Electron Pair Geometry vs Molecular Geometry
The geometry of a molecule is important in determining its properties like color, magnetism, reactivity, polarity, etc. There are various methods of determining the geometry. There are many types of geometries. Linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, octahedral are some of the commonly seen geometries.
What is Molecular Geometry?
Molecular geometry is the three dimensional arrangement of atoms of a molecule in the space. Atoms are arranged in this way, to minimize the bond-bond repulsion, bond-lone pair repulsion and lone pair-lone pair repulsion. Molecules with the same number of atoms and electron lone pairs tend to accommodate the same geometry. Therefore, we can determine the geometry of a molecule by considering some rules. VSEPR theory is a model, which can be used to predict the molecular geometry of molecules, using the number of valence electron pairs. However, if the molecular geometry is determined by the VSEPR method, only the bonds should be taken into consideration, not the lone pairs. Experimentally the molecular geometry can be observed using various spectroscopic methods and diffraction methods.
What is Electron Pair Geometry?
In this method, the geometry of a molecule is predicted by the number of valence electrons pairs around the central atom. Valence shell electron pair repulsion or VSEPR theory predicts the molecular geometry by this method. To apply the VSEPR theory, we have to make some assumptions about the nature of bonding. In this method, it is assumed that the geometry of a molecule depends only upon electron- electron interactions. Further, following assumptions are made by the VSEPR method.
• Atoms in a molecule are bound together by electron pairs. These are called bonding pairs.
• Some atoms in a molecule may also possess pairs of electron not involved in bonding. These are called lone pairs.
• The bonding pairs and lone pairs around any atom in a molecule adopt positions where their mutual interactions are minimized.
• Lone pairs occupy more space than bonding pairs.
• Double bonds occupy more spaces than a single bond.
In order to determine the geometry, first the Lewis structure of the molecule has to be drawn. Then the number of valence electrons around the central atom should be determined. All single bonded groups are assigned as shared electron pair bond type. The coordination geometry is determined by the σ framework only. The central atom electrons that are involved in the π bonding should be subtracted. If there is an overall charge to the molecule, it should also be assigned to the central atom. The total number of electrons associated with the framework should be divided by 2, to give the number of σ electron pairs. Then depending on that number, geometry to the molecule can be assigned. Following are some of the common molecular geometries.
If the number of electron pairs is 2, geometry is linear.
Number of electron pairs: 3 Geometry: trigonal planar
Number of electron pairs: 4 Geometry: tetrahedral
Number of electron pairs: 5 Geometry: trigonal bipyramidal
Number of electron pairs: 6 Geometry: octahedral
What is the difference between Electron Pair and Molecular Geometries?
• When determining the electron pair geometry, lone pairs and bonds are considered and when determining molecular geometry only bonded atoms are considered.
• If there aren’t any lone pairs around the central atom, the molecular geometry is as same as the electron pair geometry. However, if there are any lone pairs involved both geometries are different.