PowerPoint PresentationAll atoms consist of 2 regions:
1) Nucleus: very small region located near the center of the atom
Proton: positively charged particle
Neutron: neutrally charged particle
2) Electron: negatively charged particle surrounding the nucleus of an atom
The nucleus, protons, neutrons, and electrons are also known as subatomic particles
Most of the mass of the atom is in the nucleus
Most of the volume of the atom is empty space
The first discovery of a subatomic particle resulted from investigating the relationship between electricity and matter
Cathode Rays & Electrons
In 1897, J.J. Thomson used a cathode ray tube to deduce (reason) the presence of a negatively charged particle
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure
Result of Thomson’s experiment: all cathode rays are composed of identical negatively charged particles, which were late named electrons
Discovery of the Electron
Thomson was awarded a Nobel Prize in 1906 for his discovery of the electron
The discovery strongly implied that Dalton was wrong and that the atom was not the smallest particle of matter & that it can be broken down even further
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Cathode Ray Experiment
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass
Experiments have found the mass of an electron to be 9.109 x 10-31 kg
Dalton’s Atomic Model
Found to be wrong after the discovery of the electron
Thomson’s Atomic Model
Solid blob of positive charge with negative electrons embedded on the surface
Known as the “plum pudding” model
Atomic Models at the Time
In 1911, Ernest Rutherford and associates provided more detail to the atom’s structure
Scientists fired a thin, gold foil with fast moving alpha particles (positively charged particles)
Expected the particles to pass through
Discovery of the Nucleus
- Rutherford exclaimed that it was “as if you had fired a 15-inch [artillery] shell at a piece of tissue paper and it came back and hit you
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Rutherford’s findings:
A few particles bounced back
Very few were greatly deflected back toward their source
Rutherford’s explanation:
Reasoned that the particles that bounced back experienced a powerful force within the atom
That force must occupy a very small amount of space
Conclusion:
Force must be caused by a very densely packed bundle of matter with a positive electric charge at the core of atom the nucleus!
Disproved Thomson’s model of the atom!
Discovered that the volume of a nucleus was very small compared with the total volume of an atom
If the nucleus were the size of a marble, then the size of the atom would be about the size of a football field
Where were the electrons???
Rutherford suggested that the electrons surrounded the nucleus like planets around the sun
More Atomic Models at the Time
All atomic nuclei are made of two kinds of particles:
Protons: positively charged particles
Neutrons: neutral particles
Atoms are neutral because they contain equal numbers of protons and electrons
Found that a proton has a mass of 1.673 x 10-27 kg
about 1836 times bigger than the mass of an electron!
A neutron has a mass of 1.675 x 10-27 kg
slightly bigger than a proton!
Composition of the Atomic Nucleus
Subatomic Particle
Based on quantum mechanics
Electrons orbit the nucleus in orbits that have a set size and energy
More Atomic Models