The atomic hypothesis of Dalton

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

Points to Remember

The atomic theory of Dalton was the first effort to characterise all matter in terms of atoms and their properties.
Dalton's idea was founded on the laws of mass conservation and constant composition.
All matter is made up of indivisible atoms, according to the first component of his theory.
The theory's second portion states that all atoms of a given element have the same mass and characteristics.
Compounds, according to the third section, are made up of two or more different sorts of atoms.
A chemical reaction, according to the fourth part of the theory, is an atom rearrangement.
The discovery of subatomic particles and isotopes required certain changes to the theory.

Chemists are inquisitive.

There are many unresolved questions in chemistry. Since ancient times, one of the first questions people have asked is, "What is the world composed of?"

What would we observe if we zoomed in 100000000000 times (that's 11 zeros!) on the skin of your fingertip? What would it look like if you zoomed in on, say, an apple? Would we be able to cut the apple into tinier and tinier pieces with an imaginary tiny knife until the bits could no longer be sliced smaller? What would those pieces look like, and would they retain their apple-like qualities?

The answers to these problems are crucial to modern chemistry, and until a few hundred years ago, chemists couldn't agree on them. Modern chemists think of the world in terms of atoms thanks to experts like John Dalton. Even though we can't see atoms with our naked eyes, features of matter like colour, phase (solid, liquid, gas), and even smell are derived from atomic interactions. The atomic hypothesis of John Dalton, which was the first thorough attempt to define all matter in terms of atoms and their properties, will be discussed in this article.

Theory of Dalton's foundation

The law of conservation of mass and the law of constant composition were the foundations of Dalton's hypothesis.

In a closed system, the law of conservation of mass states that matter cannot be created or destroyed. That means that in a chemical reaction, the amount of each element in the starting materials and the products must be the same. Every time we balance equations, we employ the law of conservation of mass!

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.

A pure compound will always have the same proportion of the same elements, according to the law of constant composition. Table salt, for example, has the chemical formula textNaClNaClstart text, N, a, C, l, end text, and includes the same quantities of sodium and chlorine regardless of how much salt you have or where it comes from. We could manufacture more table salt with the same composition by combining sodium metal with chlorine gas (which I wouldn't recommend doing at home).

Concept check: A time-travelling scientist from the early 1700s decides to run the following experiment: he takes a 10 gram sample of ethanol (\text{CH}_3 \text{CH}_2 \text {OH}CH

OHstart text, C, H, end text, start subscript, 3, end subscript, start text, C, H, end text, start subscript, 2, end subscript, start text, O, H, end text) 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?

the system should be closed if you want to check for conservation of mass! In this case, the products of our combustion reaction are \text{CO}_2(g)CO

(g)start text, C, O, end text, start subscript, 2, end subscript, left parenthesis, g, right parenthesis and \text H_2 \text O(g)H

We'd have to be very cautious to gather the gaseous products to see if mass is conserved. O(g)start text, H, end text, start subscript, 2, end subscript, start text, O, end text, left parenthesis, g, right parenthesis

Because they didn't know about atoms yet, scientists couldn't agree on the composition of air or even the concept of a gas in the early 1700s.

Part 1 of Dalton's atomic theory: Atoms make up all matter.

The law of conservation of mass and the law of definite proportions, according to Dalton, might be explained using the concept of atoms. All matter, he suggested, is made up of small indivisible particles known as atoms, which he imagined as "solid, massy, hard, impenetrable, moving particle(s)."

Dalton did not have the proper apparatus to see or experiment on individual atoms, therefore he had no way of knowing if they had any interior structure. Dalton's atom could be seen as a piece in a molecular modelling kit, with different elements represented by spheres of various sizes and colours. While this paradigm is useful in some situations, we now know that atoms are not solid spheres.

Part 2: The mass and characteristics of all atoms of a specific element are the same.

Every atom of an element, such as gold, is the same as every other atom of that element, according to Dalton. He also pointed out that the atoms of one element are not the same as those of all other elements. We still know that this is generally true now. A sodium atom isn't the same as a carbon atom. Although elements have some qualities in common, such as boiling points, melting points, and electronegativities, no two elements have exactly the same combination of properties.

part:3 Compounds are atoms that are made up of two or more different types.

Dalton proposed that compounds are made up of two or more different sorts of atoms in the third section of his atomic theory. Table salt is one example of such a chemical. Table salt is made up of two distinct components that have distinct physical and chemical properties. Sodium, for example, is a highly reactive metal. Chlorine, the second, is a poisonous gas. When the atoms react, they generate white crystals of textNaClNaClstart text, N, a, C, l, end text, which we can sprinkle on our food in a 1:1 ratio.

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 \text{Na}_{0.5}\text{Cl}_{0.5}Na

0.5

because you can't have half an atom! start text, N, a, end text, start subscript, 0, point, 5, end subscript, start text, C, l, end text, start subscript, 0, point, 5, end subscript, start text, C, l, end text, start subscript, 0, point, 5, end subscript, start text, C, l, end text, start subscript, 0, point, 5, end subscript, start text, C, l, end text

Part 4: A chemical reaction is an atomic rearrangement.

Dalton proposed that chemical processes do not destroy or generate atoms in the fourth and final part of his atomic theory. They did little more than rearrange the atoms. Using our salt example once more, when sodium and chlorine combine to form salt, both sodium and chlorine atoms remain. They just rearrange themselves to create a new complex.

What have we learned since Dalton's idea was proposed?

The quick answer is: a great deal! For example, because protons, neutrons, and electrons make up atoms, we now know that they are not indivisible, as mentioned in section one. Dalton's "solid, massy" particle is substantially different from the contemporary notion of an atom. In fact, Ernest Rutherford, Hans Geiger, and Ernest Marsden demonstrated that atoms are largely made up of empty space in their tests.

Part two of Dalton's hypothesis had to be changed after mass spectrometry research revealed that the number of neutrons in various isotopes of the same element can vary, causing atoms of the same element to have different masses. Watch this video on atomic number, mass number, and isotopes for more information about isotopes.

Despite these limitations, Dalton's atomic hypothesis remains mostly accurate, and it serves as the foundation for contemporary chemistry. Scientists have even invented equipment that allows them to glimpse the world at the atomic level!

Summary

The atomic theory of Dalton was the first effort to characterise all matter in terms of atoms and their properties.

Dalton's idea was founded on the laws of mass conservation and constant composition.

All matter is made up of indivisible atoms, according to the first component of his theory.

The theory's second portion states that all atoms of a given element have the same mass and characteristics.

Compounds, according to the third section, are made up of two or more different sorts of atoms.

A chemical reaction, according to the fourth part of the theory, is an atom rearrangement.

Because of the presence of subatomic particles and isotopes, parts of the theory had to be changed.