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In this installment of the history of atom theory, physics professor (and my dad) Dean Zollman introduces us to John Dalton, who gave us the rules of definite proportions and simplicity to explain matter.

By Dean Zollman

Dean Zollman
In the very early 19th century, Newton’s idea of forces acting among the small particles of matter and Lavoisier’s discovery of different substances combining to create a new compound would come together to form a more coherent idea about the atomic nature of chemical reactions.

An important player in this development was John Dalton (1766-1844). Dalton’s first interest was not in chemistry but in the physical properties of gasses. This interest came about because of a strong interest in meteorology and the atmosphere. He wanted to understand why the gasses in the atmosphere were mixed evenly. They could, instead, be arranged so that the oxygen, nitrogen, water vapor, and carbon dioxide were each in separate layers. (I am using modern names for the gasses rather than the ones Dalton would have used.)

John Dalton

1895 frontispiece of John Dalton and the Rise of Modern Chemistry

Dalton came up with a rather simple idea based on Newton’s forces between particles. He concluded that each particle of a substance repelled other particles of the same substance but did not interact with the other substances in the air. Thus, oxygen particles would repel other oxygen particles but would not affect or be affected by the nitrogen, water, or carbon dioxide particles.

This idea became rather cumbersome to explain, so Dalton later modified it. He stated that each particle of an element was surrounded by heat. The heat that surrounded oxygen would interact only with the heat of another oxygen particle, nitrogen with nitrogen, and so forth. So that caused repulsion among like atoms. All of this was based on each element having different sized atoms. This investigation led Dalton to conclude that the total pressure of a gas was the sum of the pressures of the individual gasses in the mixture.

This rather strange theory (at least strange by today’s standards) led to some careful investigation of gasses. In particular, Dalton measured the weights of elements when they combined to create new substances. For example, hydrogen and oxygen combined to make water. Dalton learned that the ratio of the weight of the hydrogen to the weight of oxygen was always the same and always a simple ratio. By weight, he discovered, water was seven parts oxygen to one part hydrogen. Further, if more than one compound was made of the same two elements, each of the compounds would be a simple ratio of weights. This fact was noticeable in the two ways that nitrogen and oxygen combined. This conclusion was the rule of definite proportions.

Based on these observations, Dalton developed an atomic theory of chemistry. This theory was published in 1808 in the book A New System of Chemical Philosophy. In addition to the ideas already discussed here, the New System included:

“… all bodies of sensible magnitude … are constituted of a vast number of extremely small particles, or atoms, of matter bound together by forces of attraction.”

“… the ultimate particles of all homogenous bodies are perfectly alike in weight, figure, &c. …”

In addition, Dalton needed to explain how atoms came together to form other substances (today we call them molecules). He enunciated a rule of simplicity. If a material is made of two kinds of atoms, they will combine in the simplest way possible. For example, if A and B combine to make C, C consists of one atom of A and one atom of B. If A and B also combine to form D, then D is two As and one B. E would be one A and two Bs, and so forth.

This rule of simplicity led to an error about the composition of water. By Dalton’s rule, water would be one atom each of hydrogen and oxygen instead of two hydrogens and one oxygen. Thus, he got the ratio of the weight of oxygen to the weight of hydrogen wrong.

How and when Dalton arrived at this theory has kept historians of science busy for almost 200 years. In a recent paper, Alan J. Rocke provides about five different scenarios that led Dalton to his conclusions. And all of them are supported by writings or reports of Dalton’s own words.

While there are some flaws in Dalton’s ideas, they did form the foundation for an atomic theory of chemistry. Thus, it was very strong support for the existence of atoms. However, it was not widely accepted in its totality by many chemists of the day. Some liked the rule of simplicity but not other parts; others embraced the proportionality idea but rejected simplicity, and so forth.

Humprey Davy

Portrait of Sir Humphrey Davy, from Sketches of the Royal Society and Royal Society Club, 1849 (provided to Wikimedia Commons by Chemical Heritage Foundation)

A well-known chemist of the day was Humphrey Davy (1778-1829). He and Dalton had frequent conversations, and Dalton had said of Humphrey that he was “a very agreeable and intelligent young man … the principle failing of his character as a philosopher is that he does not smoke.” Dalton had even used some of Davy’s observations about combinations of nitrogen and oxygen in supporting his theory.

However, in 1811 Davy stated, “I shall enter no further at present into an examination of the opinions, results and conclusions of my learned friend; I am however obliged to dissent from most of them and to protest against the interpretation that he has pleased to make of my experiments … it is not, I conceive, on any speculations upon the ultimate particles of matter that the true theory of definite proportions must ultimately rest.” Thus, Davy acknowledged that the rule of definite proportions was correct, but insisted that atoms were not the way to explain it.

So, Davy was wrong about the value of atoms; Dalton missed the boat about the value of smoking.

Many more advances occurred during the first half of the 19th century. We will continue that story next time.

Images via Wikimedia Commons, public domain


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Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.