Overview
This lecture explores the nature of electrons, atomic orbitals, and electron configurations, highlighting their role in periodic trends and chemical reactivity.
Early Theories and Periodicity
- John Newlands compared periodicity of elements to musical scales, suggesting repetition in elemental properties.
- Mendeleev developed the periodic table, but did not understand the role of electrons in periodicity.
- Early models imagined electrons as particles orbiting a nucleus like planets.
Development of Quantum Theory
- Niels Bohr proposed electrons exist in quantized energy levels around the nucleus (Bohr model).
- Bohr's model worked for hydrogen but failed for more complex atoms.
- Electrons exhibit wave-particle duality; better described as standing waves, not simple particles.
- Erwin Schrodinger developed a mathematical model treating electrons as standing waves.
Atomic Orbitals and Shells
- Electrons occupy specific regions called orbitals, grouped in shells.
- The first shell has one s-orbital (holds 2 electrons).
- The second shell adds p-orbitals (3 orientations, holds 6 electrons) and another s-orbital.
- The octet rule states atoms are most stable with 8 electrons in their outer shell.
- The third shell introduces d-orbitals (5 types, holds 10 electrons), but these are filled after the 4s orbital due to energy considerations.
- f-orbitals (7 types, 14 electrons) are even more shielded, filled in larger atoms.
Electron Configuration Notation
- Electron configurations are written as: shell number + orbital letter + number of electrons (e.g., 1s2, 2s2, 2p6).
- Electrons fill orbitals in order of increasing energy, following a specific sequence (e.g., 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.).
- Some elements have irregular configurations due to energy differences between orbitals.
Chemical Reactivity and Periodic Trends
- Ionization energy is the energy needed to remove an electron from an atom.
- Removing electrons proceeds stepwise; outermost electrons are removed first and require less energy.
- Electron affinity is the energy change when an atom gains an electron.
- Elements with filled s and p orbitals (noble gases) are most stable (low reactivity).
Orbitals, Reality, and Existence
- Electrons exist as excitations (waves) in a universal electron field.
- Orbitals represent regions of high probability for finding an electron.
- The shape and filling of orbitals explain chemical properties and the structure of matter.
Key Terms & Definitions
- Electron — A subatomic particle with a negative charge, found in orbitals around the nucleus.
- Orbital — A region around the nucleus where an electron is likely to be found.
- Quantum Mechanics — The branch of physics describing atomic and subatomic systems.
- Standing Wave — A wave that remains fixed in space, used to model electrons in atoms.
- Electron Configuration — The arrangement of electrons in an atom's orbitals.
- Ionization Energy — The energy required to remove an electron from an atom.
- Electron Affinity — The energy change when an atom gains an electron.
- Octet Rule — Atoms tend to have eight electrons in their outermost shell for stability.
- Wave-Particle Duality — The concept that electrons exhibit both wave-like and particle-like properties.
Action Items / Next Steps
- Practice writing electron configurations for elements.
- Review the periodic table to connect orbital filling patterns with element placement.
- Study how ionization energy and electron affinity relate to chemical reactivity.