The postulates of Dalton's atomic theory: which points do we still use today, and what have we learned since Dalton?

Key Points

  • Dalton's 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.
  • The first part of his theory states that all matter is made of atoms, which are indivisible.
  • The second part of the theory 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 theory states that a chemical reaction is a rearrangement of atoms.
  • Parts of the theory had to be modified based on the discovery of subatomic particles and isotopes.

Chemists ask questions.

Chemistry is full of unanswered questions. One of the first questions people have been asking since ancient times is What is the world made of?
That is, if we were to zoom in ~100000000000 times—that is 11 zeros!—on the skin of your fingertip, what would we see? Would that look any different from zooming in on, say, an apple? If we then cut up the apple into tinier and tinier pieces using an imaginary tiny knife, would we reach a point where the pieces could no longer be cut any smaller? What would those pieces look like, and would they still have apple properties?
The answers to these questions are fundamental to modern chemistry, and chemists didn't agree on the answer until a few hundred years ago. Thanks to scientists such as John Dalton, modern chemists think of the world in terms of atoms. Even if we can't see atoms with our naked eye, properties of matter such as color, phase (e.g., solid, liquid, gas), and even smell come from interactions on an atomic level. This article will discuss John Dalton's atomic theory, which was the first complete attempt to describe all matter in terms of atoms and their properties.

Basis for Dalton's theory

Dalton based his theory on two laws: the law of conservation of mass and the law of constant composition.
The law of conservation of mass says that matter is not created or destroyed in a closed system. That means if we have a chemical reaction, the amount of each element must be the same in the starting materials and the products. We use the law of conservation of mass every time we balance equations!
The crystal lattice of sodium chloride shows the sodium and chloride ions in a 1:1 ratio.
A chemist thinks of table salt as sodium and chloride ions arranged in a crystal lattice structure. Image credit: "Image of salt" by OpenStax Anatomy and Physiology, CC-BY-NC-SA 4.0.
The law of constant composition says that a pure compound will always have the same proportion of the same elements. For example, table salt, which has the molecular formula N, a, C, l, contains the same proportions of the elements sodium and chlorine no matter how much salt you have or where the salt came from. If we were to combine some sodium metal and chlorine gas—which I wouldn't recommend doing at home—we could make more table salt which will have the same composition.
Concept check: A time-travelling scientist from the early 1700s decides to run the following experiment: he takes a 10 gram sample of ethanol (C, H, start subscript, 3, end subscript, C, H, start subscript, 2, end subscript, O, H) and burns it in the presence of oxygen in an open beaker. After the reaction is done, the beaker is empty. Does this result violate the law of conservation of mass?
Many scientists used to believe an experiment such as this one demonstrated that mass could be destroyed, since there was less mass in the beaker after the reaction compared to the starting materials. However, they forgot one important point: the system should be closed if you want to check for conservation of mass! In this case, the products of our combustion reaction are C, O, start subscript, 2, end subscript, left parenthesis, g, right parenthesis and H, start subscript, 2, end subscript, O, left parenthesis, g, right parenthesis, so we would need to be very careful to collect the gaseous products to test if mass is conserved.
In the early 1700s, scientists did not completely agree on the composition of air or even the definition of a gas—since they didn't know about atoms yet.

Dalton's atomic theory

Part 1: All matter is made of atoms.

Dalton hypothesized that the law of conservation of mass and the law of definite proportions could be explained 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 being solid spheres.

Part 2: All atoms of a given element are identical in mass and properties.

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 exact set of properties.
Atoms of the same element can have different masses because the number of neutrons can vary for different isotopes of a given element. They will still have the same number of protons, though, because that is what identifies them as being a particular element. We can measure the mass of atoms, and isotopes, very precisely using mass spectrometry. For more on isotopes, you can watch this video on atomic number, mass number, and isotopes.
Picture of a molecular modeling kit including multiple types of plastic spheres in different colors that represent elements and stick-like plastic "bonds".
A basic molecular modeling kit, including spherical atoms of different size and color that can be connected by sticks to represent chemical bonds. Image credit: "Photo of modeling kit" by Sonia on Wikimedia Commons, CC-BY 3.0

Part 3: Compounds are combinations of two or more different types of atoms.

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. The second, chlorine, is a toxic gas. When they react, the atoms combine in a 1:1 ratio to form white crystals of N, a, C, l, 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 N, a, start subscript, 0, point, 5, end subscript, C, l, start subscript, 0, point, 5, end subscript because you can't have half of an atom!

Part 4: A chemical reaction is a rearrangement of atoms.

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 simply rearrange to form a new compound.

What have we learned since Dalton proposed his theory?

The short answer: a lot! For instance, we now know that atoms are not indivisible—as stated in part one—because they are made up of protons, neutrons, and electrons. The modern picture of an atom is very different from Dalton's "solid, massy" particle. In fact, experiments by Ernest Rutherford, Hans Geiger, and Ernest Marsden showed that atoms are mostly made up of empty space.
Image of tungsten diselenide, W, S, e, start subscript, 2, end subscript.
Scanning transmission electron microscopy (STEM) allows us to see the atomic level structure of tungsten selenide, WSestart subscript, 2, end subscript. Image credit: "STEM image" by Kazu Suenaga et al. on Wikimedia Commons, CC BY 4.0
Part two of Dalton's theory had to be modified after mass spectrometry experiments demonstrated that atoms of the same element can have different masses because the number of neutrons can vary for different isotopes of the same element. For more on isotopes, you can watch this video on atomic number, mass number, and isotopes.
Despite these caveats, Dalton's atomic theory is still mostly true, and it forms the framework of modern chemistry. Scientists have even developed the technology to see the world on an atomic level!

Attributions:

This article was adapted from the following articles:
  1. OpenStax College. "Early Ideas in Atomic Theory." OpenStax CNX. October 2, 2014. http://cnx.org/contents/havxkyvS@9.110:HdZmYjzP@4/Early-Ideas-in-Atomic-Theory. CC-BY 4.0
The modified article is licensed under a CC-BY-NC-SA 4.0 license.

Summary

  • Dalton's 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.
  • The first part of his theory states that all matter is made of atoms, which are indivisible.
  • The second part of the theory 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 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.