…Today we will discuss about

“Dalton’s Model of Atom”

It was in the early 1800s that John Dalton, an observer of weather and discoverer of color blindness among other things, came up with his atomic theory. Let's set the stage for Dalton's work. Less than twenty years earlier, in the 1780's, Lavoisier ushered in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy. By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition ( also called the Law of Definite Proportions).

In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.

The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses.

He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

So that, briefly, is Dalton's theory. With modifications, it has stood up pretty well to the criteria that we talked about earlier. It did not convince everyone right away however. Although a number of chemists were quickly convinced of the truth of the theory, it took about a half century for the opposition to die down, or perhaps I should say die off. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

However, only 3 or 4 pages in the third chapter discussed the atomic theory he proposed. In this theory, there are four basic ideas...

1) chemical elements are made of atoms. 2) the atoms of an element are identical in their mases3) atoms of different elements have different masses 4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on. 5) atoms can be neither created nor destroyed

This is the Law of Multiple Proportions. Simply put, when elements combine to form compounds they do so in whole number ratios.

Carbon and Oxygen can form either CO ( a 1:1 ratio) or CO2 (a 1:2 ratio). It also states that if the mass of the first element is constant the second elements masses would be in a whole number ratio.

1.00g of Carbon will have combined with 1.33g in CO1.00g of Carbon will have combined with 2.66g in CO2The mass are in a 1:2 ratio

Simply put if combinations are in whole number ratios, there must be a one. This one being an atom. Here are Dalton's assumptions.All matter is composed of atoms

Atoms cannot be made or destroyedAll atoms of the same element are identical

Different elements have different types of atomsChemical reactions occur when atoms are rearrangedCompounds are formed from atoms of the constituent

elements.

Dalton’s atomic model is one of the fundamentals of physics and chemistry. This theory of atomic composition was hypothesized and partially confirmed by the English chemist and Physicist John Dalton. Dalton came with his Atomic theory as a result of his research into gases. He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements. Through deductive reasoning and experimentation, he made an interesting discovery. His findings led him to hypothesize that elements combine at the atomic level in fixed ratios. This ratio would naturally differ in compounds due to the unique atomic weights of the elements being combined.

This was a revolutionary idea but further experimentation by himself and others confirmed his theory. The findings became the basis of of Dalton’s Atomic Laws or Model. These laws focus on five basic theorems. First, Pure Elements consist of particles called atoms. Second,atoms of an element are all the same for that element. That means gold is gold and oxygen is oxygen down to the last atom. Third, atoms of different elements can be told apart by their atomic weights. Fourth, atoms of elements unite to form chemical compounds. Finally, atoms can neither be created or destroyed in chemical reaction. The grouping only changes.

The last of Dalton’s Atom Model were at the time considered true for all reactions involving atoms. This was later corrected with the discovery of nuclear fission and fusion. So we now know that this only holds true for chemical reactions.

Like other scientific theories, Dalton’s atomic model has been perfected over time with the research and discoveries of other scientists. We now know that the atom can be divided into even smaller particles and we have even discovered the actual internal atom structure, even able to view it through modern technology. We now know that atomic weight is a product of the structure of the atoms as well.

This atomic theory made possible modern chemistry and physics. Up until Dalton’s time the atom was only considered to a philosophical construct passed down by the ancient Greeks. Dalton’s ground breaking work made theory reality. This understanding the atom helped to fuel many other discoveries such as the fundamental forces and Einstein’s theory of relativity. It is especially is important when one goes into Quantum physics a discipline that looks at physics at the atomic and subatomic levels.

…Picture of “Dalton’s Model of

Atom”

"A series of Essays read before this society and afterwards published in the 5th Vol. of their Memoirs gradually led me to the consideration of ultimate particles or atoms & their combinations. Under the date of September 3rd, 1803, I

find in my notebook 'Observations on the Ultimate Particles of Bodies and their Combinations,' in which the atomic symbols I still use [were] introduced. On the 23rd

of October the same year[I] [r]ead my Essay on the absorption of gases [by water] at the conclusion of which a

series of atomic [weights] was given for 21 simple and compound elements .. . . "

- John Dalton (Excerpt from paper read to Manchester Literary and Philosophical Society, 1830)