Modern Atomic Theory

An eighteenth-century chemistry bench

Priestley, Lavoisier, and others had laid the foundations of the field of chemistry. Their experiments showed that some substances could combine with others to form new materials, other substances could be broken apart to form simpler ones, and a few key “elements” could not be broken down any further. But what could explain this complex set of observations? John Dalton, an exceptional British teacher and scientist, put together the pieces and developed the first modern atomic theory in 1803. To learn more about Priestley’s and Lavoisier’s experiments and how they formed the basis of Dalton’s theories, try the interactive experiment Dalton’s Playhouse, linked to below. Interactive Animation:Dalton’s Playhouse

Dalton made it a regular habit to track and record the weather in his hometown of Manchester, England. Through his observations of morning fog and other weather patterns, Dalton realized that water could exist as a gas that mixed with air and occupied the same space as air. Solids could not occupy the same space as each other; for example, ice could not mix with air. So what could allow water to sometimes behave as a solid and sometimes as a gas? Dalton realized that all matter must be composed of tiny particles. In the gas state, those particles floated freely around and could mix with other gases, as Bernoulli had proposed. But Dalton extended this idea to apply to all matter – gases, solids, and liquids. Dalton first proposed part of his atomic theory in 1803 and later refined these concepts in his classic 1808 paper A New System of Chemical Philosophy (which you can access through a link under the Resources tab).

Dalton’s elements

Dalton’s theory had four main concepts:

  1. All matter is composed of indivisible particles called atoms. Bernoulli, Dalton, and others pictured atoms as tiny billiard-ball-like particles in various states of motion. While this concept is useful to help us understand atoms, it is not correct as we will see in later modules on atomic theory linked to at the bottom of this module.
  2. All atoms of a given element are identical; atoms of different elements have different properties. Dalton’s theory suggested that every single atom of an element such as oxygen is identical to every other oxygen atom; furthermore, atoms of different elements, such as oxygen and mercury, are different from each other. Dalton characterized elements according to their atomic weight; however, when isotopes of elements were discovered in the late 1800s, this concept changed.
  3. Chemical reactions involve the combination of atoms, not the destruction of atoms. Atoms are indestructible and unchangeable, so compounds, such as water and mercury calx, are formed when one atom chemically combines with other atoms. This was an extremely advanced concept for its time; while Dalton’s theory implied that atoms bonded together, it would be more than 100 years before scientists began to explain the concept of chemical bonding.
  4. When elements react to form compounds, they react in defined, whole-number ratios. The experiments that Dalton and others performed showed that reactions are not random events; they proceed according to precise and well-defined formulas. This important concept in chemistry is discussed in more detail below.

Some of the details of Dalton’s atomic theory require more explanation.

Elements: As early as 1660, Robert Boyle recognized that the Greek definition of element (earth, fire, air, and water) was not correct. Boyle proposed a new definition of an element as a fundamental substance, and we now define elements as fundamental substances that cannot be broken down further by chemical means. Elements are the building blocks of the universe. They are pure substances that form the basis of all of the materials around us. Some elements can be seen in pure form, such as mercury in a thermometer; some we see mainly in chemical combination with others, such as oxygen and hydrogen in water. We now know of approximately 116 different elements. Each of the elements is given a name and a one- or two-letter abbreviation. Often this abbreviation is simply the first letter of the element; for example, hydrogen is abbreviated as H, and oxygen as O. Sometimes an element is given a two-letter abbreviation; for example, helium is He. When writing the abbreviation for an element, the first letter is always capitalized and the second letter (if there is one) is always lowercase.

Atoms: A single unit of an element is called an atom. The atom is the most basic unit of matter, which makes up everything in the world around us. Each atom retains all of the chemical and physical properties of its parent element. At the end of the nineteenth century, scientists would show that atoms were actually made up of smaller, “subatomic” pieces, which smashed the billiard-ball concept of the atom (see our Atomic Theory I: The Early Days module).

Compounds: Most of the materials we come into contact with are compounds, substances formed by the chemical combination of two or more atoms of the elements. A single “particle” of a compound is called a molecule. Dalton incorrectly imagined that atoms “hooked” together to form molecules. However, Dalton correctly realized that compounds have precise formulas. Water, for example, is always made up of two parts hydrogen and one part oxygen. The chemical formula of a compound is written by listing the symbols of the elements together, without any spaces between them. If a molecule contains more than one atom of an element, a number is subscripted after the symbol to show the number of atoms of that element in the molecule. Thus the formula for water is H2O, never HO or H2O2.

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