16 Questions
What is the approximate radius of an atom according to J.J. Thomson's model?
10^(-10) m
What is the shape of an atom according to J.J. Thomson's model?
Spherical
What is the distribution of positive charge in an atom according to J.J. Thomson's model?
Uniformly distributed
What is the mass of an atom assumed to be in J.J. Thomson's model?
Uniformly distributed
What did J.J. Thomson win the Nobel Prize for in 1906?
Theoretical and experimental investigations on the conduction of electricity by gases
What was the outcome of Rutherford's alpha-particle scattering experiment?
Most alpha particles passed through the gold foil undeflected
What was the purpose of the zinc sulphide screen in Rutherford's experiment?
To produce a flash of light when alpha particles struck it
What was the expectation of alpha particles passing through the gold foil according to Thomson's model?
They would slow down and change direction by small angles
What was the observation that led Rutherford to conclude that the positive charge of the atom is not spread throughout the atom?
A few α–particles bounced back, deflected by nearly 180°.
What is the ratio of the radius of the nucleus to the radius of the atom?
10^(-5)
What is the force that holds the electrons and the nucleus together?
Electrostatic force of attraction
What is the name given to the very small region of the atom where the positive charge and most of the mass are concentrated?
Nucleus
What is the shape of the paths in which electrons move around the nucleus?
Circular
What was the model proposed by Rutherford to describe the structure of the atom?
Nuclear model
What is the approximate size of the radius of the atom compared to a cricket ball representing the nucleus?
5 km
What was the observation that led Rutherford to conclude that most of the space in the atom is empty?
Most of the α–particles passed through the foil undeflected.
Study Notes
Thomson's Model of the Atom
- In 1898, J.J. Thomson proposed a spherical atom model with a uniformly distributed positive charge (radius approximately 10^(-10) m).
- Electrons are embedded in the sphere to create a stable electrostatic arrangement.
- The model is also known as the plum pudding, raisin pudding, or watermelon model.
Rutherford's Experiment
- Rutherford, Geiger, and Marsden bombarded thin gold foil with alpha particles in 1909.
- A stream of high-energy alpha particles from a radioactive source was directed at a thin gold foil (thickness ~ 100 nm).
- The results of the scattering experiment were unexpected:
- Most alpha particles passed through the gold foil undeflected.
- A small fraction of alpha particles was deflected by small angles.
- A very few alpha particles (~ 1 in 20,000) bounced back, deflected by nearly 180°.
Rutherford's Conclusions
- Most of the space in the atom is empty, as most alpha particles passed through the foil undeflected.
- The positive charge of the atom is concentrated in a very small volume, which repelled and deflected positively charged alpha particles.
- The volume occupied by the nucleus is negligibly small compared to the total volume of the atom.
Rutherford's Nuclear Model of the Atom
- The positive charge and most of the mass of the atom are densely concentrated in an extremely small region called the nucleus.
- The nucleus is surrounded by electrons that move around it with high speed in circular paths called orbits.
- Electrons and the nucleus are held together by electrostatic forces of attraction.
- The nucleus plays the role of the sun, and the electrons revolve around it like planets in the solar system.
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