Atomic structure

• Early ideas about atoms

  • John Dalton published his ideas about atoms in 1803
  • He thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided.
• J.J Thomson

  • Nearly 100 years later, J J Thomson carried out experiments and discovered the electron.
  • This led him to suggest the plum pudding model of the atom
  • In this model, the atom is a ball of positive charge with negative electrons embedded in it - like currants in a Christmas pudding.
• Ernest Rutherford

  • In 1909 Ernest Rutherford designed an experiment to test the plum pudding model
  • In the experiment, positively charged alpha particles were fired at thin gold foil.
  • Most alpha particles went straight through the foil. But a few were scattered in different directions.
• Nuclear Model

  • This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:
  • the mass of an atom is concentrated at its centre , the nucleus
  • the nucleus is positively charged
• Developing models of atoms

  • Niels Bohr adopted Ernest Rutherford's nuclear model
  •  Bohr did calculations that led him to suggest that electrons orbit the nucleus in shells.
  • The shells are at certain distances from the nucleus. The calculations agreed with observations from experiments.
• James Chadwick

  • Further experiments led to the idea that the nucleus contained small particles, called protons. Each proton has a small amount of positive charge.
  • In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge.
  • These particles are called neutrons. This led to another development of the atomic model
• Structure of the atom

  • An atom has a central nucleus. This is surrounded by electrons arranged in shells.
  • The nucleus is tiny compared to the atom as a whole:
  • the radius of an atom is about 0.1 nm (1 × 10-10 m)
  • the radius of a nucleus (1 × 10-14 m) is less than 1/10,000 of the radius of an atom
• Proton

  • Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1.
• Neutron

  • Uncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0.
• Remember that Protons are Positive, and Neutrons are Neutral.
• Mass

  • The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g).
• Relative mass

  • The relative mass is the number of times heavier a particle is, compared to another.
• subatomic particles

  • The nuclei of all atoms contain subatomic particles called protons. The nuclei of most atoms also contain neutrons.
• Atomic number and mass number

  • The atomic number of an atom is the number of protons in the atom
  • atoms of the same element have the same atomic number
  • atoms have equal number of protons and electrons, so overall charge=0
  • the mass number of an atom is the total number of protons and neutrons
• • mass number at the top and atomic number on the bottom
• Isotopes

  • Atoms of the same elements with different number of neutrons are called isotopes
  • Isotopes of the same element have :
  • -the same atomic number
  • -different mass numbers
• Relative atomic mass

  • The relative atomic mass of an element is the average mass of its atom
  • calculated from the mass number of its isotopes
  • The abundance of these isotopes