physically indivisible.
- Atoms are indestructible and constantly in motion.
- There is empty space between atoms.
Proust’s Law of Definite Proportions
The French scientist Joseph Louis Proust (1754-1826) studied chemical compounds and their mass proportions.
Through his experiments, Proust found chemical compounds always contain the same proportion of elements by