2Chemical Bonding is all about the electrons. in most of our discussions we will concentrate on the outer ‘s’ and ‘p’ electrons. These are called the valence electrons.since there are 4 of these orbitals in any quantum it requires 8 electrons (an octet) to fill them.the tendency of atoms to try to fill out the outer ‘s’ and ‘p’ shells is the octet rule.
3Chemical BondingElements will combine in order to fill their valence shell with electrons, like the noble gases. This can happen in one of two ways:To share electrons -The outer orbitals of 2 atoms overlap so that each atom is in the vicinity of a full set of valence electrons.This type of bonding is called covalent bonding.
5Chemical bondingAtoms gain or lose electrons to arrive at a full set of valence electrons.When atoms gain or lose electrons they become ions.Ions are attracted to ions of opposite charge and repelled by ions of the same charge.This type of bonding is called ionic bonding
9most of our discussion will be centred on covalent bonding.
10Valence level expansion Some compounds occur which cannot be easily explained:PF5, SF6, ClF7, ArF8in each case the number of chemical bonds is equal to the number of valence electrons.this can only happen if electrons are promoted to a higher energy level. In this case it is the adjacent ‘d’ orbital.
11PF5 the normal orbital diagram looks like this: with valence level expansion it looks like this:
12Valence level expansion includes elements of groups 15 to 18, from period 3 down; periods 1 and 2 do not have a ‘d’ orbital to promote to.Please note that valence level expansion is the exception, not the rule.
13Polar-Covalent Bonding Covalent bonding implies equal sharing of electrons.If sharing is not equal, the electrons in a bond will spend more time with one atom than the other.
14Polar-Covalent Bonding The atom where the electrons spend more time will have a net negative charge, while the atom at the other end of the bond will be positive.This type of bond is polar-covalent.
15Electronegativityis a measure of how strongly an atom is holding on to its valence electrons.If an atom loses an electron fairly easily it has a low electronegativity (and tends to be a cation).If an atom tends not to lose electrons, but tends to steal them from other atoms (and become an anion) it has a high electronegativity.
16To determine what type of bond exists between two atoms you subtract their respective electronegativities:if the electronegativity difference is 0.2 or less, the bond is covalentif the electronegativity difference is 1.7 or greater the bond is ionic.
17ElectronegativityIf the electronegativity difference between two atoms is between 0.3 and 1.6 the bond is polar-covalent.The greater the electronegativity difference the greater the ionic character of the bond:
19AssignmentDetermine the electronegativity difference for each chemical bond. If the bond is polar covalent draw an arrow in the direction of the dipole, from positive to negative:C - HEND = | | = 0.4polar covalent bondN - H B - F S - OP - H Si - Cl Cu - Br N - IBr - Cl O - H C - Cl C - O
23C – ClEND = |2.5 – 3.0| = 0.5polar covalent bond: C – ClC – OEND = |2.5 – 3.5| = 1.0polar covalent bond: C – O
24Covalently-Bonded Structures we now have to consider molecules made of several atoms.most of the following discussion will concern itself with molecules made with covalent or polar-covalent bonds.ionic bonds (and others) will return later in the unit.
25Lewis StructuresLewis structures allow us to predict how atoms will come together to make molecules.Lewis structures are representations of molecules showing all electrons, bonding and nonbonding.
26Writing Lewis Structures Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.If it is an anion, add one electron for each negative charge.If it is a cation, subtract one electron for each positive charge.PCl3(7) = 26
27Writing Lewis Structures The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds.Keep track of the electrons:26 6 = 20
28Writing Lewis Structures Fill the octets of the outer atoms.Keep track of the electrons:26 6 = 20 18 = 2
29Writing Lewis Structures Fill the octet of the central atom.Keep track of the electrons:26 6 = 20 18 = 2 2 = 0
30Writing Lewis Structures If you run out of electrons before the central atom has an octet……form multiple bonds until it does.
31Exceptions to Octet Rule Electron Promotiongroup 2 central atom will have 4 electronsgroup 13 central atom will have 6 electronsValence Level Expansiongroup 15 central atom will have 10 electronsgroup 16 central atom will have 12 electronsgroup 17 central atom will have 14 electronsgroup 18 central atom will have 16 electrons
32Writing Lewis Structures Write Lewis Structures for the following molecules:F2SiF4PCl3C3H8SeH2C3H6MgH2C3H4
33Coordinate Covalent bonds is defined as a covalent bond where one atom provides both of the electrons:
34Polyatomic Ionswe can use coordinate covalent bond theory to explain most ions:
35ResonanceThis is the Lewis structure we would draw for ozone, O3.+-
36ResonanceBut this is at odds with the true, observed structure of ozone, in which……both O—O bonds are the same length.…both outer oxygens have a charge of 1/2.
