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Philosophical Theories
Dalton
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 NaCl 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 CH3CH2OH 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?
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.
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 NaCl, 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 with 0.5s as subscripts 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 2WSe2
Scanning transmission electron microscopy (STEM) allows us to see the atomic level structure of tungsten selenide,
WSe_22start 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!
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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.
Thomson
Millikan and Nagaoka
Rutherford
Soddy and Chadwick
Bohr/ Quantum