Chapter 2: Atomic Structure and Chemical Bonding

1 1 Chapter 21 Chapter 2: Atomic Structure and Chemical Bonding Materials Molecules Atoms Atoms = protons (p) + neutrons (n) + electrons (e) Protons and neutrons are made of quarks Quantitative measurements need units: metricor (SystemeInternational) or mks(meter-kilogram-second) unitsmeter (m) for lengthcubic meter (m3) for volumekilogram (kg) for massKelvin (K) for temperaturesecond (s) for timemole (mol) for amount of substanceChapter 22 Typical parameters: atoms are 10-27 Neutron+ 10-27 ProtonCharge (coulomb)Mass (kg) Nuclear radius is measured in 10-15m = fm Electron radii measured in = nm = 10-10m Fundamental charge = e = 10-19C2 Chapter The Structure of Atoms Atomic number ZIn neutralatom: # of protons = # of electrons = Atomic number ZValue of Z is different for each element H (Z = 1), O (Z = 8), Fe (Z = 26) IsotopesSame element (same Z) might have different # of neutrons (N)Same chemistry (same # of electron)Hydrogen: protium H-1.

2 Deutiruim H-2 or D tritium H-3 or T Atomic mass unit (amu): 1/12 of C-12 (<1% of C-13, isotopically pure) Atomic mass AA Z + N Atomic weight (periodic table) weighted average of all isotopesXAZE lementMass numberAtomic numberH11H21H31 Chapter Periodic Table of Elements3 Chapter Electrons in Atoms One of the first models: Bohr atomBohr assumed that electrons orbited the nucleus due to Coulomb forcesElectrons could only be in one of infinitely many discrete statesExplained spectra of H, He+, Li2+ Not working for atoms with >1 electron Need a quantum mechanics approach Will summarize main can have wave-like propertiesde Broglie suggested that any particles with momentum p(=m V, non-relativistically) displays wave-like properties: = can have particle-like propertiesLight as a propagating wave of electric and magnetic fields Wavelength determines the colourEinstein showed that light is divided into packets of E (so called photons )E = h fChapter 26 The Heisenberg Uncertainty can no longer describe a particle as having a precise position x and momentum Heisenberg Uncertainty PrincipleWe cannot know both x and p simultaneously with arbitrary precisionThere is a limit !

3 X p h/ 2 We do know exactly where an electron is, we can t know how fast it is in atoms can only be in certain states The energy is quantized only certain levels are permittedThe level can change, but only if exactly the correct energy is emitted or quantum numbers: The state of an electron in an atom is described by a set of 4 quantum numbersprinciple quantum numbern= 1, 2, 3, ..nsubsidiary or azimuthal numberL = 0, 1, 2, .. n-1magnetic quantum numbermL= 0, 1, 2, .. n-1spinquantum number mS= Pauli Exclusion Principle: Two electrons in an atom can never have exactly the same set of all 4 quantumnumbers (n, L, mL, mS)4 Chapter 27 Possible electron states So .. what are the possible combinations of states? Notation nL1K0s2L1p3M2d4N3f We can describe an electron state as 1sor 3por 5fEnergy levels of the electron states the energy of an electron is determined by its state If there is only oneelectron in the atom, the energy depends on n alone If there are more electrons, they interact (repel each other), and so different states have different energiesChapter 28 Shell and Subshell EnergiesSchematic representation of the relative energies of the electrons for various shells and subshells (adopted from Callister, Figure )EPrinciple quantum number, nValence electrons The electrons in the outer shell (largest n)

4 Are called valence electrons Very important for Chemical reactions and holding matter together If the outer shell is filled, the atom is inert If not, there is a tendency for the outer shell to become filledElectronegative if it is almost full, the atom has a tendency to gain enough electrons to fill the shell Electropositive if it is almost empty, it easily losses the electrons in the outer shell5 Chapter 29 Periodic Table of ElementsChapter 210 Valence electrons The electrons in the outer shell (largest n) are called valence electrons Very important for Chemical reactions and holding matter together If the outer shell is filled, the atom is inert If not, there is a tendency for the outer shell to become filledElectronegative if it is almost full, the atom has a tendency to gain enough electrons to fill the shellElectropositive if it is almost empty, it easily losses the electrons in the outer shell6 Chapter 211 ElectronegativityThe electronegativity of the elements, adapted from Smith&HashemiElectronegativityis a degree to which an atom attracts electron to itselfChapter 212 Chemical reactivity: valence e-sNoble gases: s2p6configurationNe: 1s22s2p6 Electronegative if it is almost full, the atom has a tendency to gain enough electrons to fill the shellCl.

