4.3 Covalent Structures IB Chemistry SL Mrs. Page

Essential Idea Lewis (electron dot) structures show the electron domains in the valence shell and are used to predict molecular shape. Nature of Science Scientists use models as representatives of the real world – the development of the model of molecular shapes (VSEPR) to explain observable properties.

Understandings 1.Lewis (electron dot) structures show all the valence electrons in a covalently bonded species 2.The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of eight electrons. 3.Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons. 4.Resonance structures occur when there is more than one possible position for a double bond in a molecule. 5.Shapes of species are determined by the repulsion of electron pairs according to the valence shell electron pair repulsion (VSEPR) theory. 6.Carbon and silicon form covalent network (giant covalent) structures.

Application & Skills 1.Deduction of Lewis (electron dot) structures of molecules and ions showing all valence electrons for up to four electron pairs on each atom. 2.The use of VSEPR theory to predict the electron domain geometry and the molecular geometry for species with two, three, and four electron domains. 3.Prediction of bond angles from molecular geometry and presence of non-bonding pairs of electrons 4.Prediction of molecular polarity from bond polarity and molecular geometry 5.Deduction of resonance structures, examples include but are not limited to C 6 H 6 CO 3 2- and O 3 6.Explanation of the properties of covalent networks (giant covalent) compounds in terms of their structures.

U1 &2: LEWIS STRUCTURES Multiple ways to show the same molecule N H H H NH H H NH H H NH H H X

U1& 2: Lewis Structures & Ions  We can use Lewis structures of ionic compounds  Must use brackets around cation and anion (electrostatic force bonds ions) NH 4 Cl NH 4 NO 3

U1 & 2: Rules for Lewis Structures Add up the total number of valence electrons in the molecule. Draw the skeletal structure. Use a line between each element to symbolize an electron pair. Distribute the remaining electrons around the elements in pairs to form octets. (Hydrogen can only ever have 2 electrons.) If you do not have enough to form octets, make double or triple bonds. Ions must have square brackets around them with the charge notated in the top right hand corner. To be a correct Lewis structure, ALL electrons must be shown.

U1 &2: Lewis Structures  Tell us about covalent bonds  Bonding pairs vs. Lone pairs  Single, double or triple bonds  They do not tell us about the actual shape of the molecules

U5: VSEPR Valence Shell Electron Pair Repulsion theory. States that pairs of valence electrons repel each other and are therefore arranged as far apart from each other as possible. So far we have dealt with structural formulas which only show the types of atoms, bonds and lone pairs of electrons. They do not show the shape of the molecule. VSEPR is a model that allows us to look at shapes of molecules

U5: VSEPR – Basic Shape To determine the shape you must look at the electron domains (areas where there are pairs of electrons)

U5: VSEPR – Basic Shape Linear: central atom with two electron domains (no lone pairs) Bond angle 180° Ex: BeCl 2, CO 2

U5: VSEPR – Basic Shape Trigonal Planar: central atom with three electron domains (no lone pairs) Bond angle 120° Ex: BeF 3, NO 3 -

U5: VSEPR – Basic Shape Tetrahedral: central atom has four electron domains (no lone pairs) Bond Angle 109.5° Ex: CH 4, [NH 4 ] + Solid Wedge = bond in front Dashed Wedge = bond toward back

Effect of Lone Pairs ✘ Molecular Geometry gives the shape of the molecule ✘ Electron Domain Geometry is based on the number of electron domains ✘ Electron domains can be bonded pairs or lone pairs ✘ Lone pair electrons occupy more space than bonding pairs and therefore alter the bond angles from molecular geometry ✘ The more lone pairs, the greater the repulsion and therefore the greater the impact on the bond angle

U5: VSEPR – V-Shaped (Bent) e- Geometry: Trigonal Planar Molecular Geometry: (V-shaped or bent) : central atom has three electron domains (1 lone pair) Bond angle is <120° Ex: SO 2, SF 2, [NO 2 ] -

U5: VSEPR – Trigonal Pyramidal e- Geometry: Tetrahedral Molecular Geometry: Trigonal Pyramidal central atom has four electron domains (1 lone pair) Bond angle is <109.5° Ex: NF 3, NH 3, [H 3 O] +, [SO 3 ] 2-

U5: VSEPR – Tetrahedral Bent e- Geometry: Tetrahedral Molecular Geometry: Bent (V-Shaped) central atom has four electron domains (2 lone pairs) Bond angle is <109.5° Ex: H 2 O, [SO 3 ] 2-

Bonding Groups on Central Atom Lone Pairs on Central Atom e- Domain Geometry Molecular Geometry Bond AngleExample 20Linear Trigonal Planar Tetrahedral Trigonal Planar Bent (V- shaped)<120 31Tetrahedral Trigonal Pyramidal < Tetrahedral Bent (V- shaped) <109.5

Other Effects on Bond Angles Bond angle depends on the electron domains but is not the exact same measure for all molecules with the same domains Bond angle is effected by the type of atoms, electronegativity differences, and multiple bonds as well You do not need to know EXACT bond angles however should be able to predict which molecular and which electron domain geometry NOTE: You should also be able to predict geometries of oxoanions (polyatomic ions containing oxygen)

Using Lewis Structures; Predict the molecular geometry, the electron domain geometry and bond angles for the following CCl 4 NH 4 + NF 3 SF 2 [NO 2 ] - [SO 3 ] 2- Check answers in book, pp

