# 4.1: Evolution of the Atomic Model

Difficulty Level: At Grade Created by: CK-12

## 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.

## Lesson Vocabulary

• continuous theory of matter: The concept that matter is continuous, infinite, and comes in every form all around us, and could be divided and subdivided into smaller and smaller pieces without limit.
• discontinuous theory of matter: The concept that matter is actually finite and not limitless.
• atom: Fundamental, indivisible particles that make up matter.
• law of definite proportions: States that chemical compounds always contain the same proportion of elements by mass, regardless of amount.
• law of conservation of mass: States that the mass of a reactant must equal the mass of the product for any chemical process.
• law of multiple proportions: 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.

• What are the general properties of matter? For example, what are the properties of water that distinguish it from other substances?
• We widely accept that all matter is comprised of similar kinds of particles that are too small to detect with the unaided eye. We know that when a candle burns it gives off carbon dioxide and water vapor, but we cannot actually “see” these gases. How can we study something that we cannot directly see? Can you give an example?

## 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

Aristotle and Plato.

Our understanding of matter today is much different than it was long ago. In 440 BC, Aristotle and Plato (Figure above) 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 (Figure below) 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

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.

John Dalton

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 (Figure above). 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.

Combining Ratios in Elements in Compounds
CO and CO2 NO3 and N2O3 NO3 and N2O2
FeCl2 and FeCl3 N2O3 and N2O4

Notice in Table above 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:

1. All matter is composed of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
5. 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.

## Review Questions

1. Compare and contrast the continuous theory of matter with the discontinuous theory of matter.
2. Which of the following did Proust discover?
1. Elements combine in unpredictable ratios with other elements.
2. The mass of an element is always changing even when combined with another element.
3. Chemical compounds always contain the same proportion of elements by mass, regardless of amount.
4. Elements combine only in fixed ratios based on volume, but not mass.
3. Which of the following did Lavoisier discover?
1. When matter reacts, there is a small portion that is lost and unaccounted for.
2. For an isolated system, the mass of the reactants must equal the mass of the products.
3. When matter reacts, there can be mass that comes from nowhere and is unaccounted for.
4. Mass cannot be accurately accounted for in chemical processes.
4. What did John Dalton discover about chemical compounds? How did his theory differ from Proust's?
5. Summarize Dalton’s atomic theory.
6. What were two points made by Dalton’s atomic theory that are no longer considered correct?

• Kotz, John, and Heith Purcell. 1991. Chemistry & Chemical Reactivity. Orlando, FL: Holt, Rinehart and Winston.
• Partington, J. R. 1989. A Short History of Chemistry. 3 ed. New York: Macmillan. Reissued by Dover Publications.

## Points to Consider

• In this lesson, we discussed the early evolution of theories about the atom. More recent evidence has shown that the atom is comprised of subatomic particles that exhibit unique properties and affect the overall behavior of an atom. In our next lesson, we will look at subatomic particles, including how they were discovered and how they behave.

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