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Dalton’s Atomic Theory

John Dalton who created Dalton's Atomic Model

Introduction to the Dalton Atomic Model

Dalton’s atomic theory was proposed in 1804 and was the first attempt to describe matter in terms of atoms. He believed that all compounds were made of indivisible particles that combined in set ratios. Although Dalton didn’t get it completely correct, his theory set the foundation for today’s atomic model. His theory incorporated both the law of conservation of mass and constant composition. He also developed the law of multiple proportions.

Main Parts of Dalton’s Atomic Theory

Dalton’s model has a few key postulates. These are listed below along with a more in-depth analysis of each one.

  1. Elements consist of indivisible particles. Dalton named these particles atoms. Atoms make up all matter.
    • Atoms comes from the Greek word ‘atomos’, meaning ‘indivisible’.  Dalton did not know what an atom was exactly but knew there must be something to make up so many different elements. These were Dalton’s fundamental building blocks of matter.
  2. All atoms of the same element are identical in size, mass, and properties.
    • This idea means that all atoms of carbon will have the same mass and the same size. This was a critical component of the theory based on Dalton’s observations that lead to the law of multiple proportions (discussed below).
  3. Different chemical compounds occur because different whole-number ratios of atoms bind together.
    • Each unique molecule has a unique ratio of elements. This is the basic idea of the law of constant composition. Any pure compound will always have the same ratio of the same elements. They must be whole-number ratios because you can’t have half of an atom.
  4. Chemical reactions are a rearrangement of atoms but do not change the atom itself. Atoms can not be created or destroyed.
    • To make a new compound or molecule, atoms rearrange. The atom itself does not change, just creates new bonds to different atoms. These other atoms can be of the same element or a different element. So a carbon atom may go from being part of carbon monoxide (CO) to methane (CH4). It is now part of a new molecule, but the atom was never changed. It was always a carbon atom.

Updates to Dalton’s Theory

Although Dalton’s theory was revolutionary for the time, it was not completely right. Following the postulates for his new atomic model did not always get Dalton the correct answer. For example, Dalton thought that methane was CH2 when today we know it is CH4. This error occurred because Dalton did not know how much hydrogen weighed. In his determination, one hydrogen atom weighed twice as much as we now know it weighs.

 Below are some of the changes that would need to be made to the Dalton’s atomic model to reflect what scientists have learned since Dalton published his theory.

  1. An atom can be further subdivided into subatomic particles
  2. All atoms of the same element do not have the same mass
    • Atoms of the same element do have different masses sometimes because there are different isotopes. An isotope is an element that has a different number of neutrons, but the same number of protons and electrons.
  3. Using nuclear fission and fusion we can create and destroy atoms
    • This process requires tons of energy however and occurs only in special situations.
  4. The theory does not explain allotropes.
    • Graphite and diamond are both made of carbon but have very different properties. John Dalton’s theory would not label these two structures accurately.

Law of Multiple Proportions

The law of multiple proportions says that atoms or elements can combine to form various chemical compounds. The mass ratios will be whole-number ratios of each other.

For example:

CO has 12 g carbon per 16 g oxygen. Therefore, a mass ratio of 12:16 or 3:4.

CO2 has 12g carbon per 32g oxygen. Results in a mass ratio of 12:32 or 3:8.

Even though the same two elements are combined, they combine in different ratios. This means that they are different elements.

Also, whether there are 2 molecules of CO or 548 molecules of CO, the ratio will always come out to be the same as for 1 molecule of CO. This occurs because each molecule of the same type, has the same number of each type of atom.

Who is John Dalton?

John Dalton is an English chemist who lived from 1766 -1844. He was a teacher at a variety of schools throughout his life. He started his first teaching position assisting his brother when he was twelve years old.

An image of John Dalton who created the first atomic model with atoms (Dalton's Atomic model)
John Dalton (Source: Wikipedia Commons)

Besides his atomic theory, John Dalton also had other scientific interests. His first scientific paper discussed color blindness he had observed in himself and his brother. Colorblindness is still sometimes referred to as Daltonism due to that paper. He also enjoyed studying meteorology. He carefully recorded daily weather observations from 1787 on. His meteorological observations also helped him come up with his theory on partial pressures, and, eventually, his atomic theory.

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