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Atomic theory, ancient philosophical speculation that all things can be accounted for by innumerable combinations of hard, small, indivisible particles (called atoms) of various sizes but of the same basic material; or the modern scientific theory of matter according to which the chemical elements.
- The Editors of Encyclopaedia Britannica
- Overview
- Key Points
- Chemists ask questions.
- Basis for Dalton's theory
- Part 1: All matter is made of atoms.
- Part 2: All atoms of a given element are identical in mass and properties.
- Part 3: Compounds are combinations of two or more different types of atoms.
- Part 4: A chemical reaction is a rearrangement of atoms.
- What have we learned since Dalton proposed his theory?
- Summary
The postulates of Dalton's atomic theory: which points do we still use today, and what have we learned since Dalton?
•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.
•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.
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?
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 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 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.
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.
[Why is this only MOSTLY true?]
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 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 such as Na0.5Cl0.5 because you can't have half of an atom!
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.
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.
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 and references]
•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.
Aug 26, 2020 · The modern atomic theory, proposed about 1803 by the English chemist John Dalton (Figure \(\PageIndex{4}\)), is a fundamental concept that states that all elements are composed of atoms. Previously, an atom was defined as the smallest part of an element that maintains the identity of that element.
Atomic theory is the scientific theory that matter is composed of particles called atoms. The definition of the word "atom" has changed over the years in response to scientific discoveries.
Jun 13, 2023 · John Dalton's atomic theory: Matter is composed of atoms. Atoms come in different types, called elements. Atoms of each element have a distinct mass.
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Jun 26, 2023 · Elizabeth Gordon. Furman University. Learning Objectives. Give a short history of the concept of the atom. Describe the contributions of Democritus and Dalton to atomic theory. Summarize Dalton's atomic theory and explain its historical development. Democritus and the Greek Philosophers.
Summarize This Article. The beginnings of modern atomic theory. Experimental foundation of atomic chemistry. How scientists discovered atoms. Learn about the discovery of atoms and the instruments scientists use to see these small particles. (more) See all videos for this article.