37ResonanceOne Lewis structure cannot accurately depict a molecule such as ozone.We use multiple structures, resonance structures, to describe the molecule.
38Resonance Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.
39ResonanceIn truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.They are not localized, but rather are delocalized.
40ResonanceThe organic compound benzene, C6H6, has two resonance structures.It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.
41Molecular ShapesThe shape of a molecule plays an important role in its reactivity.By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.
42What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other.By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.
43Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”
44Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure.The geometry will be that which corresponds to that number of electron domains.
45Molecular GeometriesThe electron-domain geometry is often not the shape of the molecule, however.The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.
46Molecular GeometriesWithin each electron domain, then, there might be more than one molecular geometry.
47Group 2 GeometriesCentral AtomBonding ElectronsLone Pair ElectronsBond TypeShapeExampleMagnesium2all singlelinearMgI2one doubleMgOIn this domain, there is only one molecular geometry: linear.NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.
48Group 13 GeometriesCentral AtomBonding ElectronsLone Pair ElectronsBond TypeShapeExampleBoron3all singletrigonal planarBI3one doublelinearBIOone tripleBNbecause there are no lone pair electrons the molecular is planar (flat).
49Group 14 Geometries Central Atom Bonding Electrons Lone Pair Electrons Bond TypeShapeExampleCarbon4all singletetrahedralCH4one doubletrigonal planarCOH2one triple, ortwo doublelinearHCNCO2
50Summary for Groups 2, 13 & 14because there are no lone pair electrons:central atom bonded to 1 atom: linearcentral atom bonded to 2 atoms: linearcentral atom bonded to 3 atoms: trigonal planarcentral atom bonded to 4 atoms: tetrahedral
51Group 15 Geometries Central Atom Bonding Electrons Lone Pair Electrons Bond TypeShapeExampleNitrogen31all singletrigonal pyramidalNH3one doubleangularNOHone triplelinearN25trigonal bipyramidalNCl5
52Group 16 Geometries Central Atom Bonding Electrons Lone Pair Electrons Bond TypeShapeExampleOxygen2all singleangularH2Oone doublelinearO26octahedralOF6
53Group 17 & 18 Geometries group 17 group 18 normally makes 1 chemical bond (linear)with valence level expansion can make 7 bonds (ClF7).group 18normally makes no bondswith valence level expansion can make 8 bonds (ArF8).
54Larger MoleculesIn larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.This molecule is tetrahedral about the first carbon, planar trigonal about the second and angular about the single-bonded oxygen.
55Larger MoleculesThis approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
56Using VSEPR TheoryDetermine the shape of each molecule, or atom within the molecule:F2SiF4PCl3C3H8SeH2C3H6MgH2C3H4
57Polarity previously we discussed bond dipoles. But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.
58PolarityBy adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.In other words we can see if the dipoles reinforce each other, or cancel out.
60AssignmentPerform the following activities for the molecules at the bottom of the slide:Draw a lewis structureDraw a structural diagram and indicate shape.Determine the electronegativity difference for each bond type and determine whether the bond is covalent, polar covalent or ionicDetermine if they will be polar, or non-polar.F2 PCl3 SeH2 MgH2SiF4 C3H8 C3H6 C3H4
63Properties of Molecules are determined by how the molecules interact with each other.How they interact is determined by the forces of attraction between molecules.These are called intermolecular forces; the forces which act between molecules, to draw them together, forming the various phases of matter.
64How do we measure force ? the best way is by temperature. temperature is a measure of kinetic energy; the lowest temperature represents zero kinetic energy.for a substance to melt or boil the kinetic energy must overcome the intermolecular force.the higher the melting temperature or boiling temperature, the greater the intermolecular force.
65Van der Waals Forces occur between covalently bonded molecules. three kinds:London disersiondipole-dipole attractionhydrogen bonding
66London Dispersion Forces are the dominant forces between covalently bonded, non-polar moleculesbased on the formation of instantaneous dipoles.the more electrons in a molecule, the stronger the force.
