Covalent Compounds: Bonding Theories and Molecular Structure

Transcription

1 CHM 123 Chapter 8 Covalent Compounds: Bonding Theories and Molecular Structure 8.1 Molecular shapes and VSEPR theory VSEPR theory proposes that the geometric arrangement of terminal atoms, or groups of atoms about a central atom in a covalent compound, or charged ion, is determined solely by the repulsions between electron pairs present in the valence shell of the central atom In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible, WHY? have the least amount of repulsion of the negatively charged electrons. have a geometry around the central atom that determines molecular shape. Electrons in bonds and in lone pairs can be thought of as charge clouds that repel one another and stay as far apart as possible, this causing molecules to assume specific shapes. Working from an electron-dot structure, count the number of charge clouds, and then determine the molecular shape. The following are the parent electronic structures upon which VSEPR is based. These structures show how to minimize the energy of the structure by placing 2, 3, 4, 5 or 6 electron groups (charge clouds) as far apart around a central atom as possible in three-dimensional space. Electrons in bonds and in lone pairs can be thought of as charge clouds that repel one another and stay as far apart as possible, this causing molecules to assume specific shapes. Working from an electron-dot structure, count the number of charge clouds, and then determine the electron geometry or shape. Dang1

2 Five electrons clouds Six electron clouds The Effect of Lone Pairs on the molecular shape lone pair groups occupy more space on the central atom o because their electron density is exclusively on the central atom rather than shared like bonding electron groups relative sizes of repulsive force interactions is: Lone Pair Lone Pair > Lone Pair Bonding Pair > Bonding Pair Bonding Pair this effects the bond angles, making them smaller than expected In each of these examples, the electron pairs are arranged tetrahedrally, and two or more atoms are bonded in these tetrahedral directions to give the different geometries. Lone pairs are absolutely critical part of the electronic structure (charge clouds) that contributes to the shape of the molecule, but only the attached atoms are included in deriving the shape name. Dang2

3 How many electron groups (charge clouds) are around the central atom in the following? Identify the geometry shape SO2 PCl5 The table below summarize the electronic geometry and ideal molecular shape of a molecule Total # of e- groups on central atom Parent electronic geometry # Bond ed atoms # Lone pairs Idealized molecular shape Idealized bond angles 2 Linear 2 0 Linear 180 o 3 Trigonal Planar 3 0 Trigonal Planar 120 o 3 Trigonal Planar 2 1 Bent 120 o 4 Tetrahedral 4 0 Tetrahedral o 4 Tetrahedral 3 1 Trigonal Pyramidal o 4 Tetrahedral 2 2 Bent o 5 Trigonal Bipyramidal 5 0 Trigonal Bipyramidal 90 o, 120 o, 180 o 5 Trigonal Bipyramidal 4 1 Seesaw 90 o, 120 o, 180 o 5 Trigonal Bipyramidal 3 2 T-shaped 90 o, 180 o 5 Trigonal Bipyramidal 2 3 Linear 180 o 6 Octahedral 6 0 Octahedral 90 o, 180 o 6 Octahedral 5 1 Square Pyramidal 90 o, 180 o 6 Octahedral 4 2 Square Planar 90 o, 180 o Dang3

4 Real bond angles vs. Idealized bond angles VSEPR predicts the idealized bond angle(s) by assuming that all electron groups take up the same amount of space. Since lone pairs are attracted to only one nucleus, they expand into space further than bonding pairs, which are attracted to two nuclei. As a result, real molecules that has lone pairs on the central atom often have bond angles that are slightly different than the idealized prediction. Central atom without lone pairs has the same real bond angle as the idealized angle. The exceptions to this are square planar shapes and linear (derived from trigonal bipyramidal electronic structure) shapes where the lone pairs offset one another, thus causing no deviation from ideality. Examples: Name the shape and give the idealized bond angles for the following Lewis structure Molecular Shape Idealized bond angle Real bond angle Dang4

