Shapes of Molecules 4 2 7 Predict the

  • Slides: 26
Download presentation
Shapes of Molecules

Shapes of Molecules

• 4. 2. 7: Predict the shape and bond angles for species with

• 4. 2. 7: Predict the shape and bond angles for species with four, three and two negative charge centres on the central atom using the valence shell pair repulsion theory (VESPR).

• Before you start it would be helpful to… • know the definition

• Before you start it would be helpful to… • know the definition of a covalent bond • know what a lone pair is • know that like charges repel A lone pair is a pair of electrons in the valence shell of an atom, which are not used to form covalent bonds. We came across them when we did Lewis structures!

• Covalent molecules are not flat – even though that’s how we usually

• Covalent molecules are not flat – even though that’s how we usually draw them. • They exist in 3 dimensions and have specific shapes. • We can see these shapes using computer simulations, or by making models using Molymod kits.

• During exams(!) we won’t have either of these methods available, so we

• During exams(!) we won’t have either of these methods available, so we have to use VSEPR theory to predict the shape of the molecule. • We then use certain conventions to draw the molecule.

VSEPR • This means “valence shell electron pair repulsion” • You can probably see

VSEPR • This means “valence shell electron pair repulsion” • You can probably see why we just use the initials. . . • BUT it does tell us that the shape of a molecule is due to the number of electron pairs in the outer (valence!) shell.

• Its best to learn the principles behind this. • You can worry

• Its best to learn the principles behind this. • You can worry about all the fancy words later! • It is possible to consider the shape around any atom in a (covalently bonded) molecule • But at IB level they will usually only ask questions where its obvious which atom they want you to consider.

• The first step is to figure out how many areas of “high

• The first step is to figure out how many areas of “high electron density” there around the atom we’re interested in. • This means IF IN DOUBT DRAW A LEWIS STRUCTURE! • Areas of high electron density could be a covalent bond, a double or triple bond or a lone pair of electrons. • Notice that double and triple bonds are only ONE area of high electron density.

2 areas of high electron density 4 areas of high electron density

2 areas of high electron density 4 areas of high electron density

• Because like charges repel, • Areas of high electron density try to

• Because like charges repel, • Areas of high electron density try to get as far apart as possible. • Because valence shells usually have 8 electrons, • The commonest case is 4 areas of HED • This gives a tetrahedron

E. g. Methane BOND PAIRS 4 LONE PAIRS 0 This will be important in

E. g. Methane BOND PAIRS 4 LONE PAIRS 0 This will be important in a few minutes! BOND ANGLE. . . SHAPE. . . 109. 5° TETRAHEDRAL

• The bond angle of 109. 5 degrees is fixed as a property

• The bond angle of 109. 5 degrees is fixed as a property of a tetrahedron. • Where a central molecule is surrounded by 4 bonding electron pairs, the bond angle is ALWAYS 109. 5 degrees. • However, sometimes one of the areas of HED will be a lone pair. • Lone pairs are closer to the central atom and repel the bonding electrons a bit more strongly.

Ammonia Bond pairs 3 Lone pairs 1 Bond angle Shape 107 degrees Trigonal pyramidal

Ammonia Bond pairs 3 Lone pairs 1 Bond angle Shape 107 degrees Trigonal pyramidal

• If there are 2 lone pairs of electrons, the bonds are forced

• If there are 2 lone pairs of electrons, the bonds are forced even closer together.

Water Bond pairs 2 Lone pairs 2 Bond angle 104. 5 degrees Shape “bent”

Water Bond pairs 2 Lone pairs 2 Bond angle 104. 5 degrees Shape “bent” or “ vshaped”

• Its important to keep in mind that the shapes we have just

• Its important to keep in mind that the shapes we have just discussed are all just variations on a tetrahedron. • Its just that lone pair electrons make part of the tetrahedron “invisible”! • If you know the science involved, you can work out the bond angles. • Don’t stress too much about learning the names of the shapes. • You will need to do it, but it shouldn’t be too hard.