5 1s22s2p63s2p5or [Ne] 3s2p5Cl + 1e = Cl-negative ion (anion)Electropositive if it is almost empty, it easily losses the electrons in the outer shell (typically metals)K: 1s22s2p63s2p64s1or [Ar] 4sK 1e = K+ positive ion (cation)Oxidation number or oxidation state - not all allowed!!!7 Chapter 213 Periodic Table of ElementsChapter Types of Atomic and molecular bonds Primary Atomic (large interatomic forces, nondirectional, electron transfer, coulombic forces) (large interatomic forces, localized (directional), electron sharing) (large interatomic forces) nondirectional Secondary Atomic and molecular dipole dipole bonds8 Chapter 215 Examples and characteristics of 5 types of bondsIncrease in Bonding energy over similar molecules without hydrogen , HFHydrogenLow melting and boiling , Ar,Kr, Xe, CHCl3 Fluctuating or permanent dipoleNondirected bond, structures of very high coordination and density; high electrical conductivity; , Na, Cu, TaMetallicSpatially directed bonds, structures with low coordination; low conductivity at low temperature for pure crystals3-8 Diamond, Si, GeCovalentNondirected Bonding , giving structures of high coordination.

6 No electrical conductivity at low temperature5-10 LiF, NaCl, CsClIonicDistinct characteristicsTypical energies, eV/atomExamplesBond typeChapter Ionic bondingNaClNa1s22s2p63sCl1s22s2p63s2p5Na +1s22s2p6Cl-1s22s2p63s2p6 Typically between highly electropositive (metallic) and electronegative (nonmetallic) elements9 Chapter 217 Force and Energy DiagramThe interionic energycan be defined as the energy needed to rip a compound into its components placed far apart (ENET( ) = 0)norepulsiveattractiveNETabaeZZEEE++=+= 4221 Fnet= Fattactive+ FrepulviseChapter 218 Q.: Calculate attractive force between Na+and Cl-ion at equilibrium interionic separation distance10 Chapter 219 Interionic Energies- the energy needed to rip a compound into its components placed far apart (Enet( ) = 0norepulsionattractionnetabaeZZEEE+ +=+= 4221 Q2.: Calculate the net potential energy of a simple Na+Cl-ion pair in equilibrium (using the equation above) and assuming that n = 9 Chapter 220 Bonding Energies in Ionic Solids Typically large lattice energies (Table )600- 3000 kJ/mol High melting temperatures801oC for NaClFor NaCl: ENa+Cl-= - 10-19J = ( per ion)Compare to (elsewhere): big !)

7 11 Chapter 221 Ionic Packing X-ray crystallography tells us most of ionic compounds (NaCl, CsCl, LiF, etc) are crystals Ionic radii of selected ions are listed in the table below (note increase in size, as their principle quantum number increases) No preferred orientation (nondirectional character of the ionic bond) Geometrical arrangements (coordination) and neutrality is + + + + +Ionic radius (nm)AnionIonic radius (nm)CationChapter 222 Coordination and neutrality in ionic crystalsCsCl: 8 Cl-ions can pack around Cs+R(Cs+)/R(Cl-)== / = : 6 Cl-ions can pack around Na+R(Na+)/R(Cl-)== / = 223 Evaluation of the Madelung a cube of 8 a line of alternating in sign ions, with distance R between a lattice: lattice sum calculation can be used to estimate Madelung constant, =jijR StructureChapter 224 Madelung constant for line of x R 13 Chapter 225 ElectronegativityChapter Covalent Bonding Takes place between elements with smalldifference in electronegativity- F, O, N, Cl, H, C, s and pelectrons are commonly sharedto attain noble-gas electron configuration Multiplebonds can be formed by one atom14 Chapter 227 Hydrogen molecule2 H: 1s H21s2H +H H :H(electron pair covalent bond)++Hydrogen moleculeBonding interactionChapter 228 Other Covalent Molecules F 2s22p5 O 2s22p4 N 2s22p3 Bond energies and bond lengths of selected covalent bonds150 - 890 kJ/mol (or ~ ) (H2) (C-Cl)15 Chapter 229 Covalent Bonding by carbon C 1s22s22p2 Formation of 4 equal covalent bond: hybridization sp3hybridization other sp2, spChapter 230 Carbon-containing molecules C and H.

