IB DP Chemistry: HL复习笔记14.1.3 Further VSEPR Theory

• When an atom forms a covalent bond with another atom, the electrons in the different bonds and the non-bonding electrons in the outer shell all behave as negatively charged clouds and repel each other
• In order to minimise this repulsion, all the outer shell electrons spread out as far apart in space as possible
• Molecular shapes and the angles between bonds can be predicted by the valence shell electron pair repulsion theory known by the abbreviation VSEPR theory
• VSEPR theory consists of three basic rules:

1. 1. All electron pairs and all lone pairs arrange themselves as far apart in space as is possible.
   2. Lone pairs repel more strongly than bonding pairs
   3. Multiple bonds behave like single bonds

• These three rules can be used to predict the shape of any covalent molecule or ion, and the angles between the bonds
• The regions of negative cloud charge are known as domains and can have one, two or three pairs electrons

Molecular geometry versus domain geometry

• It is important to distinguish between molecular geometry and domain geometry in exam questions
  • Molecular geometry refers to the shape of the molecules based on the relative orientation of the atoms
  • Domain geometry refers to the relative orientation of all the bonding and lone pairs of electrons
• The Lewis structure for water enables us to see that there are four electron pairs around the oxygen so the domain geometry is tetrahedral
• However, the molecular geometry shows us there are two angled bonds so the shape is bent, angular, bent linear or V-shaped (when viewed upside down)

Diagram showing the Lewis structure of water and molecular shape from which the domain and molecular geometries may be determined