Discovery of an atom

• In 400BC, Democritus said

  that materials were made up of small particles. He called

  these particles atoms. Democritus hypothesized that atoms cannot

  be destroyed, differ in size, shape and temperature, are always

  moving, and are invisible. He believed that there are an infinite

  number of atoms. However, he had no evidence , it was just an

  idea.
• This is what Democritus' model of an atom look like

  
• In 1803, John Dalton proposed that all matter is composed of very

  small things which he called atoms. This was not a completely new concept as the ancient Greeks (notably Democritus) had proposed that all matter is composed of small, indivisible (cannot be divided) objects. He described atoms as billiard balls.
• John Dalton said

  • All atoms of the same element are identical in terms of their mass and properties
  • Atoms of different elements are different
  • Chemical changes take place when atoms link up or separate from each other
  • Atoms are not created or destroyed when chemical changes happen
• In 1897, J.J Thomson discovered the electron

  
• J.J Thomson discovered that atoms contained smaller particles, which other scientists later called electrons. He discovered these

  electrons while experimenting with a gas discharge tube.

  These are tubes filled with gases similar to a fluorescent light.

  He proposed that all atoms must contain electrons. His model

  of the atom was called the “plum pudding” model. Negatively

  charged electrons were thought to be embedded in a ball of

  positive charge, rather like the plums (the electrons) are part

  of a pudding (the ball of positive charge).
• In 1911, Rutherford and Marsden suggested that the atom had a tiny, dense, positively charged nucleus and much of the atom was empty space. The nucleus is surrounded by lighter, negatively charged electrons. Another way of thinking about this model was that the atom was seen to be like a mini solar system where the electrons orbit the nucleus like planets orbiting around the sun.
• In 1913, Niels Bohr used theoretical calculations that agreed with

  experimental evidence and adapted the nuclear model. He

  said that electrons orbited the nucleus in definite orbits at

  specific distances from the nucleus. He said that a fixed

  amount of energy is needed for electrons to move from one

  orbit to the next and that electrons only exist in these orbits.

  We call these orbits shells, orbitals or energy levels.
• Niels Bohr model

  
• By the late 1920's physicists were regularly referring to hydrogen

  nuclei as 'protons'. The term proton itself seems to have been

  coined by Rutherford, and first appears in print in 1920.
• In 1932 James Chadwick discovered the neutron and

  measured its mass
• In the Alpha Scattering Experiment, Rutherford and Marsden fired alpha particles at gold foil (which could be hammered to be 1 atom thick).

  • Most particles passed straight through, so the atoms must be mostly empty space.
  • Some bounced back, so there must be a dense nucleus with most of the mass.
  • Some were deflected, so the atom must have a positively charged nucleus
• This is what an atom looks like

  
• Atoms have a radius of about 0.1nm

  .....0,000 000 000 1m

  ….. 1 x 10(-10) m
• How do you get to namometers to milmetres?

  You multiply by 10 (-9)
• How do you get to milimetres to namometres?

  You divide by 10(-9)
• Plum pudding model

  • A sphere of positive charge with electrons dotted about ;looking like a plum pudding
• Why did scientists believe in the plum pudding model?

  Lack of experimental evidence.
• Describe what the alpha scattering experiment showed

  scientists

  Most alpha particles go straight through, some are

  scattered, some rebound off the gold foil.

  This shows that the nucleus of an atom has a very small

  radius. Most of the mass is concentrated in the nucleus.
• Why did Mendeleev leave gaps in the periodic table?

  He knew that the elements existed but they hadn’t been

  found, based on their mass.
• What happened to some of the gaps Mendeleev left?

  They have been filled. Scientists have found some of the

  elements.