7. Shapes of Covalent Molecules.pdf docs
- 1. The shape and bond angle (angle between 2 bonds) of covalent compounds depends on: • The number of electron pairs around the central atom • Whether the electron pairs are bonding electrons or lone pairs From this comes the VSEPR theory Valence shell electron pair repulsion (VSEPR) theory The VSEPR theory helps us to determine the shapes of covalent molecules through the following statements: • Electrons are negatively charged; hence they will repel each other and be as far away as possible from each other to minimize repulsion. • Electrons will not repel each other with the same strength. They follow the following trend Order of repulsion strength: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair To determine the shape of a molecule using the VSEPR theory, follow the guidelines below: 1. Determine the total number of electrons (Valence + shared) found surrounding the central atom. (Dot-and-cross diagram might be useful) 2. Find the number of electron pairs by dividing the total number of electrons by two. This is the valence shell electron pair number (VSEP / steric number) 3. Determine how many pairs is/are bond pairs and lone pairs. (A single, double or triple bond is counted as one bond pair) 4. Refer to the nice table on the next page (Knowing the table by heart or brain would be awesome BTW) to obtain the shape of the molecule. VSEP Number Shape of the Molecule 1 Linear 2 Linear 3 Trigonal Planar / Bent (non-linear) 4 Tetrahedral / Trigonal pyramidal / Bent (non-linear)/ Linear 5 Trigonal Bipyramidal / See saw / T -shaped / Linear 6 Octahedral / Square pyramidal / Square planar 7 Pentagonal Bipyramidal
- 2. VSEP / steric number Number of atoms bonded to the central atom Number of lone pairs Electron geometry Molecular shape Hybridisation Bond angle Example Representation 1 1 0 Linear Linear s 180° H2 2 2 0 Linear Linear sp 180° CO2 1 1 Linear Linear CN- 3 3 0 Trigonal planar Trigonal planar sp2 120° AlBr3 2 1 Trigonal planar Non-linear (bent) 119° SO2 1 2 Trigonal planar Linear - O2 4 4 0 Tetrahedral Tetrahedral sp3 109.5° CH4 3 1 Tetrahedral pyramidal 107° NH3 2 2 Tetrahedral Non-linear (bent) 104.5° H2O 1 3 Tetrahedral Linear - Cl2 5 5 0 Trigonal bipyramidal Trigonal bipyramidal sp3d 90°, 120°, 180° PF5 4 1 Trigonal bipyramidal See saw 173.1° and 101.6° SeH4 3 2 Trigonal bipyramidal T-shape 87.5° and <180° ICl3 2 3 Trigonal bipyramidal Linear 180° BrF2 6 6 0 Octahedral Octahedral sp3d2 90° SF6
- 3. 5 1 Octahedral Square pyramidal 81.9° IF5 4 2 Octahedral Square planar 90° XeF4 Effect of lone pair on bond angle Lone pairs cause a stronger repulsion. Comparing methane and ammonia, we see that they both have 4 pairs of electrons. They must have a tetrahedral electron geometry. Technically, the bond angle was supposed to be 109.5° but the lone pair will push more strongly on the bond pair making the angle smaller (107°). Likewise with water, the oxygen also has 4 pairs of electrons, still tetrahedral electron geometry but again the angle is not 109.5° due to the lone pairs and as it has 2 lone pairs, the push is even stronger, making the angle 104.5°. For the exams (according to the syllabus), only the following ones will be assessed. • BF3 (trigonal planar, 120°) • CO2 (linear, 180°) • CH4 (tetrahedral, 109.5°) • NH3 (pyramidal, 107°) • H2O (non-linear, 104.5°) • SF6 (octahedral, 90°) • PF5 (trigonal bipyramidal, 120° and 90°) This doesn’t mean that you will only have those 7 molecules. You could be asked a question about, for example, Carbon tetrachloride, CCl4, which is analogous to methane, CH4 and thus CCl4 has bond angle 109.5° and is tetrahedral in shape with sp3 hybridisation. Drawings The molecules represented above were drawn in 2D while in fact they are 3D molecules. To account for this, we devised a special way of representing bonds when they will not fall on the same plane. Consider for example H2O We know that water has a non-linear V-shape as follows: The ones in blue in the table
- 4. All the atoms of the water molecule can be placed on the same plane. Hence, it can be drawn as Some molecules don’t have atoms who will all fit on the same plane. Consider CH4. It is a tetrahedral compound. In 3D it looks like this: All the atoms do not fall on the same plane. Only 3 of them do: the carbon and 2 hydrogens.
- 5. You will be left with 2 hydrogen atoms out of plane, one going into the plane and one going out of the plane. To represent those two, we will use to represent “into the plane” and to represent “out of the plane”. Hence, instead of drawing methane as , it will be drawn as to show that it is in 3D and not on the same plane. The other 2 hydrogen atoms are on the same plane, so they do not change. Into the plane Out of the plane