Thomson drastically changed the billiard ball view of an atom when he discovered the electron in 1897. With the discovery of the electron Thomson suggested that the atom was not an 'indivisible' particle but a jigsaw puzzle of smaller pieces. He used a cathode ray tube and found that the glowing stream within the tube would bend toward a positively charged electric plate. This lead to the theorization that the stream was made up of small particles carrying a negative charge. Thus the electron was discovered. Based on Goldstein's 1886 discovery that atoms had positive charges, Thomson theorized that atoms were like pieces of raisin bread or chocolate chip cookies. They were a structure with clumps of small negatively charged electrons scattered inside a smear of positive charges.
Improvements on previous model:
- Describes the existence of negative and positive charged particles in an atom
- Discovery of the electron
- Does not explain the role of electrons in bonding
- Does not describe neutrons or explain the existence of isotopes
ERNEST RUTHERFORD: 1871 - 1937
Rutherford was a student of Thomson and is known for firing tiny alpha particles at solid objects. His experiment is referred to as the Gold Foil Experiment and is responsible for revolutionizing the view of the atom. In 1911 he suggested that the atom had a dense core of positively charged particles surrounded by a swirling ring of electrons. In his experiment he observed that some of the alpha particles ricocheted off a solid object while others passed through. He theorized that the nucleus was so dense that the alpha particles bounced off of it, while the electrons were very tiny and spread out allowing the alpha particles to pass right through. Rutherford expanded the view of an atom to include electrons, protons, and the nucleus and his model is often referred to as the Planetary Model.
Improvements on previous model:
- Explains why the electron spins around the nucleus
- Purposed that the atom is mostly empty space
Key problems with the model:
- Does not place electrons in definite energy levels around the nucleus
- Doesn't include neutrons in the nucleus
NIELS BOHR: 1885 - 1962
Bohr is a Danish physicist who proposed that electrons could not move continuously in the atom but only in precise steps. He hypothesized that electrons occupy specific energy levels after conducting experiments using atomic spectra. Bohr visualized these energy levels as concentric circles around the nucleus. Bohr also predicted that each energy level could only hold so many electrons. Bohr's theory states that the first electron shell can only hold 2 electrons. For elements that have more than two electrons the third electron will reside in the second electron shell. The second and third electron shells can hold a maximum of 8 electrons each. This model is easily illustrated by drawing electron shell diagrams and explains the role of valence electrons in bonding. This model is the prominent model used in grade 9 science.
Improvements on previous model:
- Explained role of valence electrons in bonding
- Relegates the number of valence electrons to the periods of a periodic table
- Explains ionic and covalent bonding
- Places electrons in definite energy levels
Key problems with the model:
- It doesn't explain the shapes of molecules or other abnormalities
Resources:
I once again used the online VisionLearning virtual text. Here are the two modules:
In my next post I will explain how to draw an electron shell diagram, it's relationship to the atomic mass and atomic number, and examples of my own diagrams. I will also briefly touch on modern Atomic Theory.
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