# 7 History of Atomic Structure

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The concept of the atom as an indivisible building block of matter was recorded as early as the 5th century BCE.

### LEARNING OBJECTIVES

Describe the early developments leading to the modern concept of the atom.

### KEY TAKEAWAYS

#### Key Points

• The ancient Greek philosophers Democritus and Leucippus recorded the concept of the atomos, an indivisible building block of matter, as early as the 5th century BCE.
• The idea of an indivisible particle was further elaborated upon and explored by a number of scientists and philosophers, including Galileo, Newton, Boyle, Lavoisier, and Dalton.
• John Dalton, an English chemist and meteorologist, is credited with the first modern atomic theory based on his experiments with atmospheric gases.

#### Key Terms

• : The smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
• : The law stating that reactants will always combine in set whole number ratios.

### Early History of the Atom

Matter is composed of indivisible building blocks. This idea was recorded as early as the 5th century BCE by Leucippus and Democritus. The Greeks called these particles atomos, meaning indivisible, and the modern word “atom” is derived from this term. Democritus proposed that different types and combinations of these particles were responsible for the various forms of matter. However, these ideas were largely ignored at the time, as most philosophers favored the Aristotelian perspective.

The concept of the atom was revisited and elaborated upon by many scientists and philosophers, including Galileo, Newton, Boyle, and Lavoisier. In 1661 Boyle presented a discussion of atoms in his The Sceptical Chymist. However, the English chemist and meteorologist John Dalton is credited with the first modern atomic theory, as explained in his A New System of Chemical Philosophy.

Dalton’s experiments with gases led to some of the earliest measurements of atomic masses and a concept of atomic structure and reactivity. Dalton’s atomic theory contained the following ideas:

• All atoms of a given element are identical.
• The atoms of different elements vary in mass and size.
• Atoms are indestructible. Chemical reactions may result in their rearrangement but not their creation or destruction.

Dalton also outlined a law of multiple proportions, which described how reactants will combine in set ratios. Like the early philosophers, Dalton’s theories were not popularly accepted for much of the 19th century, but his ideas have since been accepted, with amendments addressing subatomic particles and the interconversion of energy and mass.

## The Law of Conservation of Mass

The law of conservation of mass states that mass in an isolated system is neither created nor destroyed.

### LEARNING OBJECTIVES

Define the law of conservation of mass.

### KEY TAKEAWAYS

#### Key Points

• The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations.
• According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.
• The law of conservation of mass is useful for a number of calculations and can be used to solve for unknown masses, such the amount of gas consumed or produced during a reaction.

#### Key Terms

• : A law that states that mass cannot be created or destroyed; it is merely rearranged.
• : A chemical substance formed as a result of a chemical reaction.
• : Any of the participants present at the start of a chemical reaction. Also, a molecule before it undergoes a chemical change.

### History of the Law of the Conservation of Mass

The ancient Greeks first proposed the idea that the total amount of matter in the universe is constant. However, Antoine Lavoisier described the law of

conservation of mass (or the principle of mass/matter conservation) as a fundamental principle of physics in 1789.

This law states that, despite chemical reactions or physical transformations, mass is conserved—that is, it cannot be created or destroyed—within an isolated system. In other words, in a chemical reaction, the mass of the products will always be equal to the mass of the reactants.

### The Law of Conservation of Mass-Energy

This law was later amended by Einstein in the law of conservation of mass-energy, which describes the fact that the total mass and energy in a system remain constant. This amendment incorporates the fact that mass and energy can be converted from one to another. However, the law of conservation of mass remains a useful concept in chemistry since the energy produced or consumed in a typical chemical reaction accounts for a minute amount of mass.

We can therefore visualize chemical reactions as the rearrangement of atoms and bonds while the number of atoms involved in a reaction remains unchanged. This assumption allows us to represent a chemical reaction as a balanced equation in which the number of moles of any element involved is the same on both sides of the equation. An additional useful application of this law is the determination of the masses of gaseous reactants and products. If the sums of the solid or liquid reactants and products are known, any remaining mass can be assigned to gas.

## The Law of Definite Composition

The law of definite composition states that chemical compounds are composed of a fixed ratio of elements as determined by mass.

### LEARNING OBJECTIVES

Define the law of definite composition.

### KEY TAKEAWAYS

#### Key Points

• The law of definite composition was proposed by Joseph Proust based on his observations on the composition of chemical compounds.
• Proust proposed that a compound is always composed of the same proportions of elements by mass.
• Though initially controversial, the law of definite composition was supported by Dalton’s atomic theory.

#### Key Terms

• : Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means, and it is made up of atoms all having the same number of protons.
• : A law that states that chemical compounds are formed of constant and defined ratios of elements as determined by mass.

### History of the Law of Definite Composition or Proportions

French chemist Joseph Proust proposed the law of definite composition or proportions based on his experiments conducted between 1798 and 1804 on the elemental composition of water and copper carbonate.

