Molecular Structure, Bonding, and Hybridization: Study Notes

Molecular Structure, Bonding, and Hybridization

Electron Configurations and Bond Order in Diatomic Molecules

Understanding the electron configuration and bond order of diatomic molecules is fundamental in general chemistry. The bond order provides insight into the stability and strength of a bond between two atoms.

• Electron Configuration: The arrangement of electrons in molecular orbitals for a molecule or ion. For homonuclear diatomic molecules, electrons fill molecular orbitals in order of increasing energy.

• Bond Order: Calculated as:

$\text{Bond Order} = \frac{1}{2}[(\text{number of bonding electrons}) - (\text{number of antibonding electrons})]$

• Example: For $O_2$, the electron configuration is $(\sigma_{1s})^2(\sigma_{1s}^*)^2(\sigma_{2s})^2(\sigma_{2s}^*)^2(\pi_{2p_x})^2(\pi_{2p_y})^2(\sigma_{2p_z})^2(\pi_{2p_x}^*)^1(\pi_{2p_y}^*)^1$.

• Application: Bond order predicts magnetic properties: molecules with unpaired electrons are paramagnetic, while those with all electrons paired are diamagnetic.

Molecular Orbital Diagram for BeN and Magnetic Properties

Molecular orbital (MO) diagrams help visualize the distribution of electrons in molecules and predict properties such as bond order and magnetism.

• MO Diagram: Shows the relative energy levels of molecular orbitals formed from atomic orbitals.

• Bond Order Calculation: Use the MO diagram to count bonding and antibonding electrons.

• Paramagnetic vs. Diamagnetic:

  • Paramagnetic: Molecules with one or more unpaired electrons.

  • Diamagnetic: Molecules with all electrons paired.

• Example: If BeN has an odd number of electrons or unpaired electrons in its MO diagram, it is paramagnetic.

Lewis Structures and Resonance

Lewis structures represent the arrangement of atoms and electrons in a molecule. Resonance structures depict delocalization of electrons within molecules where more than one valid structure exists.

• Lewis Structure: Shows all valence electrons as dots or lines (bonds) around atoms.

• Resonance: When more than one valid Lewis structure can be drawn, the actual structure is a hybrid of these forms.

• Example: The carbonate ion ($CO_3^{2-}$) has three resonance structures, each with a double bond to a different oxygen atom.

Steric Number and Hybridization

The steric number determines the electron geometry around a central atom and is used to predict hybridization.

• Steric Number: The sum of the number of atoms bonded to a central atom and the number of lone pairs on the central atom.

• Hybridization: The mixing of atomic orbitals to form new hybrid orbitals suitable for bonding.

• Common Hybridizations:

  • sp: Linear geometry, steric number 2

  • sp2: Trigonal planar geometry, steric number 3

  • sp3: Tetrahedral geometry, steric number 4

• Example: In $SF_4$, sulfur has a steric number of 5 (four bonded atoms and one lone pair), leading to sp3d hybridization.

Hybrid Orbitals and Multiple Bonding

Hybrid orbitals explain the observed shapes of molecules and the formation of sigma and pi bonds in multiple bonding scenarios.

• Promotion and Hybridization: Atoms may promote electrons to higher energy orbitals before hybridizing to maximize bonding.

• Sigma (σ) Bonds: Formed by head-on overlap of orbitals (e.g., sp, sp2, sp3).

• Pi (π) Bonds: Formed by side-on overlap of unhybridized p orbitals.

• Example: In $CO_3^{2-}$, carbon is sp2 hybridized, forming three sigma bonds and one delocalized pi bond over the three oxygens.

Table: Hybridization and Molecular Geometry

Steric Number	Hybridization	Geometry	Example
2	sp	Linear	BeCl2
3	sp2	Trigonal planar	BF3
4	sp3	Tetrahedral	CH4
5	sp3d	Trigonal bipyramidal	PF5
6	sp3d2	Octahedral	SF6

Summary of Key Concepts

• Electron configurations and bond order are essential for predicting molecular stability and magnetism.

• Lewis structures and resonance illustrate electron delocalization in molecules.

• Steric number and hybridization determine molecular geometry and bonding types.

• Hybrid orbitals explain the formation of sigma and pi bonds in multiple bonding scenarios.