73Dipole-Dipole attraction is a force which acts between polar molecules (ex. H2S).results from the attraction of the opposite poles of the permanent molecular dipoles.These substances generally have higher melting and boiling points than non-polar molecules with similar molecular weights (or numbers of electrons).
75Ion-Dipole Interactions A fourth type of force, ion-dipole interactions are an important force in solutions of ions.The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.
77Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.
78How Do We Explain This?The nonpolar series (SnH4 to CH4) follow the expected trend.The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.
79Hydrogen Bonding a specialized form of dipole-dipole attraction. It occurs as when O, N, and F are bonded to H, owing to the large electronegativity difference:O - H = 1.4N - H = 1.0F - H = 2.0This is a stronger force than standard dipole-dipole attraction.Molecules with hydrogen bonding will have boiling points and melting points quite a bit higher than molecules that have only dipole-dipole or London dispersion forces.Hydrogen bonding is responsible for many of the unusual properties of water.
82Hydrogen bondingis responsible for folding, final structure and function of proteinsholds the DNA strands together, but allows them to be unzipped for copying and gene expression.
83Relationship Between Polarity, Force and Boiling Point # e- B.P. (°C) Polarity ForceC2H Non-polar LondonDispersionH2S Polar Dipole DipoleH2O Very polar Hydrogen Bonds
84Ionic Bondinggenerally occurs in compounds of metals and non-metals (salts).It is the result of the attraction of oppositely charged ions.The structures formed are very orderly and are given the name crystal lattice.Ionic solids are called crystals.No sharing of electrons occurs between the ions in the crystal lattice.As a result, ionic solids are brittle.Ionic solids conduct electricity only in the molten state, and not very well.Ionic solids are characterized by very high melting and boiling points.
86Metallic Bondingis the bonding which occurs between metals in the Periodic Table.It is characterized by close packing of the atoms, with the electrons delocalized; that is, they are free to jump from atom to atom, filling unoccupied orbitals.This free sharing of electrons allows metals to conduct electricity freely (copper conducts electricity times better than molten NaCl).The free electrons also act as a lubricant, allowing metal atoms to slide over one another without affecting the integrity of the material. Thus metals are malleable and ductile.This bond is strong, giving most metals high melting and boiling points.This bond is also variable.
89Network Covalent Bonding This is the traditional covalent bond, expanded to 2 or 3 dimensions in a network which is theoretically infinite (much like an ionic crystal lattice).Network solids include diamonds, graphite, quartz, and most rocks.Because the covalent bond is stronger than any other bond, the network solid is very hard (diamonds are the hardest substance known).Because electrons are held tightly in their bonds, network solids are brittle, and they do not conduct electricity.Because the orientation of the atoms is very specific to the bonding orientation (tetrahedral, planar trigonal), network solids form distinct crystals.Because of the strength of the bonds, network solids have very high melting and boiling points
93Intermolecular Forces Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution.
94Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other.Viscosity increases with stronger intermolecular forces and decreases with higher temperature.
95Surface TensionSurface tension results from the net inward force experienced by the molecules on the surface of a liquid.
96Vapor PressureAt any temperature, some molecules in a liquid have enough energy to escape.As the temperature rises, the fraction of molecules that have enough energy to escape increases.
97Vapor PressureAs more molecules escape the liquid, the pressure they exert increases.
98Vapor PressureThe liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.
99Vapor PressureThe boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure.The normal boiling point is the temperature at which its vapor pressure is 760 torr.
101Strength of Attraction Molecular(London dispersion)1 to 50(dipole-dipole)5(hydrogen bonding)15Ionic50Metallic20 to 80Network Covalent100
102Melting and Boiling Point Molecular(London dispersion)very low (nitrogen boils at C) VARIABLE(dipole-dipole)low (H2S boils at -61 C)(hydrogen bonding)medium(H2O boils at +100 C)Ionichigh(NaCl boils at C)Metallicvariable +357 C, C)Network Covalentvery high(SiO2 boils at 2600 C)
104Conductance of Heat and Electricity Molecular(London dispersion)non-conductor(dipole-dipole)(hydrogen bonding)Ionicconductor in liquid or dissolved phaseMetallicconductor in solid or liquid phaseNetwork Covalentnon-conductor in 3d form, some conductance in 2d
105Solubility in H2O Molecular (London dispersion) Not soluble (dipole-dipole)Soluble(hydrogen bonding)IonicMetallicNetwork Covalent