SHAPE OF MOLECULES & IONS

LEARNING OBJECTIVESAt the end of the chapter, you should be able to:1. Demonstrate an understanding of the use of electron-pair

repulsion theory to interpret and predict the shapes of simple molecules and ions.

2. Explain the shapes of BeCl2, BCl3, CH4, NH3, NH4+, H2O, CO2,

gaseous PCl5 and SF6 and the simple organic molecules.

3. Apply the electron-pair repulsion theory to predict the shapes of molecules and ions analogous.

4. Demonstrate an understanding of the terms bond length and bond angle and predict approximate bond angles in simple molecules and ions.

5. discuss the different structures formed by carbon atoms, including graphite, diamond, fullerenes and carbon nanotubes, and the applications of these, eg the potential to use nanotubes as vehicles to carry drugs into cells.

CONTENT

1.Molecular structure: VSEPR Model

2.Predicting molecular geometry

3.Arrangement of electron pair about an atom

4.Prediction of molecular & ionic structure

5.Different structure formed by carbon atom

Molecular Structure: VSEPR Model

The structure of molecules play a very important role in determining their

chemical properties.

This is particularly very important for biological molecules; a slight change in the

structure of large bimolecules can completely destroy its usefulness to a cell or may even change the cell from a normal

one to a cancerous one

Molecular Structure: VSEPR ModelVSEPR : Valence shell electron-pair

repulsion theory Useful in predicting the geometries of molecules formed from non-metals.

Postulate: the structure around a given atom is determined principally by minimizing electron-pair repulsions.

Idea: the bonding & non-bonding pairs around a given atom will be positioned as far apart as possible.

Molecular Structure: VSEPR ModelElectron pairs repel each other

Electrons are negatively charged: The electron pairs will repel each other as much as they can.

The type of electron pairs affects how much it repels other electron pairs.

Electron-pair repulsion Bond angles

Lone-pair/lone-pair Biggest

Lone-pair/bonding-pair Second biggest

Bonding-pair/bonding-pair

Smallest

Molecular Structure: VSEPR ModelThe following Lewis structures show three

molecules whose central atom is surrounded by four clouds of high electron density:

Molecular Structure: VSEPR ModelCarbon has 4 bond pairs.

The four H atoms are arranged about the C atom in a tetrahedral shape .

This shape minimizes the repulsion between the bond pairs.

The 109.5°angle is the same for all H-C-H bond angles.

Molecular Structure: VSEPR ModelThe molecule NH3 has a a lone pairing the outer

shell of the central N atom. 

In NH3 the N has 3 bond pairs and 1 lone pair, (4 total pairs).

The shape is called trigonal pyramidal 

Molecular Structure: VSEPR ModelThe O in H2O has 2 bond pairs and 2 lone pairs ( 4

total pairs).

Two corners of the tetrahedron are occupied by lone pairs.

The shape is called bent. The H-O-H bond angle is 104.4°.

This angle is less than that in NH3, due the greater repulsions felt with two lone pairs.

The following rules and figures will help discern electron pair arrangements:

Predicting Molecular Geometry

Draw the Lewis structure for the molecule

Count the electron pairs & arrange them in the way that maximize repulsion (put the pairs as part

as possible)

Use the atoms bonded to the central atom to determine the molecular shape

Steps for writing the Lewis structures:

Predicting Molecular Geometry

Sum the valence electrons from all the atoms

Use a pair of electrons to form a bond between each pair of bound atoms.

Arrange the remaining electrons to satisfy the duet rule for hydrogen & the octet rule for the second-row

elements

Writing the Lewis structures for water molecule:

Predicting Molecular Geometry

1. Sum the valence electrons for H2O1 (H) + 1 (H) + 6 (O) = 8 valence electrons

2. Using a pair of electrons per bond, draw the two O-H single bonds: H-O-H

3. Distribute the remaining electrons to achieve a noble gas electron configuration for each atom. 4 electrons have been used in forming the two bonds, 4 electrons (8-4) to be distributed.

