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Structures and Properties of Substances Structures and Properties of Substances Introducing Valence-Shell Electron- Pair Repulsion (VSEPR) Theory The VSEPR theory In 1957, the chemist Ronald Gillespie and Ronald Nyholm, developed a model for predicting the shape of molecules. This model is usually abbreviated to VSEPR (pronounced “vesper”) theory: Valence Shell Electron Pair Repulsion The fundamental principle of the VSEPR theory is that the bonding pairs (BP) and lone pairs (LP) of electrons in the valence level of an atom repel one another. Thus, the orbital for each electron pair is positioned as far from the other orbitals as possible in order to achieve the lowest possible unstable structure. The effect of this positioning minimizes the forces of repulsion between electron pairs. A The VSEPR theory The repulsion is greatest between lone pairs (LP-LP). Bonding pairs (BP) are more localized between the atomic nuclei, so they spread out less than lone pairs. Therefore, the BP-BP repulsions are smaller than the LP-LP repulsions. The repulsion between a bond pair and a lone-pair (BP-LP) is intermediate between the other two. In other words, in terms of decreasing repulsion: LP-LP > LP-BP > BP-BP The tetrahedral shape around a single-bonded carbon atom (e.g. in CH4), the planar shape around a carbon atom with two double bond (e.g. in CO2), and the bent shape around an oxygen atom in H2O result from repulsions between lone pairs and/or bonding pairs of electrons. The VSEPR theory The repulsion is greatest between lone pairs (LP-LP). Bonding pairs (BP) are more localized between the atomic nuclei, so they spread out less than lone pairs. Therefore, the BP-BP repulsions are smaller than the LP-LP repulsions. The repulsion between a bond pair and a lone-pair (BP-LP) is intermediate between the other two. In other words, in terms of decreasing repulsion: LP-LP > LP-BP > BP-BP The tetrahedral shape around a single-bonded carbon atom (e.g. in CH4), the planar shape around a carbon atom with two double bond (e.g. in CO2), and the bent shape around an oxygen atom in H2O result from repulsions between lone pairs and/or bonding pairs of electrons. Geometry of the molecules and the VSEPR theory The figure below shows the five basic geometrical arrangements that result from the interactions of lone pairs and bonding pairs around a central atom. These arrangements involve up to six electron groups. An electron group is usually one of the following: • a single bond • a double bond • a triple bond • a lone pair When all the electron groups are BP, a molecule will have one of those five geometrical arrangements. If one (or more) of the electron groups are LP, variations in the geometric arrangements result. Geometry of the molecules Each of the molecules in the following pages below has four pairs of electrons around the central atom. Observe the differences in the number of bonding and lone pairs in these molecules. Methane, CH4, has 4 BP. Ammonia, NH3, has 3 BP and 1 LP. Water, H2O, has 2 BP and 2 LP. These differences have an effect on the shapes and bond angles of the molecules. Geometry of the molecules Methane with four BP, has a tetrahedral molecular shape. The angle between any two bonding pairs in the tetrahedral electron-group arrangement is 109.5°. This angle corresponds to the most favorable arrangement of electron groups to minimize the forces of repulsion among them. Geometry of the molecules Ammonia When there are 1 LP and 3 BP around a central atom, there are two types of repulsions: LP-BP and BP-BP Since LP-BP repulsions are greater than BP-BP repulsions, the bond angle between the bond pairs in NH3 is reduced from 109.5 ̊ to 107.8 . ̊ When you draw the shape of a trigonal pyramidal molecule, without the lone pair, you can see that the three bonds form the shape of a pyramid with a triangular base Geometry of the molecules Water In a molecule of H2O, there are two BP and two LP. The strong LP-LP repulsions, in addition to the LP-BP repulsions, cause the angle between the bonding pairs to be reduced further to 104.5 . ̊ The result is the bent shape around an oxygen atom with 2 LP and two single bonds Common Molecular Shapes Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements Number of Geometric arrangement Type of electron groups of electron groups electron pairs VSEPR notationName of Molecular shape Example 2 linear 2 BP AX2 X A X BeF2 linear 3 trigonal planar 3 BP AX3 X BF3 X A X trigonal planar 3trigonal planar 2 BP, 1 LP