5 Shapes of Larger Molecules no one particular central atom Many molecules don t have a central atom but many central atoms. These molecules don t fit into the shape names that we ve learned. However, we can give an approximated shape and bond angle to each central atom at a time. Example: Give the approximate molecular shape at the numbered central atoms Drawing 3-D structures In order to draw 3-D structure, chemists use dark wedges to indicate a bond projecting forward (out of the page) and dashed to indicate a bond going away (going back into the page) and a normal line to indicate a bond in the plane of the page Examples: Draw 3-D structure for the following molecules Dang5

6 Valence Bond Theory and Hybridization When a covalent bond is formed, there is shared electron density between the nuclei of the bonded atom The simultaneous attraction of the shared electron density for both nuclei holds the atoms together, forming a covalent bond Valence Bond Theory: A quantum mechanical model which shows how electron pairs are shared in a covalent bond. Bond forms between two atoms when the following conditions are met: Covalent bonds are formed by overlap of atomic orbitals, each of which contains one electron of opposite spin. Each of the bonded atoms maintains its own atomic orbitals, but the electron pair in the overlapping orbitals is shared by both atoms. The greater the amount of overlap, the stronger the bond. The Phase of an Orbital Orbitals are determined from mathematical wave functions. A wave function can have positive or negative values. As well as nodes where the wave function = 0 The sign of the wave function is called its phase. When orbitals interact, their wave functions may be in phase (same sign) or out of phase (opposite signs). In some cases, atoms use simple atomic orbital (e.g., 1s, 2s, 2p, etc.) to form bonds. In other case, they use a mixture of simple atomic orbitals known as hybrid atomic orbitals. sigma ( ) bonds Dang6

7 Two special names for covalent bonds of Organic molecules Sigma (σ) bonds Created when head on overlap occurs of orbital Pi (π) bonds Created when side on overlaps occurs of orbital (p orbitals) Pi bonds are usually weaker than sigma bon. From the perspective of quantum mechanics, this bond's weakness is explained by significantly less overlap between the component p-orbitals due to their parallel orientation. This is contrasted by sigma bond which form bonding orbitals directly between the nucleus of the bonding atoms, resulting in greater overlap and a strong sigma bond Hybrid orbital orbitals are used to describe bonding that is obtained by taking combinations of atomic orbitals of the isolated atoms. 1. When forming hybrid orbitals, the number of hybrid orbitals formed equals the number of orbitals mathematically combined or mixed. For example, if an s orbital is combined with a p orbital the result is two sp hybrid orbitals. 2. Hybrid orbitals have orientations around the central atom that correspond to the electron-domain geometry predicted by the VSEPR Theory Dang7

8 How can the bonding in CH4 be explained? sp 3 Hybridization of Carbon - Mixing 1s and all 3p atomic orbital - Four sp 3 hybridized orbitals equal in size, energy and shape - Responsible for sigma bond ( single bond) - Tetrahedral shape Consider methane (CH4) The sp 2 Hybridization of Carbon - Mixing 1s and 2p atomic orbitals - Three sp 2 hybridized orbitals equal in size, energy and shape - One 2p unhybridized orbital - Responsible for σ bond ( single bond) - One π bond (double bond) - Trigonal planar shape Dang 8

9 3-D representation of ethane (C2H4) The sp Hybridization of Carbon - Mixing 1s and 1p atomic orbitals - Two sp hybridized orbitals equal in size, energy and shape - Two 2p unhybridized orbital - one σ bond ( single bond) - Two π bond (triple bond) - Linear Consider ethyne (C2H2) 3-D representation of ethyne (C2H2) Dang 9

10 Five types of hybrid are shown below # of e- groups around central atom Hybrid orbitals used Orientation of Hybrid Orbitals 2 sp 3 sp 2 4 sp 3 5 sp 3 d 6 sp 3 d 2 Bonding to O and N Like Carbon, O and N can participate in single bond and multiple bonds compose of σ and π *Note: the lone pair or non bonding e- pair occupies space just as bonded atom Examples: 1. Predict the hybridization, geometry, and bond angle for the carbon, oxygen and nitrogen atoms in the following molecules Dang 10

11 2. What hybrid orbitals would be expected for the central atom in each of the following? a) SF2 b) BrF3 Dang 11