Ionic Bonding Covalent Bonding VSEPR theory Molecular Polarity Edward Wen, PhD Why compounds are so Different from each other? Table sugar C12H22O11 : white crystal, soluble in water, aqueous solution not conducting electricity Table salt NaCl: white crystal, soluble in water, aqueous solution conducting electricity Butane C4H10: colorless liquid, volatile, insoluble in water, flammable Glycerol C3H6O3: colorless liquid, not volatile, soluble in water, taste sweet 2 Chapter outline
1. Ionic bonding 2. Covalent bonding Lewis structure Molecular geometry (VSEPR) Molecular polarity Bonding Theories Bonding: the way atoms attach to make molecules How and Why atoms attach together in the manner they do is central to chemistry Why studying chemical bonding? 1) predict the shapes of molecules and properties of substances 2) design and build molecules with particular sets of chemical and physical properties, for such as
drug design in pharmaceutical industries. 4 Lewis Bonding Theory Atoms bond because bonding results in a more stable Electron Configuration by either transferring or sharing electrons so that all atoms obtain an Outer Shell with 8 electrons Octet Rule Some exceptions: H, He, Li, Be. How to remember? Everyone wants to have an electron configuration like a Noble Gas 5 Lewis Symbols of Atoms also Electron Dot Symbols Symbol of element : Nucleus and Inner electrons Dots: around the Symbol as Valence electrons put one electron on each side first, then pair
elements in the same group have the same number of valence electrons same Lewis dot symbols Li Be B C Na Mg Al Si N
P O S F Cl Ne Ar 6 Lewis Symbols of Ions Cations (Cn+) have no valence electrons Valence electron(s) is lost Li+ Li + e-
Anions (An-): 8 valence electrons Electrons gained in the formation of the anion F + e- F7 Ionic Bonds Bonding : Metal + Nonmetal Metal loses electrons to form cation Nonmetal gains electrons to form anion Ionic bond results from + & - attraction (Coulombic force) larger charge = stronger attraction smaller ion = stronger attraction Na+
F- Mg2+ O2- Lewis Theory allow us to predict the correct formulas of ionic compounds 8 Ionic Compounds Ions: Metals (Cation Mx+) and Nonmetals (Anion Ny-) No individual molecules!! have a 3-dimensional array of cations and anions made of formula units: NaCl, MgO Na+ Cl- Na+ Cl- Na+ Cl Cl- Na+ Cl- Na+ Cl- Na+
Na+ Cl- Na+ Cl- Na+ Cl- 9 Example: Using Lewis Theory to Predict Chemical Formulas between Calcium (Ca) and chlorine (Cl) Use bracket  if ion has more valence electron than original. Cl Metal Cation + e Nonmetal + e Anion until Metal loses All valence electrons Nonmetal atoms have 8 electrons
Cl Ca Ca : Cl : Ca2+
Cl Lewis dot symbols of the elements : Cl :
CaCl2 10 More Example: Lewis Structures for Ionic Compound Al2S3 First, Number of valence electrons: Al = ___, S = ___ charge of ions: Al ion = ___, S ion = ___ number of valence electrons on the ION: Al ion = ___, S ion = ___. Use bracket  if ion has more valence electron than original. 11 Covalent Bonds Nonmetal atoms can form chemical bonds between them, such as water, carbon dioxide, and many others Nonmetal atoms DO NOT tend to lose its valence
electrons How two NONMETAL atoms achieve OCTET (like noble gas atoms)? Nonmetal atoms _________ their valence electrons to form chemical bond 12 Lewis Formulas of Covalent Compounds Lewis structure shows: pattern of Valence electron distribution understand the bonding in many compounds predict Shapes of molecules predict Properties of molecules and how they will interact together 13 Single Covalent Bonds Two atoms share ONE pair of electrons 2 electrons
One atom may have more than one single bond Lewis structure: F2 and H2O 14 Double Covalent Bond two atoms sharing TWO pairs of electrons 4 electrons shorter and stronger than single bond: O 2 Lewis structure: O2 15 Triple Covalent Bond two atoms sharing 3 pairs of electrons: 6 electrons shorter and stronger than single or double bond
Lewis structure: N2 16 Practice: Which one(s) violate the Octet Rule? F C C H H H F C C H H H 17
Bonding & Lone Pair Electrons Bonding pairs : Electrons shared by atoms Lone pairs : Electrons that are not shared by atoms but belong to a particular atom (also known as nonbonding pairs) Example: Find bonding pairs and lone pairs in CO2. How many Lone pair(s) on oxygen? ___ Bonding Pairs O C O ___ Lone Pairs 18 Covalent Bonding within Polyatomic Ions