In 1806 Proust summarized his observations in what is now called Proust’s law. It stated that chemical compounds are formed of constant and defined ratios of elements, as determined by mass. For example, carbon dioxide is composed of one carbon atom and two oxygen atoms. Therefore, by mass, carbon dioxide can be described by the fixed ratio of 12 (mass of carbon):32 (mass of oxygen), or simplified as 3:8.

At the time, Proust’s theory was a controversial one and disputed by a number of chemists, most notably another French chemist, Claude Louis Berthollet. Berthollet supported the concept that elements could mix in any ratio. However, the English chemist John Dalton’s formulation of atomic theory supported Proust’s idea at an atomic level, as Dalton proposed that chemical compounds were composed of set formulations of atoms from different elements. Dalton’s law of multiple proportions expanded on the law of definite composition to postulate that, in situations in which elements can combine to form multiple combinations, the ratio of the elements in those compounds can be expressed as small whole numbers.

### Applications of the Law of Definite Composition or Proportions

The law of definite composition has applications to both molecular compounds with a fixed composition and ionic compounds as they require certain ratios to achieve electrical neutrality. There are some exceptions to the law of definite composition. These compounds are known as nonstoichometric compounds, and examples include ferrous oxide. In addition, the law of definite composition does not account for isotopic mixtures.

## The Law of Multiple Proportions

The law of multiple proportions states that elements combine in small whole number ratios to form compounds.

### LEARNING OBJECTIVES

Define the law of multiple proportions.

### KEY TAKEAWAYS

#### Key Points

• The law of multiple proportions is a rule of stoichiometry.
• John Dalton formulated the law of multiple proportions as part of his theory that atoms formed the basic indivisible building block of matter.
• The law of multiple proportions says that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small, whole-number ratios.
• The law of multiple proportions is an extension of the law of definite composition, which states that compounds will consist of defined ratios of elements.

#### Key Terms

• : A law stating that if two elements form a compound, then the ratio of the mass of the second element and the mass of the first element will be small, whole-number ratios.
• : The smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.

### Dalton’s Law

The law of multiple proportions, also known as Dalton’s law, was proposed by the English chemist and meteorologist John Dalton in his 1804 work, A New System of Chemical Philosophy. It is a rule of stoichiometry. The law, which was based on Dalton’s observations of the reactions of atmospheric gases, states that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small, whole-number ratios.

For example, the reaction of the elements carbon and oxygen can yield both carbon monoxide ($\text{CO}$) and carbon dioxide ($\text{CO}_2$). In $\text{CO}_2$, the ratio of the amount of oxygen compared to the amount of carbon is a fixed ratio of 1:2, a ratio of simple whole numbers. In $\text{CO}$, the ratio is 1:1.

In his theory of atomic structure and composition, Dalton presented the concept that all matter was composed of different combinations of atoms, which are the indivisible building blocks of matter. Dalton’s law of multiple proportions is part of the basis for modern atomic theory, along with Joseph Proust’s law of definite composition (which states that compounds are formed by defined mass ratios of reacting elements) and the law of conservation of mass that was proposed by Antoine Lavoisier. These laws paved the way for our current understanding of atomic structure and composition, including concepts like molecular or chemical formulas.

## John Dalton and Atomic Theory

Dalton introduced a theory that proposed that elements differed due to the mass of their atoms.

### LEARNING OBJECTIVES

Identify the main points of Dalton’s atomic theory.

### KEY TAKEAWAYS

#### Key Points

• Dalton’s atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass.
• Dalton’s atomic theory also stated that all compounds were composed of combinations of these atoms in defined ratios.
• Dalton also postulated that chemical reactions resulted in the rearrangement of the reacting atoms.

#### Key Terms

• : The smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
• : The standard unit that is used for indicating mass of an atom.

### History of Dalton’s Atomic Theory

Although the concept of the atom dates back to the ideas of Democritus, the English meteorologist and chemist John Dalton formulated the first modern description of it as the fundamental building block of chemical structures. Dalton developed the law of multiple proportions (first presented in 1803) by studying and expanding upon the works of Antoine Lavoisier and Joseph Proust.

Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin[II] oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found an atomic theory of matter could elegantly explain this common pattern in chemistry—in the case of Proust’s tin oxides, one tin atom will combine with either one or two oxygen atoms.

Dalton also believed atomic theory could explain why water absorbed different gases in different proportions: for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in the mass and complexity of the gases’ respective particles. Indeed, carbon dioxide molecules ($\text{CO_2}$) are heavier and larger than nitrogen molecules ($\text{N}_2$).

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). Since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion, this marked the first truly scientific theory of the atom.

The main points of Dalton’s atomic theory are:

1. Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed.
2. All atoms of an element are identical.
3. The atoms of different elements vary in size and mass.
4. Compounds are produced through different whole-number combinations of atoms.
5. A chemical reaction results in the rearrangement of atoms in the reactant and product compounds.

Atomic theory has been revised over the years to incorporate the existence of atomic isotopes and the interconversion of mass and energy. In addition, the discovery of subatomic particles has shown that atoms can be divided into smaller parts. However, Dalton’s importance in the development of modern atomic theory has been recognized by the designation of the atomic mass unit as a dalton.