Overview
This lecture covers the history and quantum nature of electrons, atomic orbitals, electron configurations, and their connection to elemental reactivity and the periodic table.
Early Periodic Table and Periodicity
- John Newlands proposed element periodicity similar to musical octaves before Mendeleev's table.
- His analogy, though dismissed, was later found to relate to real patterns in atomic structure.
Models of the Atom and Quantum Mechanics
- Niels Bohr created a model with electrons in fixed circular orbits with quantized energies.
- Electrons emit or absorb energy in specific amounts called "quanta."
- Bohr’s model only worked for hydrogen; electrons actually behave as both particles and waves.
Electron Waves and Orbitals
- Erwin Schrödinger modeled electrons as standing waves, leading to the concept of atomic orbitals.
- Electrons exist in orbitals, regions where there is a high probability of finding an electron.
- Orbitals are grouped into shells; each shell can hold more complex orbitals (s, p, d, f).
Orbital Types and Electron Configurations
- The first shell has one s-orbital (holds 2 electrons).
- The second shell adds three p-orbitals (each holds 2 electrons; 6 total).
- The third shell introduces five d-orbitals (holds 10 electrons), but 4s fills before 3d due to lower energy.
- Electron configuration notation: shell number, orbital type, number of electrons (e.g., 1s2 2s2 2p6).
The Octet Rule and Element Stability
- Atoms are most stable with 8 electrons in their outer shell (octet rule), like noble gases.
- Electron arrangements affect chemical reactivity and elemental properties.
Periodic Table Connections
- The periodic table's layout matches orbital filling: s-block (left), d-block (middle), p-block (right), and f-block (bottom).
- Electron configurations can be determined using the periodic table's structure.
Energetics: Ionization Energy and Electron Affinity
- Ionization energy is the energy needed to remove an electron from an atom.
- Electron affinity is the energy change when an electron is added to an atom.
- Both properties involve discrete energy changes tied to orbital structures.
Quantum Reality of Electrons
- Electrons are excitations in the electron field, existing as probability waves around the nucleus.
- Orbitals represent regions where electrons are likely to be found, not fixed paths.
Key Terms & Definitions
- Quanta — discrete packets of energy emitted or absorbed by electrons.
- Quantum Mechanics — the field describing the behavior of particles like electrons at atomic scales.
- Orbital — a region around the nucleus where an electron is likely to be found.
- Electron Configuration — a notation describing the arrangement of electrons in an atom’s orbitals.
- Ionization Energy — energy required to remove an electron from an atom.
- Electron Affinity — energy change when an electron is added to an atom.
- Octet Rule — tendency of atoms to have eight electrons in their outer shell for stability.
Action Items / Next Steps
- Practice writing electron configurations for elements using the periodic table.
- Review the order of orbital filling (remember the diagonal rule).
- Read about the quantum mechanical model and orbital shapes.