Polyatomic ion: Atoms within the polyatomic ion are held together by covalent bonds form ionic bonds with ions of opposite charge Example: Na+NO3- ; NH4+SO42- Example: Covalent N-H bonds within NH4+ 19 Lewis Structures: common bonding patterns C = 4 bonds & 0 lone pairs 4 bonds = 4 single, or 2 double, or single + triple, or 2 single + double N = 3 bonds & 1 lone pair O = 2 bonds & 2 lone pairs H and Halogen (F, Cl, Br) = 1 bond,
Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs (Not Octet in B!) B C N O F 20 HONC Rule for most neutral molecules to help identify central atom The numbers of covalent bonds formed on #bonds: _____ _____ _____
____ H O N C Extended HONC rule: F S? P? Cl? Se? Br? Si As? 21 Lewis Structures for Covalent Molecules 1) Calculate the total number of Valence electrons
available for bonding use group number of periodic table 2) Arrange the atoms and link with _______ bonds first Usually: Atoms capable of forming (more/fewer) bonds be the center (HONC rule) 22 Lewis Structures for Covalent Molecules 3) Attach atoms with pairs of electrons Start with ______________ atoms to Octet H only wants 2 electrons then attach to Central atoms 4) Check if ALL atoms are ________tet. If central atoms are NOT ____tet, share electrons (to make multiple bonds) to complete the Octets of all the atoms
23 Practice: Drawing Lewis Structures Step by step! 1) 2) 3) 4) Count all valence electrons CO2 Lay out electrons and link with single bonds (HONC) Attach atoms with pairs of electrons to complete OCTET, starting with terminal atoms Complete OCTET for central atoms, building multiple bonds if necessary (except for H)
24 Lewis Structures for Polyatomic Ions Same procedure, #charge affect #Valence electrons Polyatomic Cations: take away electron from the total for each positive charge NH4+ #Valence electrons = 5 + 4 x 1 ____ = 8 Polyatomic Anions: add electron to the total for each negative charge SO32- #Valence electrons = 6 + 3 x 6 _____ = 26 HONC rule often does not apply!! 25 Example: Lewis structure for NO3 1) 2) 3) 4)
5) Count all valence electrons. Lay out electrons and link with single bonds (which is central atom?) Attach atoms with pairs of electrons to complete OCTET, starting with terminal atoms Complete OCTET for central atoms, building multiple bonds if necessary (except for H) Use bracket and label the charge 26 - Exceptions to the Octet Rule H & Li, lose one electron to form cation Li now has electron configuration like He
H can also share or gain one electron to have configuration like He Be : shares 2 electrons to form 2 single bonds B : shares 3 electrons to form 3 single bonds expanded octets for elements in Period 3 or below using empty valence d orbitals some molecules have odd numbers of electrons NO : N O :
27 Molecular Geometry Molecules are 3-dimensional objects Shape of a molecule like Geometric figures Molecular Geometry indicates Positions of the Surrounding atoms with the Central atom in the center of the figure. Linear, Trigonal, Tetrahedral, Pentagonal, etc. Bond Angles : angles between adjacent bonds. in water molecule H-O-H = 105H-O-H = 105 28 Valence Shell Electron Pair Repulsion (VSEPR) theory Electron pairs attached to the same atom, either bonding or nonbonding, have repulsion against each other and stay apart as much as possible
X The number of electron pairs (electron groups) on central atoms leads to specific molecular geometry (bond angles). 29 Molecular Geometry depends on #Electron Groups on Central Atom Each Bond (Shared e-) counts as 1 Electron group Single bond Double or Triple also as ________ electron group Lone Pair : counts as ONE electron group lone pairs occupy space around the central atom
take up slightly more space than bonding pairs Affects bond angles 30 Example: Use Lewis Structure to find #Electron Groups on Central Atom F C C H H H ____ electron groups on each carbon atom
31 Three Electron Group Geometries 2 Electron Groups on Central Atom: Linear 3 Electron Groups on Central Atom: Trigonal Planar 4 Electron Groups on Central Atom: Tetrahedral 32 Linear Shapes (180) 2 Electron groups around the CENTRAL atom, both Bonding
:O C O : Or two atom molecule as trivial case Bond Angle = _______ Examples: HCN, CO2 Bond Angle = 180 degrees Trigonal Planar 3 Electron groups around the CENTRAL atom bond angles = 120
Examples: H2CO, SO3 , SO2 O H C H O S O 34 Tetrahedral 4 Electron groups around the central atom bond angles = 109.5
Four identical equilateral triangles H H C H H Examples: CH4, NH4+ NH3 (three bonds + one lone pair) H2O (two bonds + two lone pair) 35 Tetrahedral with Lone Pair(s) H N H H
H O H 36 Molecular Geometry: Linear #Electron Groups on Central Atom = ___ Bonding Groups = 2 Lone Pairs = 0 Electron Geometry = Linear Angle between Electron Groups = 180 37
