Chemistry
Major Developments of the Atomic Model
- About 400BC: Greek philosopher Democritus first suggested that all substances consisted of tiny indestructible particles called atoms.
- 1808: An English chemist named John Dalton described his ideas about matter. Dalton’s ideas were based on many scientific experiments and observations. Although some of these today are to a degree, incorrect, Dalton’s developments formed the early settlements that led to the modern atomic theory. Dalton stated a number of ideas:
- All matter is composed of atoms
- Atoms cannot be made or destroyed
- All atoms of the same element are identical
- Different elements have different types of atoms
- Chemical reactions occur when atoms are rearranged
- Compounds are formed from atoms of the constituent elements.
- 1897: English Scientist Sir J.J. Thomson explained that the atom contained negatively charged particles called electrons. He suggested that atoms were positively charged spheres with negatively charged electrons embedded in them like the fruit in a plum pudding.
- 1911: Lord Rutherford proposed a nuclear model for the atom. In his model, the atom consisted of mostly space with a dense nucleus containing positively charged protons in the centre. Negatively charged electrons orbited the nucleus. Although Lord Rutherford’s model of the atom was essentially the same as today’s accepted model, its one flaw was that it proposed that the orbiting electrons would eventually lose energy and spiral in towards the nucleus.
- 1913: Neils Bohr, a scientist who had studied with Rutherford, modified the model by suggesting that electrons orbit the nucleus at different energy levels. Only electrons with specific amounts of energy could exist at each level. His model proposed that electrons could move from one level to another by gaining or losing ‘packets’ of energy. Although Bohr’s model explained why electrons did not spiral in towards the nucleus, it did not explain all the known properties of atoms.
- 1930: Schrodinger modified Bohr’s model of the atom, viewing electrons as continuous clouds.
- 1932: Sir James Chadwick discovered that the nucleus contained particles called neutrons, as well as positively charged protons. Neutrons had no electric charge and a mass about the same as a proton.
Atomic Theory:
- All elements are made up of tiny particles called atoms.
- Atoms of a given elemtn are alike
- Atoms of different elements are different.
- Chemical changes take place when atoms link up with, or separate from, one another.
- Atoms are not created or destroyed by chemical change.
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