Shapes of molecules

 The shapes of molecules and ions can be described by valence shell electron pair repulsion  VSEPR predicts the shapes and bond angles of simple molecules

It’s all about the pairs  All of the electrons in an ion or molecule are in organised into pairs  There are two types of electron pairs:  Bonding pairs  (the two shared electrons in a covalent bond)  Lone pairs  (two electrons in a pair not involved in bonding – also known as non-bonding pairs).  Electron pairs will repel each other as far as possible.  Like charges repel  This causes the shape of any molecule or ion

It’s all about the pairs  Chemists sometimes use lines to show bonds becomes H — Cl  Draw out the following molecules just using a line to show the bonds  O 2, H 2 O, CH 4, NH 3, SF 6, BF 3 and PF 5 XOXO ClH X XXXX  What is the problem when you draw these molecules onto a piece of paper?

Beryllium Fluoride Simple rules to work it out: 1. Draw a stick diagram. (displayed formula technically!) 2. Add in any lone pairs onto the stick diagram. 3. Count the total number of bonds AND lone pairs to work out the shape. LINEAR  2 Bonding pairs  0 Lone pairs  The furthest these pairs of electrons can spread apart is 180 o XOXO OXOX F Be F O OOOO OOOO

Boron Trifluoride Simple rules to work it out: 1. Draw a stick diagram. (displayed formula technically!) 2. Add in any lone pairs onto the stick diagram. 3. Count the total number of bonds AND lone pairs to work out the shape. TRIGONAL PLANAR  3 Bonding pairs  0 Lone pairs  The furthest these pairs of electrons can spread apart is 120 o provides the greatest distance EVEN IN 3-dimensional space XOXO OXOX F B F O OOOO OOOO X O O F OOOO OOOO

Further examples TETRAHEDRAL  4 Bonding pairs  0 Lone pairs HEXAGONAL  6 Bonding pairs  0 Lone pairs

Double and triple bonds TETRAHEDRAL  4 bonding pairs (even though one is double)  0 Lone pairs  The furthest these pairs of electrons can spread apart is o Double and triple bonds do not effect the shape (well not enough that we have to worry!) This phosphate ion PO 4 3- has a tetrahedral shape

5 Bonding pairs  5 bonding pairs and 0 lone pairs is called:  TRIGONAL BI-PYRAMIDAL It is a combination of the 1 st and 2 nd examples: linear and trigonal planar So what would the TWO different bond angles be?

Practice to fluency  Decide the shape and bond angle(s) of the following molecules on the right  Prepare an explanation for the rest of the class explaining the shape and bond  The fact that some of these molecules are ions is irrelevant to the shape! CO2 NO3 - PF5CH4 SO4 2 - SF6

Lone pairs vs. bonding pairs  Lone pairs are more compact as they are closer to the nucleus than bonding pairs  This causes lone pairs to provide more repulsion LP-LP The greatest repulsion is between two lone pairs LP-BP Followed by the repulsion between one lone pair and one bonding pair BP-BP The weakest repulsion is between two bonding pairs

Ammonia PYRAMIDAL  Ammonia has 3 bonding pairs and 1 lone pair  With 4 pairs in total it is similar to tetrahedral  However the LP repels the bonding pairs and reduces the bond angle by 2.5 o from o to 107 o

Water NON-LINEAR (BENT)  Water has 2 bonding pairs and 2 lone pair  With 4 pairs in total it is similar to tetrahedral  However the TWO LP’s repels the bonding pairs and reduces the bond angle by 2.5 o and 2.5 o from o to o

3D diagrams  To draw 3D diagrams on 2D paper chemists draw the bonds slightly differently  Solid wedges show a bond that comes ‘up’ from the paper or screen  (towards you)  Hashed wedges show a bond that goes ‘down’ from the paper  (away from you) methane TETRAHEDRAL

XY 6 Bond angles? “Real” shape? Name? Examples?

XY 5 Bond angles? “Real” shape? Name? Examples?

XY 4 Bond angles? “Real” shape? Name? Examples?

XY 3 Bond angles? “Real” shape? Name? Examples?

XY 2 Bond angles? “Real” shape? Name? Examples?