H-O-H....

Arrangement of Electron Pairs About an Atom

3 pairsTrigonal planar

2 pairsLinear

4 pairsTetrahedral

5 pairsTrigonal bipyramidal

6 pairsOctahedral

Presentation of Lecture Outlines, 10–16

PREDICTION OF MOLECULAR & IONIC STRUCTURE

Molecular Structure: 2 electron pairs

BeCl2: linear

Be (2 valence e-) + 2 Cl (7 valence e-) = 16 e-

Lewis structure:

Molecular geometry: linear

Bond angle: 180°

Molecular Structure: 2 electron pairs

b. recall and explain the shapes of BeCl2, BCl3, CH4, NH3, NH4+, H2O, CO2, gaseous PCl5 and SF6

CO2: linear

C (4 valence e-) + O (2 x 6 valence e-) = 16 e-

Lewis structure:

Molecular geometry : linear

Bond angle: 180°

Molecular Structure: 3 electron pairs

BF3: trigonal planar

B (3 valence e-) + F (3 x 7 valence e-) = 24 e-

Lewis structure:

Molecular geometry : trigonal planar

Bond angle: 120°

Molecular Structure: 3 electron pairs

CO32-: trigonal planar

C (4 valence e-) + O(3 x 6 valence e-) + 2 = 24 e-

Lewis structure:

Molecular geometry : trigonal planar

Bond angle: 120°

Molecular Structure: 3 electron pairs

NO3-: trigonal planar

N (5 valence e-) + O(3 x 6 valence e-) + 1 = 24 e-

Lewis structure:

Molecular geometry : trigonal planar

Bond angle: 120°

Molecular Structure: 3 electron pairs

SO2: trigonal planar

S (6 valence e-) + O (2 x 6 valence e-) = 18 e-

Lewis structure:

Molecular geometry : bent

Bond angle: 120°

Molecular Structure: 4 electron pairs

CH4: tetrahedral

C (4 valence e-) + H (1 x 4 valence e-) = 8 e-

Lewis structure:

Shape: tetrahedral

Bond angle: 109.5°

Molecular Structure: 4 electron pairs

NH4+: tetrahedral

C (5 valence e-) + H (1 x 4 valence e-) - 1 = 8 e-

Lewis structure:

Molecular geometry: tetrahedral

Bond angle: 109.5°

Molecular Structure: 4 electron pairs

NH3: tetrahedral

N (5 valence e-) + H (1 x 3 valence e-) = 8 e-

Lewis structure:

Molecular geometry: trigonal pyramidal

Bond angle: 107°

Molecular Structure: 4 electron pairs

PCl3: tetrahedral

P (5 valence e-) + Cl (3 x 7 valence e-) = 26 e-

Lewis structure:

Molecular geometry: trigonal pyramidal

Molecular Structure: 4 electron pairs

H2O: tetrahedral

H (2 valence e-) + O (6 valence e-) = 8 e-

Lewis structure:

Molecular geometry: bent

Bond angle: 104°

Molecular Structure: 4 electron pairs

The sulphate ion, SO42-

Molecular Structure: 5 electron pairs

PCl5: trigonal bipyramidal

P (5 valence e-) + Cl (5x7 valence e-) = 40 e-

Lewis structure:

Molecular geometry: trigonal bipyramidal

Bond angle:90° &120°

Molecular Structure: 5 electron pairsSF4: trigonal bipyramidal

S (6 valence e-) + F (4x7 valence e-) = 34 e-

Lewis structure:

Molecular geometry: see-saw

Bond angle:177° & 104°

Molecular Structure: 5 electron pairsClF3: trigonal bipyramidal

Cl (7 valence e-) + F (3x7 valence e-) = 28 e-

Lewis structure:

Molecular geometry: T-shape

Bond angle:87.5°

Molecular Structure: 5 electron pairs

XeF2: trigonal bipyramidal

Xe (8 valence e-) + F (2x7 valence e-) = 22 e-

Lewis structure:

