The Arrangement of Electrons in Atoms: Key Points from Niels Bohr's Studies

Spectra and Continuous Spectrum

• Niels Bohr: First to shed light on electron arrangement in atoms by studying spectra
• Continuous Spectrum:
  • Contains many colors/wavelengths with no gaps
  • Example: Rainbows, mnemonic - "Richard Of York Gave Battle In Vain"
  • Produced by white light through a prism

Emission Line Spectrum

• Bohr's Discovery:
  • Conducted experiments with hydrogen gas in a discharge tube
  • Electricity caused hydrogen gas to emit light; viewed through prism
  • Discovered a series of colored lines, termed emission line spectrum
• Unique Line Emission Spectra:
  • Each element produces unique line spectra
  • Studied using a spectrometer

Bohr's Theory and Energy Levels

• Electrons orbit nucleus in fixed paths called orbits (energy levels)
• Orbits have quantized energy (fixed amount)
• Energy Level Definition: Fixed energy value of electrons in an atom
• Energy levels represented by n:
  • 1st level: n = 1
  • 2nd level: n = 2
  • 3rd level: n = 3

Electron Excitement and Photon Emission

• Ground State: Lowest available energy level for electrons
• Excited State: Electron gains energy (e.g., heat, electricity)
  • Moves to higher energy level (excited state)
  • Unstable, electron falls back to original level, emitting a photon of light
• Formula: E2 - E1 = h * f
• Series of Electron Transitions:
  • Lyman Series: Electrons fall back to n = 1
  • Balmer Series: Electrons fall back to n = 2 (visible light)
  • Paschen Series: Electrons fall back to n = 3 (invisible to naked eye)*

Atomic Absorption Spectrometry (AAS)

• Atoms absorb light; seen as missing lines in continuous spectrum
• Corresponds to wavelengths detected in emission line spectrum
• Applications:
  • Detecting gunpowder particles in forensics
  • Detecting harmful metals in blood
  • Quality control of water and food products

Energy Sublevels

• Single energy levels are made of multiple closely spaced lines called sublevels
• Number of sublevels corresponds to main energy level (n)
  • E.g., n = 1 has 1 sublevel, n = 3 has 3 sublevels
• Sublevel Definition: Subdivision of the main energy level; one or more orbitals of the same energy

Wave-Particle Duality and Heisenberg's Principles

• Louis de Broglie (1924): Electrons move in wave motion
• Led to Heisenberg's Uncertainty Principle: Impossible to measure both velocity and position of electron simultaneously
• Orbital Definition: Region around nucleus with high probability of finding an electron
• Schrödinger's Contributions:
  • Calculated probability of electron location
  • s Orbitals: Spherical in shape
  • p Orbitals: Dumbbell shaped (X, Y, Z axis)
  • Each orbital holds max 2 electrons