AX E SnCl 2 • 2 • X A X angular 4tetrahedral 4 BP AX4 X CF4 A X X X tetrahedral 4tetrahedral 3 BP, 1LP AX3E• • PCl3 A XX X trigonal pyramidal 4tetrahedral 2 BP, 2LP AX2E2 H2S • • A • • X X angular 5trigonal 5 BP AX5 SbCl5 bipyramidal X X X A X X trigonal bipyramidal 5trigonal 4 BP, 1LP AX4EX TeCl4 bipyramidal X • • A X X seesaw 182 MHR • Unit 2 Structure and Properties Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements Number of Geometric arrangement Type of electron groups of electron groups electron pairs VSEPR notationName of Molecular shape Example 2 linear 2 BP AX2 X A X BeF2 linear 3 trigonal planar 3 BP AX3 X BF3 X A X trigonal planar 3trigonal planar 2 BP, 1 LP AX E SnCl 2 • 2 • X A X angular 4tetrahedral 4 BP AX4 X CF4 A X X X tetrahedral 4tetrahedral 3 BP, 1LP AX3E• • PCl3 A XX X trigonal pyramidal 4tetrahedral 2 BP, 2LP AX2E2 H2S Common Molecular Shapes • • A Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements • • X X Number of Geometric arrangement Type of electron groups of electron groups electron pairs VSEPR notationName of Molecularangular shape Example 25lineartrigonal 2 5 BP AX25 X A X BeFSbCl25 X bipyramidal linear X 3 trigonal planar 3 BP AX3 X A X BF3 X A X X X trigonal planar trigonal bipyramidal 3trigonal planar 2 BP, 1 LP AX E SnCl 2 • 2 5trigonal 4 BP, 1LP AX4EX • TeCl4 bipyramidal X A X • • A X angular X 4tetrahedral 4 BP AX4 XX CF4 seesaw A X 5 trigonal 3 BP, 2LP AX3E2 X • BrF3 bipyramidal X • 182 MHR • Unit 2 Structure and Properties X X A • tetrahedral• X 4tetrahedral 3 BP, 1LP AX3ET-shaped• • PCl3 5 trigonal 2 BP, 3LP AX2E3 XA • XeF2 • bipyramidal •• XXA • • X X trigonallinear pyramidal 4tetrahedral 2 BP, 2LP AX2E2 H2S •X• 6octahedral 6 BP AX6 SF6 A X • X • X A X X X angular 5trigonal 5 BP AX5 X SbCl5 bipyramidal octahedralX X 6octahedral 5 BP, 1LP AX5EX BrF5 X A X X X A X X X trigonal bipyramidal•• square pyramidal 5trigonal 4 BP, 1LP AX4EX TeCl4 bipyramidal 6octahedral 4 BP, 2LP AX E •• X XeF 4 2 • 4 • A X X A X X X X•• seesaw square planar 182PredictingMHR • Unit Molecular 2 Structure Shape and Properties You can use the steps below to help you predict the shape of a molecule (or polyatomic ion) that has one central atom. Refer to these steps as you work through the Sample Problems and the Practice Problems that follow. 1. Draw a preliminary Lewis structure of the molecule based on the formula given. 2. Determine the total number of electron groups around the central atom (bonding pairs, lone pairs and, where applicable, account for the charge on the ion). Remember that a double bond or a triple bond is counted as one electron group. 3. Determine which one of the five geometric arrangements will accommodate this total number of electron groups. 4. Determine the molecular shape from the positions occupied by the bonding pairs and lone pairs. Chapter 4 Structures and Properties of Substances • MHR 183 Predicting Molecular Shape It is possible to use the steps below to predict the shape of a molecule (or polyatomic ion) that has one central atom. 1.Draw a preliminary Lewis structure of the molecule based on the formula given. 2.Determine the total number of electron groups around the central atom (bonding pairs, lone pairs and, where applicable, account for the charge on the ion). Remember that a double bond or a triple bond is counted as one electron group. 3.Determine which one of the five geometric arrangements will accommodate this total number of electron groups. 4.Determine the molecular shape from the positions occupied by the bonding pairs and lone pairs. Sample Problem Predicting Molecular Shape for a Simpler Compound Problem Sample Problem + Determine the molecular shape of the hydronium ion, H3O . Determine the molecular shape of the hydronium ion, H O+ Plan Your Strategy 3 1.Plan Your StrategyFollow the four-step procedure that helps to predict molecular shape. Follow the four-stepUse procedure Table 4.2 forthat names helps ofto the predict electron-group molecular arrangementsshape. Use the and “Common molecular shapes” tablemolecular on the shapes. previous pages for names of the electron-group arrangements and molecular shapes. Act on Your Strategy 2.Act on Your Strategy Step 1 + + Step 1: A possible LewisA structure possible forLewis H3 Ostructure is: for H3O is: H + H OH •• Step 2: The Lewis structure shows 3 BPs and 1 LP. That is, there are a total of four Step 2 The Lewis structure shows 3 BPs and 1 LP. That is, there are a electron groups around the central O atom.
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