1.3.2 The discovery of the electron, nucleus, and proton

• What did early atomic models attempt to describe?

  What atoms might be like
• How many oxygen atoms are in the molecule shown?

  1
• What was missing in the early attempts to describe atoms?

  Experiments that could show atomic structure
• How does the total mass of the water molecule compare to the individual masses of the atoms that make it up?

  • The total mass of the water molecule (18 mass units) is greater than the mass of the individual oxygen atom (16 mass units) and the 2 hydrogen atoms (2 mass units).
  • This is because the atoms bond together to form the water molecule, resulting in a higher total mass.
• How do the sizes of electrons compare to atoms?

  Electrons are much smaller than atoms
• What is the purpose of the positively and negatively charged metal plates in the image?

  • The positively charged metal plate attracts electrons
  • The negatively charged metal plate repels electrons
  • Together they create an electric field that can accelerate charged particles
• What characteristic do all cathode ray particles share?

  All particles are identical, regardless of metal
• How are cathode ray particles affected by electric and magnetic fields?

  They are deflected by electric and magnetic fields
• What is the name of the particle shown flowing through the device?

  α-particle
• What did Thomson's cathode ray experiment demonstrate about cathode rays?

  Cathode rays are made of negatively charged particles
• How does the detector in the image work to detect the α-particles?

  • The detector measures the deflection of the α-particles as they pass through the thin gold foil, providing information about their charge and mass.
• What insights can you gain about the structure and composition of the water molecule from the information provided?

  • The water molecule contains 1 oxygen atom and 2 hydrogen atoms
  • The oxygen atom has a mass of 16 mass units
  • Each hydrogen atom has a mass of 1 mass unit
  • The total mass of the water molecule is 18 mass units
• What are the components of the device shown in the image?

  • Po Source
  • Vacuum pump
  • Unknown radiation
  • Protons
  • Amplifier
  • Oscillograph
• What were early atomic models lacking?

  Details about electrons, nucleus, and protons
• Why were early atomic models considered basic?

  They lacked detailed scientific knowledge
• Why are neutrons significant in atomic structure?

  They explain why atoms have more mass
• What was the purpose of early atomic models?

  To describe what atoms might be like
• Who discovered the atom?

  Democritus
• What characterized the early atomic models?

  They were very basic and simplistic
• How could this setup be used to accelerate charged particles?

  • Charged particles (e.g. electrons) would be attracted to the positively charged plate
  • This would accelerate the particles through the electric field between the plates
  • The accelerated particles could then be used for various applications like particle physics experiments
• What are the main postulates of Dalton's atomic theory?

  1. All matter is made of indestructible atoms.
  2. Atoms of the same element are identical.
  3. Atoms of different elements have different masses.
  4. Atoms can be combined in fixed ratios.
• How many hydrogen atoms are in the molecule shown?

  2
• What are the key features of Rutherford's atomic model?

  • Developed in 1912
  • Describes the atom as a nuclear model
  • Contains a dense nucleus with orbiting electrons
• What model succeeded Dalton's, Thomson's, and Rutherford's atomic models?

  The modern quantum cloud model
• What are the key features of Thomson's atomic model?

  • Developed in 1897
  • Describes the atom as a plum pudding
  • Electrons are embedded in positive dough
• What are neutrons?

  Uncharged particles in the nucleus
• What is the name of the radiation shown in the image?

  Unknown radiation
• What does it mean that atoms are indestructible according to Dalton's theory?

  Atoms cannot be created or destroyed in reactions
• What is the purpose of the thin gold foil shown in the image?

  • The thin gold foil is used to allow the α-particles to pass through and be detected by the device.
• What is the name of the positively charged metal plate in the image?

  positively charged metal plate
• How can you calculate the total mass of a water molecule given the individual masses of the atoms?

  • Add the mass of the oxygen atom (16 mass units)
  • Add the mass of the 2 hydrogen atoms (2 mass units)
  • The total mass of the water molecule is 18 mass units
• How many mass units do the 2 hydrogen atoms have in total?

  2
• What did Chadwick show about the penetrating radiation he detected?

  It contained particles with mass similar to protons
• If the radius of the circular path of the α-particles is $r$, what is the formula to calculate the centripetal force acting on the α-particle?


  $F_c =$$\frac{mv^2}{r}$
• How does Dalton's atomic theory explain water formation?

  • 16 mass units of oxygen
  • 2 mass units of hydrogen
  • Combine to form 18 mass units of water (H₂O)
• What is the name of the device shown in the image?

  Detector
• What type of particles did Rutherford fire at gold foil?

  Alpha particles
• How do neutrons contribute to atomic mass?

  They add mass beyond protons and electrons
• What experimental setup did Chadwick use to discover neutrons?

  Fired alpha particles at beryllium
• What are the three major atomic models?

  Dalton's, Thomson's, and Rutherford's