Discovery of Dalton’s Atomic Model
By far, Dalton’s most influential work in chemistry was his atomic model. Attempts to trace precisely how Dalton developed this model have proved futile. Even Dalton’s own recollections on the subject are incomplete.
He based his theory of partial pressures on the idea that only like atoms in a mixture of gases repel one another. Whereas unlike atoms appear to react indifferently toward each other. This conceptualization explained why each gas in a mixture behaved independently.
Although this view was later shown to be erroneous. It served a useful purpose in allowing him to abolish the idea held by many previous atomists. From the Greek philosopher Democritus to the 18th-century mathematician and astronomer Ruggero Giuseppe Boscovich, that atoms of all kinds of matter are alike.
Dalton claimed that atoms of different elements vary in size and mass, and indeed this claim is the cardinal feature of his atomic model. His argument that each element had its own kind of atom was counterintuitive to those who believed that having so many different fundamental particles would destroy the simplicity of nature. But Dalton dismissed their objections as fanciful.
Instead, he focused on determining the relative masses of each other type of atom. This process could be accomplished, he claimed, only by considering the number of atoms of each element present in different chemical compounds. Although Dalton had taught chemistry for several years, he had not yet performed actual research in this field.
History of Dalton 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 ad 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 would 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 the atomic model 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 (CO2) molecules are heavier and larger than nitrogen molecules (N2).
Dalton proposed that each chemical element is composed of atoms of a single, unique type. 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 examining the results in an empirical fashion. This marked the first truly scientific model of the atom.
Postulates of Dalton Atomic Model
Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his model. Dalton’s model quickly became the theoretical foundation in chemistry.
- Dalton’s atomic model was the first complete attempt to describe all matter in terms of atoms and their properties.
- Dalton based his model on the law of conservation of mass and the law of constant composition.
- The first part of his model states that all matter is made of indivisible atoms.
- The second part of the model says all atoms of a given element are identical in mass and properties.
- The third part says compounds are combinations of two or more different types of atoms.
- The fourth part of the model states that a chemical reaction is a rearrangement of atoms.
- Parts of the model had to be modified based on the existence of subatomic particles and isotopes.
Merits of Dalton Atomic Theory
- The atomic theory explains the laws of chemical combination (the Law of Constant Composition and the Law of Multiple Proportions).
- Dalton was the first person to recognize a workable distinction between the fundamental particle of an element (atom) and that of a compound (molecule).
Limitations of Dalton’s Atomic Model
- It does not account for subatomic particles: Dalton’s atomic model stated that atoms were indivisible. However, the discovery of subatomic particles (such as protons, electrons, and neutrons) disproved this postulate.
- It does not account for isotopes: As per Dalton’s atomic model, all atoms of an element have identical masses and densities. However, different isotopes of elements have different atomic masses (For example, hydrogen, deuterium, and tritium).
- It does not account for isobars; This model states that the masses of the atoms of two different elements must differ. However, two different elements can share the same mass number. Such atoms are called isobars (Example: 40Ar and 40Ca).
- Elements need not combine in simple, whole-number ratios to form compounds: Certain complex organic compounds do not feature simple ratios of constituent atoms. For Example: sugar/sucrose (C11H22O11).
- The model does not account for allotropes: The differences in the properties of diamond and graphite, both of which contain only carbon, cannot be explained by Dalton’s atomic model.