Structures and Properties of Substances. Introducing Valence-Shell Electron- Pair Repulsion (VSEPR) Theory

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1 Structures and Properties of Substances Introducing Valence-Shell Electron- Pair Repulsion (VSEPR) Theory

2 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

3 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.

4 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.

5 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.

6 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.

7 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 This angle corresponds to the most favorable arrangement of electron groups to minimize the forces of repulsion among them.

8 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 to 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

9 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 The result is the bent shape around an oxygen atom with 2 LP and two single bonds

10 Common Molecular Shapes Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements Number of electron groups Geometric arrangement of electron groups Type of electron pairs VSEPR notation Name of Molecular shape Example 2 3 linear trigonal planar 2 BP A 2 A BeF 2 linear 3 BP A 3 BF 3 A trigonal planar 3 trigonal planar 2 BP, 1 LP A 2 E SnCl 2 A angular 4 tetrahedral 4 BP A 4 CF 4 A tetrahedral 4 tetrahedral 3 BP, 1LP A 3 E PCl 3 A trigonal pyramidal 4 tetrahedral 2 BP, 2LP A 2 E 2 H 2 S A angular

11 tetrahedral Common Molecular Shapes 4 2 BP, 2LP A 2 E 2 H 2 S A 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 notation Name of Molecular angular shape Example 25 linear trigonal 25 BP A 25 A BeF SbCl 25 bipyramidal linear 3 trigonal planar 3 BP A 3 A BF 3 A trigonal planar trigonal bipyramidal 3 trigonal planar 2 BP, 1 LP A 2 E SnCl 2 5 trigonal 4 BP, 1LP A 4 E TeCl 4 bipyramidal A A angular 4 tetrahedral 4 BP A 4 CF 4 seesaw A 5 trigonal 3 BP, 2LP A 3 E 2 BrF 3 bipyramidal 182 MHR Unit 2 Structure and Properties A tetrahedral 4 tetrahedral 3 BP, 1LP A 3 E T-shaped PCl 3 5 trigonal bipyramidal 2 BP, 3LP A 2 E 3 A ef 2 trigonal linear pyramidal 4 tetrahedral 2 BP, 2LP A 2 E 2 H 2 S 6 octahedral 6 BP A 6 SF 6 A A A angular

12 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.

13 Sample Problem Problem Sample Determine the molecular shape of the hydronium ion, H 3 O +. Determine the molecular shape of the hydronium ion, H3O Plan Your Strategy + 1.Plan Your Strategy Follow the four-step procedure that helps to predict molecular shape. Follow the four-step Use procedure Table 4.2 for that names helps of to the predict electron-group molecular arrangements shape. Use the and Common molecular shapes. molecular shapes table on the previous pages for names of the electron-group arrangements and molecular shapes. Act on Your Strategy 2.Act on Your Strategy Step 1: A possible Step Lewis 1 A structure possible for Lewis H3O structure + is: for H 3 O + is: H + H O H Step 2: The Lewis structure Step 2 The shows Lewis 3 structure BPs and shows 1 LP. That 3 BPs is, there and 1 are LP. That a total is, of there four are a electron groups around the central O atom. total of four electron groups around the central O atom. Step 3: The geometric arrangement of the electron groups is tetrahedral. Step 4: For 3 BP and Step 13 LP, The the geometric molecular arrangement shape is trigonal of the pyramidal. electron groups is tetrahedral. Step 4 For 3 BPs and 1 LP, the molecular shape is trigonal pyramidal. This molecular shape corresponds to the VSEPR notation for this ion, A3E. Check Your Solution This molecular shape corresponds to the VSEPR notation for this ion, A 3 E.

14 Practice Problem Use VSEPR theory to predict the molecular shape for each of the following: (a) HCN (b) SO2 (c) SO3 (d) SO4 2- Use VSEPR theory to predict the molecular shape for each of the following: (a) CH2F2 (b) NH4 + (c) BF4 - Use VSEPR theory to predict the molecular shapes of NO2 + and NO2 -.

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