Physics A (Combined)
Matter
The History of the Atom and Atomic Structure
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The History of the Atom and Atomic Structure
The History of the Atom
- Democritus proposed the first atomic theory in 400BC, suggesting that matter is composed of small, indestructible particles.
- English chemist John Dalton further developed the atomic theory in the 1800s, claiming all matter is made up of atoms, which are indestructible and can't be divided.
- Towards the end of the 19th century, J.J. Thomson discovered the electron using cathode rays, leading to the 'plum-pudding' model.
- In 1911, Ernest Rutherford conducted his gold foil experiment, discovering the nucleus of the atom, which led to the 'planetary' model of the atom.
- Later, Niels Bohr revised Rutherford’s model to include distinct electron shells, a theory which partially stands today.
Atomic Structure
- Atoms are comprised of protons, neutrons, and electrons.
- The nucleus at the centre of the atom contains both protons and neutrons. It is positively charged due to the protons.
- Electrons are negatively charged particles that orbit the nucleus within energy levels or electron shells.
- Atoms are electrically neutral, meaning the number of protons (positive) equals the number of electrons (negative).
- The number of protons an atom possesses defines the atomic number and the type of element it is.
- The mass number (or atomic mass) of an atom is the sum of protons and neutrons.
Atomic Models
- Plum-pudding model: J.J. Thomson suggested the atom was a positively charged sphere with negative electrons embedded within it.
- Planetary model: Following his discovery of the nucleus, Rutherford compared atoms to a mini solar system, with electrons orbiting the nucleus.
- Bohr model: Bohr improved upon Rutherford’s model by organising the electrons into fixed orbits or 'shells'.
Isotopes
- Isotopes are atoms of the same element (having the same number of protons) but with different numbers of neutrons, leading to different mass numbers.
- Isotopes of the same element share chemical properties but may have different physical properties.
- Some isotopes are unstable, or radioactive, and can emit radiation through alpha, beta, and gamma decay.