8 Hydrocarbons Structural formulas: CH4(methane), C2H6(ethane), and C4H10(normal butane) Saturated CnH2n+2- strong bonds inside molecule, weak between molecules Unsaturated CnH2n, CnH2n-2- generally more reactive16 Chapter 231 Benzene (C6H6)important for some polymeric materialsChapter Metallic Bonding Consider Na metal: what holds it together? In solid state metal atoms are packed in a systematic pattern or crystal Structure The valence electrons are weakly bonded to the positive-ion cores and can readily move as free electrons ++++++++++++++++Valence electrons in the form of electron charge clouds or electron gas Positive ion cores17 Chapter 233 Nonlocalized behavior of electrons in metals High electrical conductivity- electrons move easily when E-field applied High thermal conductivity- the electrons can carry energy through metal High density- outer shell removed from atoms, so can be packed together High ductility- if metal distorted, bent, electrons can quickly move to compensate- metal bond is not directionalChapter Secondary Bonding Fluctuating or permanent dipoles (also called physical bonds, or van der Waals bonds or forces) weak relatively to the primary Bonding (2-5eV/atom or ion)

9 ~ or ~ 10 kJ/mol always present, but overwhelmed by other interactionmost easily observed in inert gases dipoles to be 235 Electric dipoleaxkexkeaxkeU+ + =2222consider a + and - charge (e) separated by a distance a+--a/2+ a/20 Dipole moment [debye]: = e aWhat is the PE (U) of two dipoles in a distance xapart?+--a/2+ a/20+-x-a/2x+ a/2xChapter 236 Fluctuation Dipole Noble-gas elements: s2p6 Stronger effect for larger electron shells Boiling temperature increases as a function of ZSchematic representation of how the van-der-Waals bond is formed by interaction of induced 237 Permanent Dipoles When a covalent molecule has permanent dipole? Depends on geometry of the CH4? CH3Cl?H2O?Q.:Calculate the dipole moment associated with the ionic model of the water molecule. The length of the O-H bond is and the angle between the bonds is Hydrogen bond: permanent dipole-dipole interaction for the molecules with a hydrogen atoms bonded to a highly electronegative element (F, Cl, O, N) : H2O, polymeric materialsPermanent dipole bond: a secondary bond created by the attraction of molecules that have permanent dipolesChapter 238Q.

10 :Calculate the dipole moment associated with the ionic model of the water molecule. The length of the O-H bond is and the angle between the H-O-H bonds is Mixed Bonding Ionic - Covalent Mixed BondingAB:where XAand XBare the electronegativity of the atoms A and B (Table above) ZnSe, GaAs Metallic - Ionic Mixed BondingTypical for intermetallic compounds between elements with significant difference in : NaZn13() = 2411%100__%BAXX echaracterionicChapter 240 Summary Primary Atomic (large interatomic forces, nondirectional, electron transfer, coulombicforces) (large interatomic forces, localized (directional), electron sharing) (large interatomic forces, nondirectional) Secondary Atomic and molecular bonds dipole dipole bondsnorepulsionattractionnetabaeZZEEE+ +=+= 4221 Dipole moment [debye]: = e a21 Chapter 241 Periodic Table of ElementsChapter How many atoms are there in 1 g of copper?