Dalton's Atomic Theory | General Chemistry 1

What are the 5 Points of Dalton's Atomic Theory?

Dalton's Atomic Theory was formulated by John Dalton in 1808, and it remains a fundamental tenet of chemistry to this day. The five main points are:

  • Matter is made up of atoms, small and indivisible particles.
  • All atoms of the same element are identical and have the same mass.
  • Atoms of different elements vary in size, mass, and chemical behavior.
  • Chemical compounds are made up of at least 2 atoms of different elements. The resulting particle is called a molecule.
  • In a chemical reaction, atoms are rearranged, separated, or recombined to form new compounds but no atom is created or destroyed.

While Dalton was considered a pioneer of modern chemistry, some of his theories were later proven inaccurate. Despite this, many of his ideas became the foundation for discoveries in chemistry in the coming decades.


Which Part of Dalton's Theory Was Wrong?

The indivisibility of an atom is no longer accepted, as it was discovered that atoms can be further divided into protons, neutrons, and electrons. 

  • Dalton also proposed that atoms of different elements combine in simple whole-number ratios such as 2:3 or 3:5 for Hydrogen and Oxygen respectively. In contrast, complex organic compounds like sugar (C12H22O11) defy this theory.
  • Dalton's theories were also wrong when it came to, for example, calcium and argon. In the case of these two elements, they share a 40 amu atomic mass. This means Dalton's theory of atoms of different elements being different in all respects was incorrect in certain cases.
  • Atoms of the same element typically will have similar properties as Dalton proposed. However, there are a few exceptions. These are known as isotopes. Isotopes are atoms with different masses, which goes against Dalton's original theory. For example, chlorine has two isotopes: one at mass number 35 and another at 37.
  • Finally, allotropes are a poorly explained phenomenon that cannot be accounted for by Dalton's theory. Charcoal, graphite, and diamond have many differences in properties from one another despite having similar structures which suggest there may exist an underlying explanation as to why this happens.


Who Proved Dalton's Theory Wrong?

In a world-shattering discovery, English physicist J.J Thomson disproved the idea that atoms are indivisible.  Thomson discovered that an atom consists of a heavy and dense core, known as the nucleus, which is orbited by much lighter particles: protons and electrons.


How Did Thomson Prove This?

Thomson entered a modified Crookes tube into an electrical field. He noticed that the tube gave off a green glow as cathode rays hit the opposite electrode. Thomson concluded that the glowing particles were not simply light, but had mass because they were deflected in opposite directions by an electrical field.

Thomson realized that cathode rays could be made up of only one kind of particle if he was able to deflect them using an electric field without changing their properties, which was possible with a magnet. When the rays were deflected by a magnet, Thomson concluded that they must be made up of negatively charged particles since only particles with negative charges would curve in the opposite direction to an electric field.

Thomson also found that these particles could be both absorbed and released by the element neon, which meant they were not stable atoms. This led Thomson to conclude that cathode rays were streams of particles, each with a negative charge and mass.

Thomson's discoveries led him to propose the first complete model of the atom: an atom has a positively charged nucleus surrounded by negatively charged electrons in orbit.