The History of the Atom and Atomic Structure

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.