Atomic theory

English chemist and physicist John Dalton (1766–1844) is recognized as the father of modern atomic theory. Dalton’s theory, first developed in 1803, is based on the premises that all matter is made up of atoms and that all atoms of the same element are identical to each other in size and mass. As such, according to this theory, atoms of different elements could be distinguished from each other by inherent differences in their size and mass. When atoms of separate elements are united chemically to form compounds, a unique atomic mass ratio can be discerned. Dalton’s insight on atomic masses and fixed ratios of atoms in compounds gave scientists important knowledge about chemical compositions of matter, leading to the creation of the indispensable periodic table of elements and to new explorations and advances within the fields of chemistry and physics.

Background

The idea behind atomic theory originated in ancient Greece, when a philosopher named Democritus (ca. 460–ca. 370 BCE) proposed that matter is composed of miniscule particles in constant motion. While some of his contemporaries embraced the theory, the leading philosopher of the day, Aristotle (384–322 BCE), did not. As a result, early atomic theory was given no scientific credence until Dalton’s ideas were published centuries later.

Dalton’s interest in natural sciences was ignited when a friend introduced him to meteorology in 1787. He began recording his daily weather observations, which prompted him to consider the composition of Earth’s atmosphere. After careful study and some two hundred thousand recorded observations, Dalton determined that when different gases combine, the resulting pressure is the sum of the pressure of each individual gas. From that, he postulated that gases must be made up of tiny particles, or atoms. He further theorized that the atoms composing a given element have identical characteristics, such as mass, which enable the atoms to be identifiable as that specific element and to be differentiated from other elements. Dalton published his New System of Chemical Philosophy, a list of atomic masses for each element, starting in 1808. He was awarded a Royal Medal in 1826 in recognition of his work on atomic theory.

Dalton’s atomic theory gained credence over the ensuing decades through scientific experimentation and evidentiary examples that supported the main tenets of his hypothesis. The creation of a periodic table of elements in the 1860s was a significant boon for Dalton’s theory. In particular, a version of the periodic table, by Russian chemist and inventor Dmitry Mendeleyev (1834–1907), arranged the elements in order of increasing atomic mass so that elements with similar chemical properties became grouped. It provided a visual depiction of Dalton’s ideas about the existence and characteristics of atoms in matter. By the early 1900s, Dalton’s theory was the generally accepted principle underlying the scientific study of atoms.

Overview

Dalton’s atomic theory included the following postulates: (1) that all matter consists of atoms; (2) that atoms cannot be subdivided, created, or destroyed; (3) that all atoms of the same element are identical in size, mass, and other characteristics; (4) that atoms of different elements can combine in whole-number ratios to form various compounds; and (5) that atoms are the smallest units of matter that can participate in chemical reactions.

In the late nineteenth and early twentieth centuries, scientists delved deeper into the structure of the atom itself. In 1897, English physicist J. J. Thomson (1856–1940) discovered the existence of negatively charged subatomic particles called electrons, disproving Dalton’s postulate that atoms could not be further subdivided into smaller particles. Later, Thomson, along with English chemist and physicist Francis William Aston (1877–1945), determined that some atoms of the same element may have different masses. These atoms, known as isotopes, were first observed by Aston in 1919, thus disproving another of Dalton’s postulates. However, much of Dalton’s theory remained applicable and formed the basis of further developments in atomic theory.

In 1911, New Zealand–born British physicist Ernest Rutherford (1871–1937) showed that the atom’s structure consists of a positively charged core called the nucleus, orbited by the negatively charged electrons. Based on classical theory of electron acceleration, however, the electrons in Rutherford’s model would eventually lose their energy and crash into the nucleus. In 1913, Danish physicist Niels Bohr (1885–1962) applied Albert Einstein’s theory of quantum physics to the issue and proposed that electrons could circle a nucleus without losing energy because when they drop from an orbit of higher energy into one of lower energy, the difference in energy output is emitted in the form of photons to offset the change and to maintain a constant of equivalent energy.

With the development of quantum mechanics during the 1920s, a viable explanation for the role of electrons in atoms emerged. The discovery of the neutron in 1932 completed the modern picture of the atom. By that time, English physicist Henry Moseley (1887–1915) had found that each nucleus was characterized by an atomic number equal to the number of positive charges associated with it. The discovery sparked the creation of the modern version of the periodic table of elements, which arranges the elements according to their atomic number rather than their atomic mass, as had been done in the past.

Dalton’s theory of the atom expanded scientific understanding in physics and chemistry. In both disciplines, the atom is the foundation for quantitative analysis, from understanding matter and force to uncovering the relationships between substances during chemical reaction. Over the years, scientists have learned a great deal about the structure and behavior of atoms, and modern scientists have been expanding their research efforts to focus on larger as well as smaller atomic scales. For example, some researchers have looked into the behavior of large groups of atoms versus individual atomic activity in efforts to delve deeper into the basic nature of matter, while others have experimented with the creation of novel types of new atoms made from elementary particles discovered after the proton, neutron, and electron.

Bibliography

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