QUIZ (15 minutes) XeO 3 CH 3 + ClO 4 - Using Lewis Structures (2); Predict the molecular geometry (1), the electron domain geometry (1) and bond angles (1) for the following (15 marks total)

XeO 3 ED geometry: tetrahedral Molecular geometry: trigonal pyramidal Bond angles: <109.5 CH 3 + ED geometry: trigonal planar Molecular geometry: trigonal planar Bond angles: 120 ClO 4 - ED geometry: tetrahedral Molecular geometry: tetrahedral Bond angles:109.5

U3:Exceptions to Octet Rule Hydrogen will never have more than 2 electrons. Some elements such as Be and B may have an incomplete octet when bonding Be has only 4 electrons in BeCl 2 bond Boron only has 6 valence electrons in BF 3 Some elements like S and P can have expanded octets which hold more than 8 electrons.

U3:Exceptions to Octet Rule Coordinate covalent bonds are formed when both electrons originate from the same atom. An arrow is used to denote the direction in a coordinate covalent bond showing the atom from which both electrons originated.

Your Turn Draw the Lewis structures showing the coordinate (dative) bond 1.CO CO 3.NO 3 -

U4 & A5: Resonance Structures  Resonance is a concept used to describe the structures when there are multiple ways to depict the same molecule.  If you can put a double bond in more than one position, you will be expected to draw the resonance structures.  The electrons are actually delocalized in the areas of the double bonds and are spread out equally among all bonding positions.  Bond strength and length are in between that of single and double bonds.

U4 & A5: Resonance Structures  Resonance structures allow us to depict all the possible positions of the double bonds.  The true structure, however, is an intermediate form known as a resonance hybrid.  Double arrows are placed between all resonance structures.

A5:Resonance & Benzene

A5:Resonance & Carbonate

A5:Resonance & Ozone

A5: Resonance Practice Draw all resonance structures for the following polyatomic molecules and ions. Be sure to draw polyatomic ions in brackets and include the charge on each. 1.Formate ion, CHO Cyclobutadiene, C 4 H 4 3.Ozone, O 3 4.Sulfur Dioxide, SO 2 5.Carbonate ion, CO Chlorate ion, ClO 3 -

A4: Molecular Polarity Bond polarity refers to a specific bond within a compound Molecular polarity is the polarity of the molecule as a whole We can have a non-polar molecule even if the molecule contains polar bonds

A4: Molecular Polarity Steps to deduce molecular polarity: Determine molecular geometry with VSEPR Determine polarity of each bond using electronegative values ∆  p, show with vectors and dipole moment  -  + Add vectors to determine if there is a net dipole moment, . If so the molecule is polar

A4: Molecular Polarity Try it: SF 2 BF 3 CO 2 NH 3 H 2 O CS 2 Check solutions in book pp

U6 & A6: Covalent Networks Allotropes: different structures of the same element Carbon has 4 allotropes; graphite, diamond, graphene, and C 60 fullerene 3 of these allotropes, graphite, diamonds and graphene are examples of covalent network solids – also known as giant 3-D covalent structures C 60 fullerene is a molecule * Another example of a covalent network solid is quartz (SiO 2 )

U6 & A6: Covalent Networks Properties of Covalent Network Solids High melting points (>1000°C) due to MANY strong covalent bonds Generally poor conductors (except graphite and graphene) Usually insoluble in most substances Generally very hard (except graphite – sheets slide)

U6 & A6: Covalent Networks Graphite Each carbon is bonded to 3 other carbon atoms in a trigonal planar geometry – these covalent bonds are strong Carbon atoms form layers of hexagonal rings Layers connected by weak intermolecular forces called London forces Electrons delocalized allowing conduction Often used as a lubricant because layers can slide past each other due to weak London forces Pencil lead

U6 & A6: Covalent Networks Diamonds Each carbon is bonded to 4 other carbon atoms in a tetrahedral geometry Form a lattice structure One of hardest substance on earth due to bonding and interlocking tetrahedrons Very high melting & boiling point (strong bonds) No delocalized electrons (no conduction) Insoluble

U6 & A6: Covalent Networks Graphene Each carbon is bonded to 3 other carbon atoms Form a lattice structure that is densely packed Atoms arranged hexagonally and one atom thick One of thinnest and strongest materials known Excellent conductor Transparent Flexible

U6 & A6: Covalent Networks C 60 fullerene (buckyballs) 60 carbon atoms 20 hexagonal surfaces 12 pentagonal surfaces Each carbon bonded to 3 other carbons A molecule (not a covalent network solid) Black solids Insoluble in water but soluble in non-polar solvants Not conductive

U6 & A6: Covalent Networks Silicon Dioxide (Quartz) Another example of a covalent network solid Each silicon bonded to 4 oxygens Each oxygen bonded to 2 silicon atoms High melting and boiling points (strong covalent bonds) Not conductive

EXTRA PRACTICE Formula Lewis Structure (Show Resonance Structures) Molecular Geometry e- Domain Geometry Bond Angle Polar Molecule? SO 3 BeCl 2 PO 4 3- SO 2 N3 -N3 - XeO 3 NH 2 - NO 2 - ClO 2 - H3O+H3O+ NH 3 Complete the following: