3 Octet RuleAtoms tend to gain, lose, or share electrons in order to acquire a full set of valence electronsA very important ruleAn Octet consist of eight electronsThis refers to the outermost eight electronsCompounds form to satisfy the Octet RuleIonic or Covalent Bonds
4 Types of IntramolecularBonds IonicFormed by transfer of electronsHeld together by electrostatic attractionCovalentFormed by sharing of electronsHeld together by shared electronsMetallicFormed when electron(s) become detached from metal atomsSea of electrons among + ions metal of atoms
9 Na+1 + Cl-1 NaCl (table salt) Ionic BondsIonic bonding occurs when one atomtransfers an electron to another atomBoth atoms involved become chargedOne is negatively chargedOne is positively chargedThis occurs when a metal reacts with a nonmetalNa+1 + Cl-1 NaCl (table salt)
11 Ions and Charges Cations Positive Ions Anions Negative Ions Group 1, 2, 3Form ions with charges equal to their group numberGroup 5, 6, 7Form ions with charges equal to group number minus eight
12 Ionic Compounds and Formulas The formula of a compound describes what elements are in the compound and in what proportions.Compounds that are held together by ionic bonds are called ionic compounds.
13 Common polyatomic ion names FormulaNameNH4+Ammonium ionCO32-Carbonate ionPO43-Phosphate ionSO42-Sulfate ionOH-Hydroxide ionNO3-Nitrate ion
14 Formation of Ionic Compounds Al SO42-Al2(SO4)3Ionic compounds forms such that total of ionic charges is zero.For above case:2 x (+3) = 63 x (-2) = -6Total
15 O The Lewis dot structure for Oxygen Oxygen is in group VIA so it has 6 valence electrons
16 Cl The Lewis dot structure for Chlorine chlorine is in group VIIA so it has 7 valence electrons
17 Ca The Lewis dot structure for calcium calcium is in group IIA so it has 2 valence electrons
19 Covalent BondsA covalent bond is a chemical bond that is formed when two atoms share a pair of electrons.H. + H. H:HCovalent Compounds and FormulasIn the above example, each hydrogen has a filled valence shell simulating the electron configuration of helium.Compounds that are held together by covalent bonds are called covalent compounds.Covalent compounds form from atoms on the right side of the periodic table
20 Multiple Bonds.In electron dot notations, a pair of electrons can be represented by a pair of dots : .This can be a bonding pair or a lone pair (non-bonding pair).Bonding pairs can also be represented by lines connecting atoms.H:H = H-HWhen one pair of electrons is shared, it is called a single bond.H-H
21 When two pairs of electrons are shared it is called a double bond. When three pairs of electrons are shared it is called a triple bond.
22 Bond LengthBond Length varies for covalent bonds between different elementsSee p. 187Two important trendsAs one moves down a group, bond length between atoms increasesF—F nmCl—Cl nmBr—Br nmMultiple bonds are shorter than single bondsExample: two carbon atoms bonded togetherSingle covalent bond nmDouble covalent bond nmTriple covalent bond nm
24 Exceptions to the Octet Rule Exceptions to the octet rule include those for atoms that cannot fit eight electrons, and for those that can fit more than eight electrons, into their outermost orbital.Hydrogen forms bonds in which it is surrounded by only two electrons.Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons.Main-group elements in Periods 3 and up can form bonds with expanded valence, involving more than eight electrons, e.g. PF5 and SF6
25 Electron-Dot Notation To keep track of valence electrons, it is helpful to use electron-dot notation.Electron-dot notation is an electron-configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol. The inner-shell electrons are not shown.
26 Lewis Structures Chapter 6 Electron-dot notation can also be used to represent molecules.The pair of dots between the two symbols represents the shared electron pair of the hydrogen-hydrogen covalent bond.For a molecule of fluorine, F2, the electron-dot notations of two fluorine atoms are combined.
27 Chapter 6 Lewis Structures The pair of dots between the two symbols represents the shared pair of a covalent bond.In addition, each fluorine atom is surrounded by three pairs of electrons that are not shared in bonds.An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom.See Example, p. 185
28 Multiple Covalent Bonds Chapter 6Multiple Covalent BondsDouble and triple bonds are referred to as multiple bonds, or multiple covalent bonds. (See Table 2, p. 187)In general, double bonds have greater bond energies and are shorter than single bonds.Triple bonds are even stronger and shorter than double bonds.When writing Lewis structures for molecules that contain carbon, nitrogen, or oxygen, remember that multiple bonds between pairs of these atoms are possible.
29 Molecular Shape Arises because electrons repulse one another Called VSEPRValence Shell Electron Pair RepulsionVSEPR states that in a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible
30 Summary of Molecular Shapes TypeBondAngleUnshared PairsExampleLinear180°Balanced(2 each side)CO2TrigonalPlanar120°noneBCl3Tetrahedral109.5°CH4Pyramidal107°oneNH3Bent105°twoH2O
31 Bent molecular structure of the water molecule.
35 8.3Molecular OrbitalsWhen two atoms combine, the molecular orbital model assumes that their atomic orbitals overlap to produce molecular orbitals, or orbitals that apply to the entire molecule.Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole.A molecular orbital that can be occupied by two electrons of a covalent bond is called a bonding orbital.
36 Molecular Orbitals 8.3 Sigma Bonds When two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei, a sigma bond is formed.
37 8.3Molecular OrbitalsWhen two fluorine atoms combine, the p orbitals overlap to produce a bonding molecular orbital. The F—F bond is a sigma bond.Two p atomic orbitals can combine to form a sigma-bonding molecular orbital, as in the case of fluorine (F2). Notice that the sigma bond is symmetrical around the bond axis connecting the nuclei.
38 Molecular Orbitals 8.3 Pi Bonds In a pi bond (symbolized by the Greek letter ), the bonding electrons are most likely to be found in sausage-shaped regions above and below the bond axis of the bonded atoms.
39 Polarity Bonds can be polar or nonpolar Depends on electronegativity differenceMolecules can be polar or nonpolarPolar molecules are called dipolesOne end of a polar molecule has “+” charge; other end has “-” chargeWill align in electric fieldWill be attracted or deflected by magnetic fieldPolarity of a molecule is determined by shape and the type of bonds between its atoms
40 The chlorine atom attracts the electron cloud more than the hydrogen atom does. Why?
41 Polar Molecules 8.4 A hydrogen chloride molecule is a dipole. When polar molecules, such as HCl, are placed in an electric field, the slightly negative ends of the molecules become oriented toward the positively charged plate and the slightly positive ends of the molecules become oriented toward the negatively charged plate. Predicting What would happen if, instead, carbon dioxide molecules were placed between the plates? Why?
42 Intermolecular Forces Attraction between moleculesHolds groups of molecules togetherIntermolecular forces are known as van der Waals forcesDipole-Dipole attractionDispersion ForcesHydrogen Bonds
43 van der Waals Forces Dipole-Dipole attraction Dispersion Forces Electrostatic attraction between polar moleculesMolecules line up like magnetsDispersion ForcesPolarity arises due to electron imbalanceWeak electrostatic forces ariseHydrogen BondsStrong dipoles arising from covalent bonds of hydrogen atom to a very electronegative atomH electronegativity = 2.1 F=4.0Results in liquids with high boiling points
44 Probability representations of the electron sharing in HF.
46 Water molecule behaves as if it had a positive and negative end.
47 Polarity Formaldehyde CH2O Carbon Dioxide CO2 Water H2O Forms dipole due to high electronegativity of OCarbon Dioxide CO2Has polar bond but they cancel outCO2 is nonpolarWater H2OWater is a bent molecule due to unshared pairsunlike CO2 bond polarities do not cancel outWater is polar and forms dipoleWhy is water liquid and Carbon Dioxide a gas?
48 Large Molecules Examples: protein, Subunits linked together in a chain Often bend and twist to form 3D shapeChains, rings, balls
62 Bond Energies and Bond Lengths for Single Bonds Section 2 Covalent Bonding and Molecular CompoundsChapter 6Bond Energies and Bond Lengths for Single Bonds
63 Hybrid Orbitals When atoms bond, their outer orbitals become distorted Orbitals do not look like those of unbonded atomHybrid orbitals are formed which are a cross between the bonding orbitalsspsp2sp3Orbital hybridization contributes to shape of molecule