2.2

• Proton


  Symbol: p

  Relative charge = +1

  Relative mass = 1
• Neutron


  Symbol: n

  Relative charge = 0

  (Neutral)

  Relative mass = 1
• Electron


  Symbol: e

  Relative charge = –1

  Relative mass = 1/1840
• The subatomic particles found in an atom which is made up of protons, neutrons and electrons.
• Proton and neutrons are located at the nucleus of the atom. 
• • Electrons move around the nucleus at a high speed. 
• The mass of the atom is concentrated at the centre of the atom. 
• Studies on atoms started since the introduction of atomic theory by

  Democritus, a Greek philosopher, around 500 B.C.
• John Dalton


  • All matter is composed of extremely small particles called atoms.
  • All atoms are the same for one kind of element.
  • Atom can not be created or divided into smaller particles.
  • In a chemical reaction, atoms are separated, combined, or rearranged.
• J.J Thomson


  • An atom consists of small negatively charged particles called electrons. 
  • The atom is a positively charged spherical cloud with electrons positioned uniformly throughout the atom.
• Ernest Rutherford


  • Proton, which is a positively charged particle, is discovered. 
  • Proton and most of the mass of the atom is located at the nucleus, which is the centre of the atom. 
  • The atom is mostly empty space. 
• Neils Bohr


  • Electrons move around the nucleus in their fixed shells.
  • The shells formed are circles with a fixed radius from the nucleus. 
  • Electrons at each shell have their own fixed energy level. 
• James Chadwick


  • Discovered the presence of the neutrons in the nucleus through an experiment where the beryllium-9 atom is bombarded with alpha particles. 
  • Proposed that half of the mass of an atom is contributed by the neutron. 
• James Chadwick

  
• Ernest Rutherford

  
• Neils Bohr

  
• J.J Thomson

  
• John Dalton

  
• Dalton's Atomic Model

  
• Thomson's Atomic Model

  A) Electron

  B) Positively-charged sphere

  
• Rutherford's Atomic Model

  A) Electrons move outside the nucleus

  B) Nucleus consists of protons

  
• Bohr's Atomic Model

  A) Nucleus that contains protons

  B) Shell

  C) electron

  
• Chadwick's Atomic Model

  A) Nucleus that contains protons and neutrons

  B) shell

  C) Electron