Molecular Geometry: Trigonal Planar Electron Groups Around Central Atom = 3 Bonding Groups = 3 Lone Pairs = 0 Electron Geometry = Trigonal Planar Angle between Electron Groups = 120 38 Molecular Geometry: Trigonal Bent
Electron Groups Around Central Atom = 3 Bonding Groups = 2 Lone Pairs = 1 Electron Geometry = Trigonal Planar Angle between Electron Groups = 120 39 Three Electron Groups on Central Atom (bond angle _________) Three terminal atoms (no lone pair electrons) Two terminal atoms (ONE lone pair electrons)
40 Molecular Geometry: Tetrahedral Electron Groups Around Central Atom = 4 Bonding Groups = ___ Lone Pairs = ___ Electron Geometry = Tetrahedral Angle between Electron Groups = _____ 41 Molecular Geometry: Trigonal Pyramid
Electron Groups Around Central Atom = 4 Bonding Groups = ___ Lone Pairs = ___ Electron Geometry = Tetrahedral Angle between Electron Groups = 109.5 42 Molecular Geometry: Tetrahedral Bent Electron Groups Around Central Atom = 4
Bonding Groups = 2 Lone Pairs = 2 Electron Geometry = Tetrahedral Angle between Electron Groups = 109.5 43 Four Electron Groups on Central Atom (bond angle _________) Four Terminal Atoms (Tetrahedral; no LPE) Three Terminal Atoms (Trigonal pyramidal; One LPE) Two Terminal Atoms (Bent; Two LPE)
44 Example: Shape and Bond Angle sulfur trioxide sulfite ion carbon disulfide ammonia Trigonal planar, 120 Trigonal pyramidal, 109.5 Linear, 180 Trigonal pyramidal, 109.5
45 Electronegativity Measure of the pull an atom has on bonding electrons increases across period (left to right) decreases down group (top to bottom) 46 Bond Polarity d+ HF d- bonding between unlike atoms results in unequal sharing of the electrons Atom with high electronegativity pulls the electrons in the
bond (closer, further) to its side Atom that pulls electrons stronger has (larger, lower) electron density than the other Bond Polarity the end with the higher electronegativity gets a partial negative charge (d-) and the end that is electropositive gets a partial positive charge (d+) 47 Electronegativity (EN) affects Bond Polarity Larger Difference in EN between bonded atoms (DEN), more polar bond 3.0-3.0 = 0.0 4.0-2.1 = 1.9
Bond Polarity: Dipole Moments Dipole: a material with positively and negatively charged ends Polar bonds or molecules have one end slightly positive, d+; and the other slightly negative, d not full charges, come from nonsymmetrical electron distribution Dipole Moment (m) : a measure of the size of the polarity measured in Debyes, D 49 Dipole Moment: Torque produced by nearby electric charge + + -
- Polar molecules can be attracted to each other like magnets: The repulsion (same charge) and attraction (opposite charges) leads to the torque that eventually will cause the molecule to rotate Higher charges on both ends of Polar molecules usually result in higher dipole moment 50 Polarity of Molecules Polarity of molecule requires
1) Polar bonds electronegativity difference - theory bond dipole moments - measured 2) Unsymmetrical shape so that Bond Polarity wont offset each other Bent and Trigonal Pyramidal molecules are POLAR vector addition Polarity affects the intermolecular forces of attraction 51 Why molecular polarity matters? N N
S S Polar molecules attract each other stronger than nonpolar molecules, similar to magnets Such behavior affects how molecules aggregate together (for example, alcohol mixes well with water, but oil not dissolve in water) 52 Static charges attract Polar Molecule Polar molecules can be attracted to Static charge Example: Styrofoam attracts to our clothing
because the attractive force, due to its closer distance to the charge, overcomes the repulsion. Online Demonstration (at 2:27) 53 Bond Polarity vs. Molecular Polarity: Addition of Forces as Vector Combined Force Force 1 Combined Force Force 2 Force 1 Force 1 Force 2
54 Polarity of Molecules: Symmetry matters! O H H Polar bonds, + Unsymmetrical shape water molecule as POLAR :O
C O : Polar bonds + Symmetrical shape polarity cancel CO2 molecule as Nonpolar 55 Cl Cl Cl Cl C
C CCl4 : NONPOLAR Cl H H Cl CH2Cl2: POLAR 56 Adding Dipole Moments 57 How to Determine the Molecular Polarity
Step 1: First, draw Lewis structure Step 2: use VSEPR theory to determine the molecular geometry/shape Step 3: consider the bond dipole moment addition. 58 Example: Determine if NH3 is Polar. Information Given: NH3 Find: if Polar Design a Solution Map. formula of compound molecular polarity
Lewis Structure bond polarity & molecular shape 59 Practice: Polar or Nonpolar?
