Summary for Ground-State Electron Configurations and Related Topics

Tips for Determining Ground State Electron Configurations

1. Follow the Aufbau Principle: Electrons fill orbitals in order of increasing energy (e.g., 1s → 2s → 2p → 3s, etc.).

2. Apply Pauli’s Exclusion Principle: Each orbital can hold a maximum of two electrons with opposite spins.

3. Use Hund’s Rule: Within the same subshell, electrons occupy separate orbitals with parallel spins before pairing up.

4. Electron Configuration Notation: Write the subshells with the number of electrons in superscripts, e.g., 1s22s22p6\text{1s}^2 \text{2s}^2 \text{2p}^61s22s22p6.

3.1.4 Ground-State Configurations of Monatomic Ions

• Cations: Remove electrons starting with the outermost shell. For transition metals, remove from 4s4s4s before 3d3d3d.

  • Example: Fe→[Ar]3d64s2\text{Fe} \to \text{[Ar]} 3d^6 4s^2Fe→[Ar]3d64s2 → Fe2+→[Ar]3d6\text{Fe}^{2+} \to \text{[Ar]} 3d^6Fe2+→[Ar]3d6.

• Anions: Add electrons to the next available orbital.

  • Example: O→[He]2s22p4\text{O} \to \text{[He]} 2s^2 2p^4O→[He]2s22p4 → O2−→[He]2s22p6\text{O}^{2-} \to \text{[He]} 2s^2 2p^6O2−→[He]2s22p6.

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3.2 Effective Nuclear Charge (ZeffZ_\text{eff}Zeff​)

• Definition: The net positive charge experienced by an electron.

• Trend:

  • Increases across a period: More protons are added, pulling electrons closer.

  • Slightly decreases down a group: Increased shielding reduces ZeffZ_\text{eff}Zeff​.

3.3 Atomic and Ionic Size

• Atomic Size:

  • Decreases across a period: Higher ZeffZ_\text{eff}Zeff​ pulls electrons closer.

  • Increases down a group: Additional shells increase the size.

• Ionic Size:

  • Cations: Smaller than their parent atoms (loss of outer electrons reduces repulsion).

  • Anions: Larger than their parent atoms (addition of electrons increases repulsion).

3.4 Ionization Energy (IE)

• Definition: Energy required to remove an electron from an atom in the gas phase.

• Trend:

  • Increases across a period: ZeffZ_\text{eff}Zeff​ increases, making electrons harder to remove.

  • Decreases down a group: Outer electrons are further from the nucleus and less tightly bound.

• Key Points:

  • Successive IEs increase for the same element.

  • Large jumps in IE occur after removing all valence electrons.

3.5 Electron Affinity (EA)

• Definition: Energy change when an electron is added to a neutral atom in the gas phase.

• Trend:

  • Generally becomes more negative (more favorable) across a period.

  • Becomes less negative down a group due to increased distance and shielding.

• Exceptions: Noble gases and elements with half-filled or filled subshells (e.g., nitrogen) have lower electron affinities.

3.6 Electronegativity

• Definition: The ability of an atom to attract shared electrons in a bond.

• Trend:

  • Increases across a period: Higher ZeffZ_\text{eff}Zeff​.

  • Decreases down a group: Larger size weakens the pull on shared electrons.

• Scale: Fluorine is the most electronegative element (χ=4.0\chi = 4.0χ=4.0).

3.7 Photoelectron Spectroscopy (PES)

• Definition: A technique used to determine the binding energy of electrons in atoms or molecules.

• Key Points:

  • Binding energy is the energy required to remove an electron from an atom.

  • Higher peaks in a PES spectrum indicate more electrons in that energy level.

  • The x-axis represents binding energy, and the y-axis represents relative intensity.

• Applications:

  • Confirming electron configurations.

  • Identifying differences in ZeffZ_\text{eff}Zeff​ between elements.