Evolution of the Atomic Model

Lesson Objectives Describe the development of the concept of the atom from Democritus to the modern day. Compare and contrast the continuous and discontinuous theories of matter. State the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. Summarize the five essential points of Dalton’s atomic theory. Describe the relationship between Dalton’s atomic theory and the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

Introduction For centuries, humans have been fascinated with the behavior of matter. They have recognized that certain things, like candles, burn while other things, such as metals, do not readily burn. Humans have noticed that certain substances react with one another. For instance, iron will rust over time in the presence of air or water. Similarly, 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 Our understanding of matter today is much different than it was long ago. In 440 BC, Aristotle and Plato 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 proposed an alternate view, referred to as the discontinuous 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 called atoms. The essential ideas behind his theory are the following: 1. Everything is composed of “atoms,” which are physically indivisible. 2. Atoms are indestructible and constantly in motion. 3. 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 mass, no matter the amount. Based off of this idea, Proust developed the law of definite proportions which was published in 1799. To illustrate this, suppose compound AB is made of element A and element B. Regardless of how much AB is present, the ratio between the mass of A and the mass of B will be the same for any sample. In other words, elements combine in fixed ratios based on their mass. Water, H2O, is always 1/9 by mass hydrogen and 8/9 by mass oxygen, regardless of whether we are looking at one drop or an entire lake.

Lavoisier’s Law of Conservation of Mass At the same time, another French scientist named Antoine Lavoisier was studying mass relations in chemical reactions. He noticed that for an isolated system, the mass of the reactants must equal the mass of the products for any chemical process. This discovery was later called the law of conservation of mass. This law greatly influenced chemistry because it quantified gaseous chemical species, which were often viewed as “missing matter” that was not involved in chemical processes.

While it must be assumed that many more scientists, philosophers and others studied the composition of matter after Democritus, a major leap forward in our understanding of the composition of matter took place in the 1800s with the work of John Dalton. John Dalton, a school teacher from England, studied the weights of various elements and compounds.

He noticed that matter always combined in fixed ratios based on weight (and volume, in the case of gases). Chemical compounds always contain the same proportion of elements by mass, regardless of amount, which provided further support for Proust’s law of definite proportions. Dalton also observed that there was more than one mass ratio in which two elements could combine.

Notice in this table that the elements combine in different but predictable ratios with other elements. Carbon can combine in a 1:1 ratio with oxygen to form carbon monoxide –a poisonous gas. Change the ratio to 1:2 carbon to oxygen, and you get carbon dioxide, which is a gas that we exhale. His work led to the development of the law of multiple proportions. This law states that if two elements form more than one compound between them, then the ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small whole numbers.

Dalton’s Atomic Theory (1804) From his experiments and observations, as well as the work of contemporary scientists, Dalton proposed a new theory of the atom. This later became known as Dalton’s atomic theory. The general tenets of this theory were as follows: All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created, or destroyed. Atoms of different elements can combine in simple whole number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged.

Dalton’s atomic theory has been largely accepted by the scientific community, although a couple of modifications have been made since its conception. We know now that (1) an atom can be further sub-divided, and (2) not all atoms of an element have identical masses.

Lesson Summary Early Greek philosophers thought that matter could be divided and subdivided into smaller and smaller pieces without limit. In 400 BC, Democritus proposed that there was a point at which matter could no longer be divided any further. He suggested that all matter was composed of tiny indivisible particles, which he called atoms. Joseph Proust found that compounds always contain the same proportion of elements by mass, regardless of amount. This was later called the law of definite proportions. Antoine Lavoisier proposed the law of conservation of mass, which states that in a chemical reaction mass is not created nor destroyed. John Dalton discovered that certain combinations of elements could combine in multiple ratios. This was called the law of multiple proportions. In 1804, John Dalton proposed a modern atomic theory. This theory is still largely accepted by the scientific community, with a couple modifications.