复旦大学：《普通化学 General Chemistry》课程教学资源（课堂讲义）Chap 3 - Chemical Bonds, Valence Bond Method, and Molecular Shapes

Part 3- Chemical Bonds. Valence Bond Method, and Molecular Shapes Reference: Chapter 9-10 in textbook

1 PART 3 – Chemical Bonds, Valence Bond Method, and Molecular Shapes Reference: Chapter 9—10 in textbook

∨ alence electrons Valence electron Define: the outer shell electrons Important for determination of chemical properties Valence electrons AⅢAⅢ AA VA VIAⅦAWA Li·Be·B :N·:O::F::Ne In general the number of valence electrons of a representative element is equal to the group number

Valence Electrons  Valence Electron  Define: the outer shell electrons  Important for determination of chemical properties 2

lonic Bond lonic bond Define: Attraction between cation (s )and anion(s) Cl Sodium atom Chlorine atom Sodium ion Chloride ion (a cation) (an anion) Sodium chloride(nacd Usually ionic bond is formed between an active metal atom and an active non-metal atom 3

Ionic Bond  Ionic bond  Define: Attraction between cation(s) and anion(s)  Usually ionic bond is formed between an active metal atom and an active non-metal atom. 3

lonic Bonds Which compounds have ionic bonds? Cation: formed from active metal atoms, ( Group IA and lIA),e.g. Li, Na, Mg, K, Ca Anion: formed from active non-metal atoms, (Group VIA and VIIA), e.gO, F, S, Cl, Br, I How to represent ionic bonds? Cation and anions are written in brackets with the outmost shell of electrons and charges

Ionic Bonds  Which compounds have ionic bonds?  Cation: formed from active metal atoms, (Group IA and IIA), e.g. Li, Na, Mg, K, Ca, …  Anion: formed from active non-metal atoms, (Group VIA and VIIA), e.g. O, F, S, Cl, Br, I, …  How to represent ionic bonds?  Cation and anions are written in brackets, with the outmost shell of electrons and charges. 4

Covalent bond Covalent bond formed between two(or more)non-metals, or one non-metal and one metal with low activity Atoms are connected by one pair(or more pairs)of shared electrons Examples H2, O2, N2, HCl, H2O, Co2, Co, C2 Br2 GaAs, ZnSe, Hgs

Covalent Bond  Covalent bond:  formed between two (or more) non-metals, or one non-metal and one metal with low activity.  Atoms are connected by one pair (or more pairs) of shared electrons.  Examples:  H2, O2, N2, HCl, H2O, CO2, CO, C2Br2.  GaAs, ZnSe, HgS 5

Lewis structure Octet rule Lewis structure A simple way to show valence electrons and molecular structure Octet Rule(useful but not always true) Octet rule: Groups of 8 electrons around each atom are most stable Additional important concepts Single bond, Double bond, Triple bond Lone pair electrons(Nonbonding electrons 6

Lewis Structure & Octet Rule  Lewis structure  A simple way to show valence electrons and molecular structure.  Octet Rule (useful but not always true)  Octet rule: Groups of 8 electrons around each atom are most stable.  Additional important concepts  Single bond, Double bond, Triple bond  Lone pair electrons (Nonbonding electrons) 6

How to Write lewis structures Step S:(see book, pg. 330) (1)Write the atoms in the correct arrangement (2) Calculate the total valence electrons (3)Place one pair of e between each atoms; (4)Beginning at structure outside, place e in pairs until reaching octet: (5)Extra e are put in the central atoms;(Attention! (6)If not enough e, make double/triple bonds (7) Check possible resonance structures 7

How to Write Lewis Structures  Steps: (see book, pg. 330)  (1) Write the atoms in the correct arrangement;  (2) Calculate the total valence electrons;  (3) Place one pair of e between each atoms;  (4) Beginning at structure outside, place e in pairs until reaching octet;  (5) Extra e are put in the central atoms; (Attention!)  (6) If not enough e, make double/triple bonds.  (7) Check possible resonance structures. 7

Questions Please write the following molecules with Lewis structure H2, O2, n2, HCL, HCIO, H2o, CO2, Co, C2Br23 HNO. HCO H2SO4, H3PO4, HCIO4 (Does the octet rule work here? Why this rule can be broken? 8

Questions  Please write the following molecules with Lewis structure:  H2, O2, N2, HCl, HClO, H2O, CO2, CO, C2Br2;  HNO3, H2CO3;  H2SO4, H3PO4, HClO4. (Does the octet rule work here? Why this rule can be broken?) 8

Additional Concepts in Lewis structure Coordinate bond: a covalent bond in which both electrons are provided by one atom e.g. CO, Lone pair e: a valence e pair without bonding or sharing with other atoms e.g.H2O, NH3, co2 Bond order, Bond length, Bond energy High bond order← Short bond length← High bond energy(requiring more energy to break a bond)

Additional Concepts in Lewis Structure  Coordinate bond: a covalent bond in which both electrons are provided by one atom.  e.g. CO,  Lone pair e: a valence e pair without bonding or sharing with other atoms.  e.g. H2O, NH3, CO2.  Bond order, Bond length, Bond energy  High bond order ↔ Short bond length ↔ High bond energy (requiring more energy to break a bond) 9

Bond Length, Energy Order Bond order Bond length bond Ave Length ve Energy/k mol- H-H 74 pm 432 Average distance H-C 109p 415 H-N 101 390 between two 9 460 H-Cl l27 428 bonding atoms H-Br 141 362 154 345 Bond energy C=C 133 615 C≡ 120 Energy required to N≡N 110 942 Cl-cl 240 break a given bond Br-Br 190 267 149 (in moles)

Bond Length, Energy & Order  Bond order  Bond length  Average distance between two bonding atoms.  Bond energy  Energy required to break a given bond (in moles). 10