The English scientist John Dalton, working in the early 1800s, developed the first comprehensive theory describing the nature of matter in terms of atoms. Dalton’s ideas provided a logical mechanism that explained the established laws of chemical combination, such as the conservation of mass and definite proportions. The resulting atomic theory established a foundation that remains a powerful framework for understanding chemical reactions today. This model is built upon a set of fundamental principles concerning the composition and behavior of matter.
Postulate One: Atoms are Indivisible and Indestructible
Dalton’s model asserted that all matter is composed of extremely small particles termed atoms. He proposed that these particles represent the ultimate, smallest unit of any substance. In Dalton’s view, atoms were considered indivisible building blocks that could not be broken down into smaller components, created, or destroyed.
This postulate directly explained the Law of Conservation of Mass, which states that matter’s total mass remains unchanged during a chemical process. If atoms cannot be created or destroyed, the total count and mass of all particles before a reaction must equal the total count and mass afterward. Although modern science has since discovered subatomic particles, demonstrating that atoms are divisible, Dalton’s historical claim of indivisibility was foundational to early chemistry.
Postulate Two: Atomic Identity Within an Element
Dalton proposed that all atoms belonging to a specific element, such as gold or oxygen, are identical in every measurable way. This includes having the same mass, size, and chemical properties. The consistency of these attributes ensures that every sample of a pure element will behave identically in chemical reactions.
Conversely, the atoms of one element are fundamentally different from the atoms of any other element. This distinction in mass and properties is what gives each element its unique chemical identity. While the later discovery of isotopes showed that atoms of the same element can have slightly different masses, Dalton’s original idea of elemental distinction remains central to chemistry.
Postulate Three: Formation of Compounds Through Combination
Dalton proposed that compounds are formed when atoms of two or more different elements chemically combine. This combination must occur in simple, fixed, whole-number ratios. For example, a molecule of water is always formed by exactly two hydrogen atoms and one oxygen atom; a formula like H\(_{1.5}\)O is impossible because atoms cannot be divided.
The requirement for simple, whole-number ratios provided a direct, atomic-level explanation for the Law of Definite Proportions. This law states that a specific chemical compound always contains the same elements in the same proportions by mass, regardless of the sample’s size or source. Since every atom of a given element has a constant mass, the fixed count of atoms in a molecule results in the observed fixed mass ratio for the compound.
Postulate Four: Conservation During Chemical Reactions
Chemical reactions do not involve the creation or destruction of atoms, nor do they convert atoms of one element into another. Instead, a reaction is understood as the separation, combination, or rearrangement of the atoms already present. The atoms simply change their partners to form new substances.
This concept ensures that the total mass remains constant throughout the process, reaffirming the Law of Conservation of Mass. For example, when sodium and chlorine react to form table salt, the original atoms merely rearrange to form the new compound sodium chloride. This rearrangement of existing particles is the fundamental mechanism by which all chemical transformations occur.