• Exactly the same science is involved for shapes which aren’t based on

• Exactly the same science is involved for shapes which aren’t based on a tetrahedron.

Carbon dioxide There are 2 areas of high electron density round the carbon The

Carbon dioxide There are 2 areas of high electron density round the carbon The furthest apart that 2 areas of HED can get is 180 degrees Bond angle 180 degrees Shape Linear

BF 3 There are 3 areas of high electron density round the boron The

BF 3 There are 3 areas of high electron density round the boron The furthest apart that 3 areas of HED can get is 120 degrees Bond angle 120 degrees Shape Trigonal planar

SO 2 There are 3 areas of high electron density round the sulfur The

SO 2 There are 3 areas of high electron density round the sulfur The lone pair forces the bonding electrons slightly closer together Bond angle 117 degrees Shape Bent or v-shaped

• These are the only shapes that SL students need to know! •

• These are the only shapes that SL students need to know! • HL need a few more! • Notice that in some cases the actual bond angles given are only approximate! • The “real” angle changes depending on the environment.

September 2021 Shapes of Molecules LO Predict shapesSeptember 2021 Shapes of Molecules LO Predict shapes
SHAPES OF MOLECULES SHAPES OF MOLECULES CONTENTS PriorSHAPES OF MOLECULES SHAPES OF MOLECULES CONTENTS Prior
SHAPES OF MOLECULES SHAPES OF MOLECULES Before youSHAPES OF MOLECULES SHAPES OF MOLECULES Before you
Congruent and similar shapes Congruent shapes Similar shapesCongruent and similar shapes Congruent shapes Similar shapes
Shapes of Molecules All shapes came from httpShapes of Molecules All shapes came from http
Shapes of Molecules All shapes came from httpShapes of Molecules All shapes came from http
Shapes of Molecules All shapes came from httpShapes of Molecules All shapes came from http
Shapes of Molecules All shapes came from httpShapes of Molecules All shapes came from http
Shapes of Molecules Intermolecular Forces Shapes of covalentShapes of Molecules Intermolecular Forces Shapes of covalent
ORGANIC MOLECULES Macromoleculeslarge molecules Organic Molecules l FoundORGANIC MOLECULES Macromoleculeslarge molecules Organic Molecules l Found
Organic Molecules Organic Molecules Organic molecules are thoseOrganic Molecules Organic Molecules Organic molecules are those
Carbon and Organic Molecules Organic Molecules Organic moleculesCarbon and Organic Molecules Organic Molecules Organic molecules
Predict changes Predict continuity An important economic continuityPredict changes Predict continuity An important economic continuity
i Predict Breast cancer Assays The i Predicti Predict Breast cancer Assays The i Predict
Molecular Shapes In order to predict molecular shapeMolecular Shapes In order to predict molecular shape
Ordinary Shapes and FourDimensional Shapes Ingvar Johansson InstituteOrdinary Shapes and FourDimensional Shapes Ingvar Johansson Institute
Compound Shapes Compound shapes Can you describe whatCompound Shapes Compound shapes Can you describe what
Threedimensional Shapes 3 D These shapes are solidThreedimensional Shapes 3 D These shapes are solid
TERRITORY SHAPES Companies uses different kinds of shapesTERRITORY SHAPES Companies uses different kinds of shapes
Shape PositiveNegative Shapes OrganicGeometric Shapes FigureGround Shape AShape PositiveNegative Shapes OrganicGeometric Shapes FigureGround Shape A
State Shapes Homework Debra Troxell NBCT State ShapesState Shapes Homework Debra Troxell NBCT State Shapes
Visio Shapes basics Course contents Overview Shapes fulfillVisio Shapes basics Course contents Overview Shapes fulfill
Visio Shapes basics Course contents Overview Shapes fulfillVisio Shapes basics Course contents Overview Shapes fulfill
Building Background Shapes and solids outdoors Shapes huntBuilding Background Shapes and solids outdoors Shapes hunt