4.1.10 Changes in the Atomic Model

Alpha-Scattering Experiment

Historical Context

• Ancient Greeks:
  • Believed everything is made of atoms.
  • Atoms are tiny spheres that cannot be divided.
• 1897: Discovery of Electrons
  • Scientists discovered that atoms contain tiny negative particles called electrons.
  • This showed that atoms are not spheres that cannot be divided and have an internal structure.

Plum-Pudding Model

• Suggested that an atom is a ball of positive charge with negative electrons embedded in it.
• The alpha-scattering experiment was carried out to test if this model was correct.

Alpha-Scattering Experiment

• Setup:
  • Scientists took a piece of gold foil, which can be hammered into a very thin foil, just a few atoms thick.
  • They then fired tiny alpha particles at the gold foil. Alpha particles have a positive charge.
• Observations:
  • Most alpha particles passed straight through the gold foil without changing direction.
  • Some alpha particles were deflected (changed direction as they passed through).
  • Sometimes an alpha particle bounced straight back off the gold foil.

• Impact:
  • Scientists replaced the plum-pudding model with the nuclear model.

The Nuclear Model

The First Nuclear Model

• Components:
• Negative electrons
• Positive nucleus
• Empty space
• Description:
• Most of the atom is simply empty space.

Niels Bohr's Model

• Electrons:
  • Electrons orbit the nucleus at specific distances.
  • These orbits are now called energy levels or shells.

Updated Nuclear Model

• Components:
• Positive nucleus
• Electron energy levels (shells)

Protons and Neutrons

• Protons:
• The positive charge in the nucleus is due to tiny positive particles called protons.
• Neutrons:
• Discovered by James Chadwick; the nucleus also contains neutral particles called neutrons.

Atomic Structure

• Atoms have no overall charge because the number of electrons is the same as the number of protons.
• The radius of an atom:
  • $0.1 nm (1X10^-10m)$
• The radius of the nucleus:
  • $1\times 10^{-14} \text{m}$

Subatomic Particles

Particle	Relative Charge	Relative Mass
Proton	+1	1
Neutron	0	1
Electron	-1	Very small