Chemical Bonding and VSEPR 1

Chemical Bonding and VSEPR 1

The Shapes of Molecules The shape of a molecule has an important bearing on its reactivity and behavior. The shape of a molecule depends a number of factors. These include: 1. Atoms forming the bonds 2. Bond distance 3. Bond angles 2

Valence Shell Electron Pair Repulsion Valence Shell Electron Pair Repulsion (VSEPR) theory can be used to predict the geometric shapes of molecules. VSEPR is revolves around the principle that electrons repel each other. One can predict the shape of a molecule by finding a pattern where electron pairs are as far from each other as possible. 3

Bonding Electrons and Lone Pairs In a molecule some of the valence electrons are shared between atoms to form covalent bonds. These are called bonding electrons. Other valence electrons may not be shared with other atoms. These are called non-bonding electrons or they are often referred to as lone pairs. 4

VSEPR In all covalent molecules electrons will tend to stay as far away from each other as possible The shape of a molecule therefore depends on: 1. the number of regions of electron density it has on its central atom, 2. whether these are bonding or non-bonding electrons. 5

Lewis Dot Structures Lewis Dot structures are used to represent the valence electrons of atoms in covalent molecules Dots are used to represent only the valence electrons. Dots are written between symbols to represent bonding electrons 6

Writing Dot Structures Writing Dot structures is a process: 1. Determine the number of valence electrons each atom contributes to the structure 2. The number of valence electrons can usually be determined by the column in which the atom resides in the periodic table 7

Writing Dot Structures 3. Add up the total number of valence electrons 4. Adjust for charge if it is a poly atomic ion – Add electrons for negative charges – Reduce electrons for positive charges Example SO32 1 S 6e 3 0 6x3 18 e (2-) charge 2 e --------Total 26 e 8

Electron Dot Structures 5. Make the atom that is fewest in number the central atom. 6. Distribute the electrons so that all atoms have 8 electrons. 7. Use double or triple pairs if you are short of electrons 8. If you have extra electrons put them on the central atom 9

Electron Dot Structures Example 2: SO3 1 S 6e 3 O 6x3 18 e no charge 0e --------Total 24 e Note: a double bond is necessary to give all atoms 8 electrons 10

Lewis Dot Stucture for SO3 The diagram below shows the dot structure for sulfur trioxide. The bonding electrons are in shown in red and lone pairs are shown in blue. 11

Electron Dot Structures Example 3: NH4 1 N 5 e4 H 4x1 4 e( ) charge -1 e--------Total 8 e- Note: Hydrogen atoms only need 2 e- rather than 8 e12

Example: Carbon Dioxide 1. 2. 3. Central atom Valence electrons Form bonds. C O 4 e6 e- x 2 O’s 12 eTotal: 16 valence electrons This leaves 12 electrons (6 pairs). 4. Place lone pairs on outer atoms. 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.

Carbon Dioxide, CO2 C 4 e- How many are in the O 6 e- X 2 O’s 12 eTotal: 16 valence electrons drawing? There are too many electrons in our drawing. We need to take away 4 electrons and then form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 pairs. Take away the lone pairs from the carbon. So one pair is taken away from each oxygen atom and replaced with another bond.

Violations of the Octet Rule Violations of the octet rule usually occur with B and elements of higher periods. Some common examples include: Be, B, P, S, and Xe. Be: 4 BF33 B: 6 P: 8 OR 10 S: 8, 10, OR 12 Xe: 8, 10, OR 12 SF44

VSEPR Predicting Shapes

VSEPR: Predicting the shape Once the dot structure has been established, the shape of the molecule will follow one of basic shapes depending on: 1. The number of regions of electron density around the central atom 2. The number of regions of electron density that are occupied by bonding electrons 17

VSEPR: Predicting the shape The number of regions of electron density around the central atom determines the electron skeleton. The number of regions of electron density that are occupied by bonding electrons and hence other atoms determines the actual shape. 18

Basic Molecular shapes The most common shapes of molecules are shown at the right 19

Linear Molecules Linear molecules have only two regions of electron density. 20

Angular or Bent Angular or bent molecules have at least 3 regions of electron density, but only two are occupied 21

Triangular Plane Triangular planar molecules have three regions of electron density. All are occupied by other atoms 22

Tetrahedron Tetrahedral molecules have four regions of electron density. All are occupied by other atoms 23

Trigonal Bipyramid Some molecules have expanded valence shells around the central atom. In PCl5 there are five pairs of bonding electrons. The structure of such molecules with five pairs around one is called trigonal bipyramid. 24

Octahedron A few molecules have valence shells around the central atom that are expanded to as many as six pairs or twelve electrons. Sulfur hexafluoride, SF6 is and example These shapes are known as octahedrons 25

Molecular Polarity Molecular Polarity depends on: 1. the relative electronegativities of the atoms in the molecule 2. The shape of the molecule 3. Molecules that have symmetrical charge distributions are usually non-polar 26

Non-polar Molecules The electron density plot for H2. Two identical atoms do not have an electronegativity difference The charge distribution is symmetrical. The molecule is non-polar. 27

Polar Molecules The electron density plot for HCl Chlorine is more electronegative than Hydrogen The electron cloud is distorted toward Chlorine The unsymmetrical cloud has a dipole moment HCl is a polar molecule. 28

Molecular Polarity To be polar a molecule must: 1. Have polar bonds 2. Have these polar bonds arranged in such a way that their polarity is not cancelled out 3. When the charge distribution is nonsymmetrical, the electrons are pulled to one side of the molecule 4. The molecule is said to HF and H2O are both have a dipole moment polar molecules, but and therefore polar CCl4 is non-polar 29

Bond angles The angle formed between two peripheral . atoms and a central atom is known as a bond angle.

Bond Angles Bond angles are determined by the geometry of the electron skeleton. The number of regions of electron density determine the basic bond angle Skeleton Shape Linear Triangular Plane Tetrahedron Trigonal Bipyramid Electro Basic Bond n Angle Regions 2 180o 3 120o 4 109o 5 90o and 120o

Bond Angles and Lone Pairs When there are lone pairs present they tend to . repel slightly more. Hence the bond angles are slightly smaller. Methane Ammonia Water