Process of Theoretical Change: Dalton’s Atomic Theory

Over centuries many theories were developed but today only a few remain valid. One of the most “talked-about” theories was the one made by John Dalton, a physicist, meteorologist and a chemist. One of the main reasons why his work is so popular today is because some of his discoveries have helped in the development of the modern atomic theory. There are however cases were his theory is a little different from modern theories.

John Daltons Theory Stated that:

1. All matter is made up of atoms.
2. Atoms are indestructible and indivisible.
3. Atoms of the same element have identical properties and mass.
4. A chemical reaction is a rearrangement of atoms.
5. A compound is formed by a combination of two or more different kinds of atoms.

Cases where Daltons Theory is different:

• John Dalton’s theory stated that atoms of the same element have identical masses but modern theories have proved that atoms of the same element can have different masses, these atoms are called isotopes however they do have the same chemical properties.
• In modern research it was discovered that atoms can be destroyed by nuclear reactions but not by chemical reactions.

The following table summarizes the changes in theories over time:

Name	Discovery/Theory
Democritus, 460 BC	Came up with the idea of atoms
John Dalton, 1808	After the idea of atoms was dismissed for over 2000 years, he reintroduced the idea of atoms and through many experiments he proved the existence of atoms.
Joseph .J.Thompson, 1898	Stated that positively charged substances were distributed uniformly over the surface of the atom containing electrons embedded within them.
Ernest Rutherford, 1910	From J.Thompson’s model and through his own experiments he was able to determine the true model of the atom. Instead of the positively charged particles being distributed over the atom they were gathered together in the nucleus.
Niels Bohr, 1913	He came up with these rules: Electrons can only orbit a fixed distance from the nucleus. Atoms lose energy when electrons go from higher energy to lower energy orbits and the opposite happens from lower energy to higher energy orbits.
Wolfgang Pauli, 1924	Stated that electrons spin while orbiting the nucleus. He also came up with his own principle known as the Pauli Exclusion Principle which stated that no two electrons of a given atom can have the same set of quantum numbers.