4.4.1.3 The Development of the Model of the Atom

Question 1

What may lead to a scientific model being changed?

Answer 1

new experimental evidence

Question 2

What was an atom thought to be before the electron was discovered by John Dalton 1880?

Answer 2

thought to be tiny solid spheres that could not be divided

Question 3

What did the disovery of the electron lead to?

Answer 3

• 1897 - J.J Thomson discovered the electron and the plum pudding model was formed
• the model suggested that teh atom is a ball of charge which negative electrons embedded in it to cancel out the positive charge and make the atom neutral
• spread of mass and charge throughout atom

Question 4

What was the alpha particle scattering experiment?

Answer 4

• 1911 - carried out by Ernest Rutherford
• positively charged alpha particles directed towards thin gold sheet and expected most of them to go through
• most of the alpha particles did pass through the gold foil - so most of the atom is empty space
• some alpha particles were slightly deflected (greater than expected) - so nucleus must be a positively charged core deflecting the positive alpha particles
• few alpha particle were deflected by 90 degrees - so most of the mass of an atom was concentrated in the centre in the nucleus

Question 5

How did Niels Bohr change the model of an atom?

Answer 5

• 1913 - adapted the nucleur model by suggesting that electrons orbit the nucleus at specific distances
• the theoretical calculations of Bohr agreed with experimental observations
• if RUtherford was right, the electrons in the cloud close to the nucleus would get attracted and cause the atom to collapse and release energy as they spiralled into the centre of the atom - atoms don’t release this sort of energy
  
  • now the electrons are in fixed orbitals

Question 6

How was the proton discovered?

Answer 6

• later experiments led to the idea that the positive charge of any nucleus could be subdivide into a whole number of smaller particles - each particle having the same amount of positive charge
• 20 years after the nucleus became an accepted scientific idea

Question 7

How did James Chadwick change the model of an atom?

Answer 7

• experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus
• about 20 years after the nucleus became an accepted scientifc idea

Question 8

What led to the plum pudding of the model being replaced by the nuclear model of the atom?

Answer 8

• plum pudding model was replaced due to the alpha particle scattering experiment carried out by Ernest Rutherford
• plum pudding model stated that an atom was a positive ball of charge containing electrons embedded in it
• alpha particle experiment - alpha particles (positive) directed at gold foil (Rutherford expected alpha particles to pass straight through with them being deflected by a few fractions of a degrees at most)
  
  • most alpha particles passed through - shows most of an atom is empty space
  • some alpha particles deflected at small and large angles showing how most of an atom was concentrated in the centre of the atom and that it had a positive core (aka nucleus) as the alpha particles were deflected due to being repelled
• this disproved plum pudding model causing it to be replaced by nuclear model as the plum pudding model shows that mass and charge was spread throughout the atom which goes against results of alpha particle experiment