Molecular Shapes and VSEPR Theory

Molecular Shapes

• Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, determined by the repulsive forces between electron pairs surrounding the central atom.
• Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes.
• VSEPR theory predicts that electron pairs (bonding and lone pairs) arrange themselves as far apart as possible to minimize repulsion.
• Molecular shapes are classified by the number of bonding and lone pairs around the central atom.
• Common molecular shapes include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.
• The difference between predicted and observed shapes arises from the stronger repulsive forces exerted by lone pairs compared to bonding pairs. Lone pairs are more concentrated on a single atom.
• Electron geometry describes the arrangement of all electron pairs (bonding and lone) around a central atom, while molecular geometry considers only the positions of atoms.
• These shapes are crucial in determining properties like polarity, reactivity, and intermolecular forces.
• Examples include:
  • Linear (CO2): two bonding pairs, 180° bond angle.
  • Trigonal planar (BF3): three bonding pairs, 120° bond angle.
  • Tetrahedral (CH4): four bonding pairs, 109.5° bond angle.
  • Trigonal bipyramidal (PCl5): five bonding pairs, 90° and 120° bond angles.
  • Octahedral (SF6): six bonding pairs, 90° bond angles.
• Factors affecting molecular shape include the number of bonding and lone pairs, and electronegativity differences.

Bonding Ions

• Ionic bonds form when a metal and nonmetal react, resulting in the transfer of electrons.
• Metals lose electrons (cations) and nonmetals gain electrons (anions).
• The electrostatic attraction between oppositely charged ions (cations and anions) creates the ionic bond.
• Ionic compounds are typically solid at room temperature, with high melting and boiling points. The strength depends on the charges of the ions and their size. Larger charges and smaller sizes lead to stronger bonds.
• Ionic compounds arrange into crystal lattices.
• Lattice energy is the energy needed to separate ions in a crystal lattice. The size and charges of ions determine the crystal structure.
• Ionic bonding is a significant force in geological processes, forming minerals such as NaCl (halite) and MgO.
• Ionic compounds tend to be hard and brittle, and soluble in polar solvents like water. They conduct electricity when molten or dissolved. This is because the ions are free to move.
• Examples include NaCl, MgO, and KCl.

Relationship between Molecular Shape and Bonding Ions

• Ionic bonds involve the strong electrostatic attraction between oppositely charged ions.
• Molecular shapes are due to the arrangement of atoms in covalent bonds (electron repulsion).
• Ionic compounds form extended crystal lattices, not discrete molecules. The shape is defined by the geometry of the ion arrangement.
• Intermolecular forces that influence molecular shapes play some part in holding ions together, but electrostatic interactions are dominant in ionic compounds.
• The charge and size of ions significantly impact melting and boiling points of ionic compounds.