Molecular geometry: linear

Molecular Structure: 6 electron pairs

SiF6: Octahedral

Si (4 valence e-) + F (6x7 valence e-) = 46 e-

Lewis structure:

Molecular geometry: octahedral

Molecular Structure: 6 electron pairs

IF5: Octahedral

I (7 valence e-) + F (5x7 valence e-) = 42 e-

I

FFF

:

FF

square pyramidal

Molecular Structure: 6 electron pairs

XeF4: Octahedral

I (8 valence e-) + F (4x7 valence e-) = 36 e-

square planar

DIFFERENT STRUCTURE FORMED BY CARBON ATOM

Allotropes are different forms of the same

element in the same state

Carbon forms 3 allotropes – diamond, graphite & fullerenes

Diamond is the hardest substance

Because of strong covalent bonds:

1. Diamond has a very high melting point

2. Diamond is extremely hard – it’s used in diamond-tipped drills & saws

3. Vibration travel easily through the stiff lattice – good thermal conductor

4. It can’t conduct electricity – all the 4 outer electrons are held in localised bonds

5. It won’t dissolve in any solvent.

Diamond is the hardest substance

Uses of diamond:

1. The unusual brilliant shine of diamond makes it an invaluable precious stone in jewellery.

2. Making high precision cutting tools for use in medical field.

3. Because of it's hardness it is used in manufacturing tools/cutting drills for cutting glass and rock.

4. In making dyes for drawing very thin wires of harder metals. Tungsten wires of thickness 1/6th that of human hair, can be drawn using diamond dyes.

Graphite an allotrope of carbon

1. The carbon atoms are arranged in sheets of flat hexagons covalently bonded with three bonds each.

2. The 4th outer electron of each carbon atom is delocalized.

3. In graphite, the carbon atoms are arranged in flat parallel layers as regular hexagons.

4. Each layer is bonded to adjacent layers by weak Van der Waals forces.

Graphite an allotrope of carbonProperties of graphite:1. The weak bonds between the layers in

graphite are easily broken, so the sheets can slide over each other – graphite feels slippery & is used as dry lubricant & in pencils.

2. The delocalised electrons in graphite aren’t attached to any particular carbon atoms and are free to move along the sheets, so an electrical current can flow.

3. The layers are quite far apart compared to the length of the covalent bonds, so graphite is less dense than diamond & is used to make strong, lightweight sports equipment.

Graphite an allotrope of carbon

Properties of graphite:

4. Because of the strong covalent bonds in the hexagon sheets, graphite also has a very high melting point.

5. Insoluble in any solvent. The covalent bonds in the sheets are difficult to break.

The fullerenes – 1. Hollow Balls

Fullerenes are molecules of carbon shaped like hollow balls or tubes.

Each carbon atom forms 3 covalent bonds with its neighbors, leaving free electrons that can conduct electricity.

Fullerenes are nanoparticles (<100 nm scale)

1st fullerene – Buckminsterfullerence, which has 60 carbon atoms joined to make a ball, C60

Occurs naturally in soot.

The fullerenes – 1. Hollow Balls

Soluble in organic solvents – form brightly coloured solutions.

Hollow – can be used to ‘cage’ other molecules. The fullerene structure forms around another molecule, which is then trapped inside. This could be used as a way of delivering a drug into specific cell in the body.

Fullerenes are used in nanotechnology - materials & made from nanoparticles.

The fullerenes – 2. Tubes A carbon nanotube is like a single

layer of graphite rolled up into a tiny hollow cylinder.

All those bonds make carbon nanotubes very strong. They can be used to reinforce graphite in tennis rackets & to make stronger, lighter building materials. Nanotubes conduct electricity, so they can be used as tiny wires in circuits for computer chips.

The ends of a nanotube can be ‘capped’, or closed off, to create a large cage molecular structure.