Tetrahedral Bent Trigonal pyramidal Bent Trigonal planar 60 Linear tetrahedral More Lewis Structures for Ionic Compounds Li+ LiF CaO
Ca S Al3+ Al3+ Al2S3 F - S S 2+ 222-
H * NH4Cl O 2- H N H H + Cl 61 Draw Lewis Structure Stepby-step Information
Given: CO2 Write the Lewis structure of CO2. Find: Lewis structure SM: formula skeletal electron distribution Lewis Apply the Solution Map. Count and Distribute the Valence Electrons attach atoms O C O C=4 O=26 total CO2 = 16 start = 16 euse = 4 eleft = 12 e62
Write the Lewis structure of CO2. Information Given: CO2 Find: Lewis structure SM: formula skeletal electron distribution Lewis Apply the Solution Map. C=4 Count and Distribute the Valence ElectronsO = 2 6 total CO2 = 16 complete octets outside atoms first start = 16 e
:O C O : use = 4 eleft = 12 estart = 12 euse = 12 eleft = 0 e63 Example: Write the Lewis structure of CO2. Information Given: CO2 Find: Lewis structure
SM: formula skeletal electron distribution Lewis Apply the Solution Map. Count and Distribute the Valence Electrons complete Octets if not enough electrons to complete octet of central atom, bring in pairs of electrons from attached atom to make multiple bonds start = 12 e- :O
C O : : O C O : use = 12 eleft = 0 e64 Example: Write the Lewis structure of CO2. Information Given: CO2 Find: Lewis structure SM: formula skeletal electron distribution Lewis
Check Start C = 4 eO = 2 6 etotal CO2 = 16 eEnd bonding = 4 2 elone pairs = 4 2 etotal CO2 = 16 e- : O C O : The skeletal structure is symmetrical. All the electrons are accounted for. 65 Example: Determine if NH3 is Polar. Information Given: NH3
Find: if Polar SM: formula Lewis Polarity & Shape Molecule Polarity Apply the Solution Map. Draw the Lewis Structure Count all the VE Lay out the atoms Single bond first complete octets: from terminal atoms first; double or triple bond for the central atoms H N
H H 66 Example: Determine if NH3 is Polar. Information Given: NH3 Find: if Polar SM: formula Lewis Polarity & Shape Molecule Polarity Apply the Solution Map. Determine Shape of Molecule H
N H 4 areas of electrons around N; H 3 bonding areas 1 lone pair N H H H shape = trigonal pyramid 67
Example: Determine if NH3 is Polar. Information Given: NH3 Find: if Polar SM: formula Lewis Polarity & Shape Molecule Polarity Apply the Solution Map. Determine if Bonds are Polar H N H H
Electronegativity N = 3.0 H = 2.1 3.0 2.1 = 0.9 polar covalent 68 Example: Determine if NH3 is Polar. Information Given: NH3 Find: if Polar SM: formula Lewis Polarity & Shape Molecule Polarity Apply the Solution Map.
Determine Molecular Polarity bonds = polar shape = trigonal pyramid N H H H molecule = polar 69 Example: Determine if NH3 is Polar. Information Given: NH3 Find: if Polar
SM: formula Lewis Polarity & Shape Molecule Polarity Check. H N=5 H=31 total NH3 = 8 bonding = 3 2 elone pairs = 1 2 etotal NH3 = 8 e- N H H bonds = polar shape = trigonal pyramid
H N H H The Lewis structure is correct. The bonds molecule = polar are polar and the shape is unsymmetrical, so it should be polar. 70
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