RECAP Molecules, or ions, possessing ONLY BOND PAIRS of electrons fit into a set of standard shapes. All the bond pair-bond pair repulsions are equal. All you need to do is to count up the number of bond pairs and chose one of the following examples... C 2LINEAR 180ºBeCl 2 3TRIGONAL PLANAR 120ºAlCl 3 4TETRAHEDRAL 109.5ºCH 4 5TRIGONAL BIPYRAMIDAL 90º & 120º PCl 5 6OCTAHEDRAL 90ºSF 6 BOND PAIRS SHAPE ANGLE(S)EXAMPLE A covalent bond will repel another covalent bond

IRREGULAR SHAPES If a molecule, or ion, has lone pairs on the central atom, the shapes are slightly distorted away from the regular shapes. This is because of the extra repulsion caused by the lone pairs. BOND PAIR - BOND PAIR < LONE PAIR - BOND PAIR < LONE PAIR - LONE PAIR O O O As a result of the extra repulsion, bond angles tend to be slightly less as the bonds are squeezed together.

Practice to fluency  Decide the shape and bond angle(s) of the following molecules on the right  For each example include:  Name  Bond angle(s)  3-D diagram  The number of bonding pairs  The number of lone pairs AlCl3 BeCl2 CH4 NH4 +

BERYLLIUM CHLORIDE Cl Be Cl Beryllium - has two electrons to pair up Chlorine - needs 1 electron for ‘octet’ Two covalent bonds are formed Beryllium still has an incomplete shell

BERYLLIUM CHLORIDE Cl Be 180° BOND PAIRS2 LONE PAIRS0 BOND ANGLE... SHAPE ° LINEAR Cl Be Cl Beryllium - has two electrons to pair up Chlorine - needs 1 electron for ‘octet’ Two covalent bonds are formed Beryllium still has an incomplete shell

Al ALUMINIUM CHLORIDE Cl Al 120° Cl Al Cl BOND PAIRS3 LONE PAIRS0 BOND ANGLE... SHAPE ° TRIGONAL PLANAR Aluminium - has three electrons to pair up Chlorine - needs 1 electron to complete ‘octet’ Three covalent bonds are formed; aluminium still has an incomplete outer shell.

Al ALUMINIUM CHLORIDE Cl Al 120° Cl Al Cl BOND PAIRS3 LONE PAIRS0 BOND ANGLE... SHAPE ° TRIGONAL PLANAR Aluminium - has three electrons to pair up Chlorine - needs 1 electron to complete ‘octet’ Three covalent bonds are formed; aluminium still has an incomplete outer shell.

METHANE C H C H H H H Carbon - has four electrons to pair up Hydrogen - 1 electron to complete shell Four covalent bonds are formed C and H now have complete shells

METHANE BOND PAIRS4 LONE PAIRS0 BOND ANGLE... SHAPE ° TETRAHEDRAL C H C H H H H 109.5° H H C H H Carbon - has four electrons to pair up Hydrogen - 1 electron to complete shell Four covalent bonds are formed C and H now have complete shells

METHANE BOND PAIRS4 LONE PAIRS0 BOND ANGLE... SHAPE ° TETRAHEDRAL C H C H H H H Carbon - has four electrons to pair up Hydrogen - 1 electron to complete shell Four covalent bonds are formed C and H now have complete shells

AMMONIA H N N H H H BOND PAIRS3 LONE PAIRS1 TOTAL PAIRS4 Nitrogen has five electrons in its outer shell It cannot pair up all five - it is restricted to eight electrons in its outer shell It pairs up only three of its five electrons 3 covalent bonds are formed and a pair of non-bonded electrons is left As the total number of electron pairs is 4, the shape is BASED on a tetrahedron

AMMONIA ANGLE ° SHAPE... PYRAMIDAL H N N H H H BOND PAIRS3 LONE PAIRS1 TOTAL PAIRS4 H H N H H H N H 107° H H N H The shape is based on a tetrahedron but not all the repulsions are the same LP-BP REPULSIONS > BP-BP REPULSIONS The N-H bonds are pushed closer together Lone pairs are not included in the shape

AMMONIA H N N H H H BOND PAIRS3 LONE PAIRS1 TOTAL PAIRS4

Spot the errors  Read this set of instructions and write it out again correcting any mistakes as you go  To predict the shape of different molecules: A. Draw a dot and cross diagram for the molecule B. Count the total number of electrons around the central atom (include electrons in the lone pairs) C. Decide on the starting shape based on the total number of lone pairs (eg. 3 lone pairs = trigonal planar) D. If lone pairs are present just ignore them E. Decide on the name