4.1.1 Atomic Structure

Subatomic Particle	Relative Mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron	0 (0.0005)	-1

Typical radius of an atom: $1 × 10^−10$ metres

• And the radius of the nucleus is 10,000 times smaller

• Most (nearly all) of the mass of the atom is concentrated in the nucleus Electron Arrangement:

• Electrons lie at different distances from the nucleus (different energy levels). The electron arrangements may change with the interaction with EM radiation.

Atomic Structure: Developing the Model of the Atom

Historical Development

• Greeks:
  • Atoms are tiny spheres which cannot be divided.
• Thomson's Plum Pudding Model:
  • An atom is a ball of positive charge with negative electrons embedded in it.

1909 - Rutherford's Alpha Scattering Experiment

• Fired a beam of alpha particles (positively charged) at a thin sheet of gold foil.
  • Most particles went straight through → Atom is mostly empty space.
  • Some were deflected → The Center of the atom must have a positive charge, repelling particles that come close to it.
  • Some bounced straight back → Mass is concentrated in the centre.
  • This research led to the development of the first nuclear model.

Bohr's Model

• Electrons orbit the nucleus at specific distances.
  • Orbits are called energy levels or shells.

Updated Nuclear Model

• Positive Nucleus: Contains protons and neutrons.
• Electron Energy Levels: Electrons occupy energy levels or shells around the nucleus.

James Chadwick

• Proved the existence of the neutron which explained the imbalance between atomic and mass numbers.

Current Model of the Atom

• Nucleus: Contains protons and neutrons.
• Electrons: Occupy energy levels or shells around the nucleus.

Atomic Structure and Energy Levels

Subatomic Particles

• Protons (p)
  • Positive charge
• Neutrons (n)
  • Neutral charge
• Electrons (e)
  • Negative charge
• Overall Charge
  • Atoms have no overall charge.

Atomic Representation

• Atomic Number (p, e):
  • Number of protons and electrons in the atom.
• Atomic Mass (p + n):
  • The sum of protons and neutrons in the atom.
• Symbol:
  • Element symbol (e.g., Li for Lithium).

Energy Levels

• Higher Energy Levels:
  • Energy levels further from the nucleus are at a higher energy.
• Absorption and Emission of Electromagnetic Radiation:
  • If the atom absorbs electromagnetic radiation, an electron can move from a lower energy level to a higher energy level.
  • The atom can then emit electromagnetic radiation, causing the electron to return to the lower energy level.