Fundamental Particles
Chemistry: AQA A Level, topic 1.1.1
History of Atomic Theory
Our understanding of atoms has changed over time. Important models were developed by John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr.
John Dalton
- John Dalton was an English schoolteacher, who first wrote that atoms were the basic unit of chemistry.
- He thought that atoms were spherical, and different spheres constituted different elements.
- Dalton thought atoms were the fundamental units of matter and were indivisible.
J.J. Thomson
- 90 years after Dalton's ideas, J.J. Thomson discovered that atoms were divisible.
- Thompson discovered and measured the mass of the electron.
- The electron was measured to be several orders of magnitude lighter than an atom, proving that smaller particles than atoms existed.
- This lead to the plum pudding model, where the atom was viewed to be a large positively charged sphere, with embedded smaller, negatively charged, electrons.
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Ernest Rutherford
- Ernest Rutherford fired alpha particles at a thin film of gold.
- Were the plum pudding model true, the particles would have been deflected by the gold.
- Instead, most particles passed straight through the film.
- This led to the nuclear model, where the atom was viewed as being mostly empty space, with a positive nucleus and orbiting electrons.
Niels Bohr
- The nuclear model had a problem; classically, an electron should spiral into the nucleus, and atoms should collapse.
- Niels Bohr solved this problem by developing the first quantum theory of the atom, with electrons assigned to fixed orbits of defined energy.
- When electrons move between orbits, they must emit or absorb electromagnetic radiation of a particular frequency.
Subatomic Particles
Atoms are made of protons, neutrons, and electrons. These different subatomic particles have different properties.
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Units
- Subatomic particles are so small that conventional SI units aren't very useful.
- For example, a proton weighs 1.6726219 × 10-27kg - this is not a nice number to use!
- We define certain units for use with atoms - the atomic mass unit (amu) is defined as one twelfth the mass of a single carbon-12 atom.
- The elementary charge unit (e) is equal to the charge on an electron.
Protons
- A proton has a mass of 1.0073amu.
- It has a charge of +1e.
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Neutrons
- A neutron has a mass of 1.0087amu.
- A neutron has no electric charge.
- A neutron is very slightly heavier than a proton, but the difference is so small we often take the masses to be the same, and equal to 1amu.
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Electrons
- An electron has a charge of -1e.
- The mass of an electron is so small it is usually approximated to zero.
- The mass of an electron is approximately 0.00055amu.
Subatomic Particles
Atoms are made of a nucleus and shells of electrons.
Nucleus
- The nucleus of an atom contains protons and neutrons.
- Most of the mass of an atom is in the nucleus.
- The nucleus is positively charged.
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Electron shells
- Electron shells are further split into sub-shells.
- Each sub-shell has a slightly different energy.
- Electron shells occupy most of the space of the atom.
Test yourself on Fundamental Particles
Who disproved the plum pudding model of the atom as a result of an experiment with gold foil?
Evolution of the Atomic Model
What is the definition of the atomic mass unit?
What is the mass of an electron?