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Introduction. This section explores how we predict the molecular and electron-pair shapes of molecules using the VSEPR (Valence Shell Electron Pair Repulsion) theory. We will first go over what VSEPR theory is and how it defines an electron-pair geometry and a molecular geometry. Then we will go over the steps for determining the electron-pair ...
May 20, 2018 · This shape is called bent or angular. A molecule with four electron groups about the central atom orients the four groups in the direction of a tetrahedron, as shown in Figure 10.7.1 10.7. 1 Tetrahedral Geometry. If there are four atoms attached to these electron groups, then the molecular shape is also tetrahedral.
Apr 13, 2023 · Figure 11.7.2 11.7. 2 (left to right): A three-dimensional rendering of CH 4, a tetrahedron, and the three-dimensional structure of CH 4 drawn inside a tetrahedron. The oxygen atom in a water molecule is still surrounded by four electron groups. It's just that two of those groups are lone pairs and those lone pairs still have repulsions and ...
H 2 O is an example of a molecule whose central atom has four electron groups, but only two of them are bonded to surrounding atoms. Figure 4.8.8 4.8. 8: Water bonding. Although the electron groups are oriented in the shape of a tetrahedron, the shape of the molecule is bent 109° or angular.
- VSEPR Theory
- Linear Molecules According to VSEPR Theory
- Trigonal Planar Molecules According to VSEPR Theory
- Tetrahedral Molecules According to VSEPR Theory
- Pyramidal Molecules According to VSEPR Theory
- Bent Or Angular Molecules According to VSEPR Theory
- Summary: Shapes of Molecules
- Predicting Shapes of Molecules According to VSEPR Theory
– The Lewis structure of a molecule tells us the number of pairs of electrons in the valence shell of the central atom. – These electron pairs are subject to electrostatic attraction between them. – On this basis, R.G. Gillespie (1970) proposed a theory called the Valence-Shell Electron Pair Repulsion or VSEPR (pronounced as ‘Vesper’) theory. – VSE...
(a) Beryllium chloride, BeCl2
– It has the Lewis structure: – The central atom Be has two bonding electron pairs and no unshared electron. – According to VSEPR theory, the bonding pairs will occupy positions on opposite sides of Be forming an angle of 180º. – An angle of 180º gives a straight line. Therefore, BeCl2molecule is linear. – In general, all molecules as A–B–A which have only two bonds and no unshared electrons are linear.
(b) Carbon dioxide, CO2
– It has the following structure: – The central C atom has no unshared electron. – We know that a double bond counts the same as a single bond in the VSEPR model. – Thus CO2is a linear molecule. – Similarly, it can be shown that hydrogen cyanide (H – C ≡ N) and acetylene (H – C ≡ C – H) are linear molecules.
(a) Boron trifluoride, BF3
– Its Lewis structure shown that the central atom B has three bonding electron pairs and no unshared electrons. – VSEPR theory says that the three bonding electron pairs will be as far apart as possible. – This can be so if these electron pairs are directed to the corners of an equilateral triangle. – Thus VSEPR model of BF3molecule has three F atoms at the corners of the triangle with B atom at its centre. – All four atoms (three F and one B) lie in the same plane. – Therefore, the shape of...
(b) Sulphur trioxide, SO3
– In the Lewis structure of SO3 – The central S atom is joined with two O atoms by covalent bonds. – The third O atom is joined with S by a double bond. – But a double bond is counted as a single electron pair for the purpose of the VSEPR model. – Therefore, in effect, S has three electron pairs around it. – Thus like BF3, SO3has trigonal planar geometry.
(a) Methane, CH4
– The Lewis structureof methane shows that the central C atom has four bonding electron pairs. – These electron pairs repel each other and are thus directed to the four corners of a regular tetrahedron. – A regular tetrahedron is a solid figure with four faces which are equilateral triangles. – All bond angles are 109.5º. – Similarly, CCl4in which the central C atom is bonded to four other atoms by covalent bonds has tetrahedral shape.
(b) Ammonium ion, NH4+, and Sulphate ion SO4-2
– The N atom in NH4+ and S atom in SO4-2have four electron pairs in the valence shell. – These are directed to the corners of a tetrahedron for maximum separation from each other. – Thus both NH4+ and SO4-2 have tetrahedral shape.
(a) Ammonia molecule, NH3
– The Lewis structure of NH3shows that the central N atom has three bonding electrons and one lone electron pair. – The VSEPR theory says that these electron pairs are directed to the corners of a tetrahedron. – Thus we predict that H–N–H bond angle should be 109.5º. – But the shape of a molecule is determined by the arrangement of atoms and not the unshared electrons. – Thus, if we see only at the atoms, we can visualise NH3molecule as a pyramid with the N atom located at the apex and H atom...
(b) Phosphorus trichloride, PCl3
– The structural formula indicates that the central phosphorus atom has three bonding electron pairs and one lone electron pair. – Thus, like NH3it has a pyramidal shape and the observed bond angle Cl–P–Cl is 100º.
(a) Water, H2O
– In the structural formula of H2O, the O atom is bonded to two H atoms by covalent bonds and has two lone pairs. – Thus O is surrounded by two bonding electron pairs and two unshared electron pairs. – VSEPR theory says that to secure maximum separation between them, the four electron pairs are directed to the corners of a tetrahedron. – If we look at the three atoms (and ignore the unshared pairs), the atoms HOH lie in the same plane and the predicted bond angle is 109.5º. – But with two uns...
(b) Sulphur dioxide, SO2
– The Lewis structure of SO2is given below. – The S atom is bonded to one O by a double bond and to the other O by a single bond. – It has an unshared electron pair. – In the VSEPR model a double bond is counted as a single electron pair. – That way, the S atom is surrounded by three electron pairs, two bonding pairs and one unshared pair. – For maximum separation, the three electron pairs are directed to the corners of an equilateral triangle. – The predicted bond angle is 120º. – But with t...
– The directional nature of covalent bonds is shown in the diagrams of molecules above. – The shape of the methane molecule is tetrahedral because the four bonding pairs of electrons repel each other equally, and the equilibrium position of all four bonding electron pairs is tetrahedral.
– It is possible to work out the shape of a small molecule that has the formula XYnby applying a few simple rules. – We will use ammonia as an example to illustrate the idea.
The shapes of these molecules can be predicted from their Lewis structures, however, with a model developed about 30 years ago, known as the valence-shell electron-pair repulsion (VSEPR) theory. The VSEPR theory assumes that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of ...
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3.6 Predicting Molecular Shape. Utilising VSEPR theory, predict the geometries of molecules through the presence of lone-pairs and electron-dense groups. Determine the polarity of a molecule, considering bond types and geometry. You may have noticed that when drawing or representing molecules, we sometimes draw them with unexpected geometry.
