Atomic Theory

Early Atomic Theory

History of Early Atomic Theory

Atomic theory is based on the work of John Dalton, whose experiments led to the understanding of atomic properties and behavior.

Throughout human history, people have wondered what makes up the objects around them. What makes stones different from rivers? What makes the sky different from soil? As people sought answers to these questions, the questions became more refined. What makes one stone different from another? What makes copper different from iron? People were now asking the questions that would lead to the development of atomic theory.

Prior to the 18th century, people had not yet developed the tools required to adequately answer these questions. Around the late 1780s, French chemist and philosopher Antoine-Laurent Lavoisier developed the law of conservation of mass, which states that matter is neither created nor destroyed during a chemical reaction. It was well known at the time that the mass of a substance decreases when it is burned. A reasonable conclusion was that matter was destroyed during the reaction. Lavoisier, however, conducted a series of oxidation experiments in which he burned tin, lead, and mercury in a closed container. Through careful measurements, he was able to show that the total mass of the system did not change. He proposed that the decrease in mass scientists had observed when a substance is burned occurs because a gas is released. When the mass of this gas is included, the total mass is conserved.

Shortly after this discovery, another French chemist, Joseph Louis Proust, proposed the law of definite proportions, also called the law of constant composition, which states that compounds have the same proportions of masses of their constituent elements regardless of how small they are broken down. Thus, 10 kilograms of sodium chloride, NaCl, has the same ratio of sodium to chloride as 1 kilogram of sodium chloride.

English chemist John Dalton considered both of those laws. He suspected that these observations occurred because of particles called atoms that behave in specific, predictable ways. The concept of atoms had been proposed years earlier, but Dalton was the first to develop a theory about their properties and behavior. Today, scientists have determined that an atom is the smallest particle of an element that has the properties of that element. An atom has an equal number of protons and electrons, giving it an overall neutral charge.

Four Points of Dalton's Atomic Theory

John Dalton proposed that elements are composed of identical, indivisible atoms, compounds are formed by combining atoms, and chemical reactions change compounds by rearranging the atoms.

Dalton's atomic theory had four parts:

  • All matter is made of atoms that are indivisible and indestructible.
  • All atoms of a given element are identical to one another in terms of mass and properties.
  • Compounds are formed by combinations of two or more different kinds of atoms.
  • A chemical reaction is a rearrangement of atoms.

The first part of Dalton's theory was based on measurements made of compounds before and after they reacted with each other. Dalton assumed both the law of conservation of mass and the law of definite proportions were true, despite controversy among scientists regarding the truth of each law. Instruments of the time were unable to produce images of individual atoms and their behavior. Since Dalton's time, scientists have discovered that atoms are indeed made up of smaller particles: electrons, protons, and neutrons.

The second part of Dalton's theory was easier to demonstrate. Any atom of a particular element, such as lead, has the same mass and properties as any other atom of that same element, and different mass and properties from any other element. Thus, all lead atoms have properties unique to lead, just as all carbon atoms have properties unique to carbon. This part of Dalton's theory has been modified since Dalton's time because of the discovery of isotopes. An isotope is one of two or more atoms of an element that have the same number of protons but different numbers of neutrons. Isotopes have different masses despite being atoms of the same element, which allows isotopes to have different physical properties, such as a boiling point.

The third part of Dalton's theory explains the wide variety of substances found on Earth and beyond. Atoms of different elements join to form compounds, which have different properties from the elements in pure form. For example, sodium is a highly reactive metal, and chlorine is a toxic gas. But when they combine, they form sodium chloride, NaCl, more commonly known as table salt. More importantly, since atoms are indivisible, they form compounds in whole number ratios of atoms. Thus, the formula for water is H2O rather than HO1/2 because half an atom is not possible.

The fourth part of Dalton's theory explains Lavoisier's observations that led him to develop the law of conservation of mass. In a chemical reaction, all the atoms that exist before the reaction still exist after the reaction is completed. The atoms have simply rearranged. For example, heating calcium carbonate (CaCO3) causes it to decompose into calcium oxide (CaO) and carbon dioxide (CO2).

Decomposition of Calcium Carbonate

The law of conservation of mass is represented in the equation because the number of each type of atom is the same on the left side of the equation as the right.
Dalton originally believed that atoms would combine in their simplest forms. He thought fewer combined atoms were favorable over more combined atoms but was unable to find evidence supporting or disproving this hypothesis. Yet his observations did still align with all four parts of his theory. This led to the development of the law of multiple proportions, which states that when two elements combine to form more than one compound, the mass of one element combines with a fixed mass of the other in a ratio of small whole numbers. Dalton noted that two oxides exist for carbon. A fixed mass of carbon (for example, 100 grams) may form compounds with either 133 grams of oxygen or 266 grams of oxygen. This ratio, 133:266, can be reduced to 1:2. Therefore, one carbon atom forms compounds with either one oxygen atom or two oxygen atoms: carbon monoxide (CO) or carbon dioxide (CO2).