A-level Chemistry/OCR (Salters)/Molecular geometry

molecular geometry The shapes of molecules is the title of Section 3.3 in Chemical Ideas and it covers the topic of molecular geometry. Introductory examples Methane molecule Ammonia molecule Water molecule Hydrogen fluoride molecule Polyhedra Tetrahedra You have probably come across tetrahedra before in maths, although you most likely called them triangle-based pyramids. Tetrahedra have four vertices (corners), four faces and six edges. Each face is an equilateral triangle. The tetrahedron is one of the most important shapes in chemistry because a very great many molecules contain them. Tetrahedral molecules don't actually contain little pyramids. What they do contain is a central atom bonded to four other atoms. The four atoms surrounding the central atom occupy positions that you can imagine as the vertices of a tetrahedron. In the image gallery below, the central atom is coloured magenta and the surrounding atoms are coloured white. The angle between any two bonds in a tetrahedral molecule is approximately 109.5°. The tetrahedral angle can be calculated as accurately as required because it is equal to cos−1(–⅓). Octahedra You may or may not have met an octahedron before. Octahedra have six vertices (corners), eight faces and twelve edges. Each face is an equilateral triangle. Octahedra are very important in chemistry because many transition metal-based molecules are octahedral. Common molecular geometries Further examples Example molecules Example ions Molecular geometry and lone pairs You can use the so-called AXE method to calculate the shape of a molecule. It is based on molecules that have a central atom, which we label A. Atoms or groups bonded to A are labelled X. Lone pairs are labelled E. A molecule with three lone pairs and two atoms/groups bonded to it would be denoted AX2E3. The table below shows how X and E and molecular shape are related. Valence shell electron pair repulsion theory (VSEPR) is used to predict the shape of a molecule once X and E are known. This sounds more complicated than it is. You consider any X's and E's to be regions of charge that position themselves as far apart from each other as possible, in order to minimize the forces of electrostatic repulsion between each other. AXE label X(substituents) E(lone pairs) Shape 2D diagramlone pairs shown 2D diagram lone pairs not shown 3D modellone pairs shown 3D modellone pairs not shown Examples AX1E0 1 0 Linear 100px 100px 100px 100px H2 AX1E1 1 1 Linear 100px 100px 100px 100px CN− AX1E2 1 2 Linear 100px 100px 100px 100px O2 AX1E3 1 3 Linear 100px 100px 100px 100px HCl AX2E0 2 0 Linear 100px 100px 100px 100px BeCl2HgCl2CO2 AX2E1 2 1 Bent 100px 100px 100px 100px NO2−SO2O3 AX2E2 2 2 Bent 100px 100px 100px 100px H2OH2SOF2 AX2E3 2 3 Linear 100px 100px 100px 100px XeF2I3− AX3E0 3 0 Trigonal planar 100px 100px 100px 100px BF3CO32−NO3−SO3 AX3E1 3 1 Trigonal pyramidal 100px 100px 100px 100px NH3PCl3 AX3E2 3 2 T-shaped 100px 100px 100px 100px ClF3BrF3 AX4E0 4 0 Tetrahedral 100px 100px 100px 100px CH4NH4+PO43−SO42−ClO4− AX4E1 4 1 Seesaw 100px 100px 100px 100px SF4 AX4E2 4 2 Square Planar 100px 100px 100px 100px XeF4 AX5E0 5 0 Trigonal Bipyramidal 100px 100px 100px 100px PCl5 AX5E1 5 1 Square pyramidal 100px 100px 100px 100px ClF5BrF5 AX5E2 5 2 Pentagonal planar 100px 100px 100px XeF5- AX6E0 6 0 Octahedral 100px 100px 100px 100px SF6 AX6E1 6 1 Pentagonal pyramidal 100px 100px 100px 100px IF6- AX7E0 7 0 Pentagonal bipyramidal 100px 100px 100px 100px IF7 AX8E0 8 0 Square antiprismatic 100px 100px IF8- AX8E1 8 1 Distorted square antiprismatic 100px XeF82- AX9E0 9 0 Tricapped trigonal prismatic OR capped square antiprismatic 100px 100px ReH92- (tricapped trigonal prismatic)