John Dalton Atomic Theory | Discovery, History, Example

Discovery of John Dalton Atomic Theory

John Dalton Atomic Theory: By far, Dalton’s most influential work in chemistry was his atomic theory. Attempts to trace precisely how Dalton developed this theory have proved futile; even Dalton’s own recollections on the subject are incomplete.

john dalton atomic theory

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 theory.

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 upon determining the relative masses of each different kind of atom, a process that 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. In a memoir read to the Manchester Literary and Philosophical Society on October 21, 1803. He claimed: “An inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new; I have lately been prosecuting this inquiry with remarkable success.”

He described his method of measuring the masses of various elements, including hydrogen, oxygen, carbon, and nitrogen, according to the way they combined with fixed masses of each other. If such measurements were to be meaningful, the elements had to combine in fixed proportions.

Dalton took the fixed proportions for granted, disregarding the contemporary controversy between French chemists Joseph-Louis Proust and Claude-Louis Berthollet over that very proposition.

Dalton’s measurements, crude as they were, allowed him to formulate the Law of Multiple Proportions. When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.

Thus, taking the elements as A and B, various combinations between them naturally occur according to the mass ratios A: B = x:y or x:2y or 2x:y, and so on. Different compounds were formed by combining atomic building blocks of different masses.

As the Swedish chemist, Jöns Jacob Berzelius wrote to Dalton. “The law of multiple proportions is a mystery without the atomic theory”. And Dalton provided the basis for this theory.

However, the problem remained that knowledge of ratios was insufficient to determine the actual number of elemental atoms in each compound.

For example, methane was found to contain twice as much hydrogen as ethylene. Following Dalton’s rule of “greatest simplicity,” namely, that AB is the most likely combination for which he found a meretricious justification in the geometry of close-packed spheres.

He assigned methane a combination of one carbon and two hydrogen atoms and ethylene a combination of one carbon and one hydrogen atom. This, we now know, is incorrect, for the methane molecule is chemically symbolized as CH4 and the ethylene molecule as C2H4.

Nevertheless, Dalton’s atomic theory triumphed over its weaknesses because his foundational argument was correct.

However, overcoming the defects of Dalton’s theory was a gradual process, finalized in 1858 only after the Italian chemist Stanislao Cannizzaro pointed out the utility of Amadeo Avogadro’s hypothesis in determining molecular masses.

Since then, chemists have shown the theory of Daltonian atomism to be a key factor underlying further advances in their field.

Organic chemistry, in particular, progressed rapidly once Dalton’s theory gained acceptance. Dalton’s atomic theory earned him the sobriquet “father of chemistry.”

After the age of 50, Dalton performed little scientific work of distinction, although he continued to pursue research in various fields. When faced with the Royal Society’s rejection of his 1838 paper “On the Arseniates and Phosphates,” he had it printed privately, noting bitterly that Britain’s chemistry elites, “Cavendish, Davy, Wollaston, and Gilbert are no more.”

His atomic theory eventually began to prove its worth, and its author gained widespread recognition. He was elected into the fellowship of the Royal Society of London and the Royal Society of Edinburgh, awarded an honorary degree from the University of Oxford. And elected as one of only eight foreign associates of the French Academy of Sciences, taking place vacated by the death of Sir Humphry Davy.

He also received a pension on the Civil List from the British crown. In Manchester, he was elected president of the Literary and Philosophical Society in 1817, continuing in that office for the rest of his life.

The society provided him with a laboratory after the New College moved to York. Dalton remained in Manchester and taught private pupils.

Despite his growing affluence and influence, his frugality persisted. He died of a stroke and was accorded the equivalent of a state funeral by his fellow townsmen.

Modern atomic theory is, of course, a little more involved than Dalton’s theory. But the essence of Dalton’s theory remains valid.

Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element known as “isotopes”. But isotopes of an element have the same chemical properties.

History of Dalton’s 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 and 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 typical 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 atomic theory 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’ particles. Indeed, carbon dioxide molecules (CO2) 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 experimenting and empirically examining the results. This marked the first truly scientific theory of the atom.

Postulates of John Dalton Atomic Theory

Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton’s theory quickly became the theoretical foundation in chemistry.

  • John Dalton atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.
  • Dalton based his theory on the law of conservation of mass and the law of constant composition.
  • First part of his theory states that all matter is made of indivisible atoms.
  • The second part of the theory says all atoms of a given element are identical in mass and properties.
  • Third part says compounds are combinations of two or more different types of atoms.
  • The fourth part of the theory states that a chemical reaction is a rearrangement of atoms.
  • Parts of the theory had to be modified based on the existence of subatomic particles and isotopes

Key Points of John Dalton Atomic Theory

Here are some key points of John Dalton Atomic theory:

1.All Matter is Made of Atoms

dalton's atomic theory

Dalton hypothesized that the law of conservation of mass and the law of definite proportions could explain using the idea of atoms.

He proposed that all matter is made of tiny indivisible particles called atoms. Which he imagined as “solid, massy, hard, impenetrable, movable particle(s).”

It is important to note that since Dalton did not have the necessary instruments to see or otherwise experiment on individual atoms. He did not have any insight into whether they might have any internal structure.

We might visualize Dalton’s atom as a piece in a molecular modeling kit, where different elements are spheres of different sizes and colors. While this is a handy model for some applications, we now know that atoms are far from solid spheres.

2. All Atoms of a given Element are Identical in Mass and Properties

atomic theory

Dalton proposed that every single atom of an element, such as gold, is the same as every other atom of that element. He also noted that the atoms of one element differ from the atoms of all other elements.

Today, we still know this to be mostly true. A sodium atom is different from a carbon atom.

Elements may share some similar boiling points, melting points, and electronegativities. But no two elements have the same set of properties.

3. Compounds are Combinations of Two or More Different Types of Atoms

dalton's theory

In the third part of Dalton’s atomic theory, he proposed that compounds are combinations of two or more different types of atoms.

An example of such a compound is table salt. Table salt is a combination of two separate elements with unique physical and chemical properties.

The first, sodium, is a highly reactive metal. Second, chlorine is a toxic gas. When they react, the atoms combine in a 1:1 ratio to form white crystals of NaCl start a text, N, a, C, l, end text, which we can sprinkle on our food.

Since atoms are indivisible, they will always combine in simple whole-number ratios. Therefore, it would not make sense to write a formula such as Na0.5 Cl0.5 because you can’t have half of an atom.

4. A Chemical Reaction is a Rearrangement of Atoms

atomic theory formula

In the fourth and final part of Dalton’s atomic theory, he suggested that chemical reactions don’t destroy or create atoms. They merely rearranged the atoms.

Using our salt example again, when sodium combines with chlorine to make salt, both the sodium and chlorine atoms still exist. They rearrange to form a new compound.

Merits of Dalton’s 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 to recognize a workable distinction between the fundamental particle of an element (atom) and that of a compound (molecule).

Limitations of Dalton’s Atomic Theory

  • It does not account for subatomic particles: Dalton’s atomic theory 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 theory, 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).
  • Does not account for isobars. This theory 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. Example: sugar/sucrose (C11H22O11).
  • The theory 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 theory.

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