Chapter 12 Structures and Characteristics of Bonds Objectives

Transcription

1 Objectives 1. To learn about ionic and covalent bonds and explain how they are formed - what holds compounds together? 2. To learn about the polar covalent bond are all covalent bonds equal? 3. To understand the nature of bonds and their relationship to electronegativity why some atoms form ionic and covalent compounds at different times 4. To understand bond polarity and how it is related to molecular polarity how the bonds in a molecule impact how the molecule behaves + - Bonding Summary

2 A. Types of Chemical Bonds Bond force that holds groups of atoms together and makes them function as a unit Bond energy energy required to break a chemical bond Covalent and Ionic bonds are the strongest types Representations of the methane molecule

3 A. Types of Chemical Bonds Ionic Bonding (a) (b) Ionic compound results when a metal reacts with a nonmetal

4 Properties of Ionic Compounds Typical State of Matter? Found as..? Electrolytes? Conductivity? Soluble in Water? Melting/Boiling Points?

5 A. Types of Chemical Bonds Covalent Bonding A covalent bond results when electrons are shared by nuclei

6 Energy Released as H 2 Molecule Forms

7 A. Types of Chemical Bonds Covalent Bonding Gives rise to small or large molecules or 3-D structures Water molecule Naphthalene Part of a Protein Diamond

8 Properties of Covalent Compounds Typical State of Matter? Found as..? Electrolytes? Conductivity? Soluble in Water? Melting/Boiling Points?

9 A. Types of Chemical Bonds Covalent Bonding A polar covalent bond results when electrons are shared unequally by nuclei One atom attracts the electrons more than the other atom

10 B. Electronegativity Electronegativity the relative ability of an atom in a molecule to attract shared electrons to itself Increases from left to right across a period Decreases down a group of representative elements

11 Electronegativity Values

12 B. Electronegativity The polarity of a bond depends on the difference between the electronegativity values of the atoms forming the bond

13 B. Electronegativity

14 C. Bond Polarity and Dipole Moments A dipole moment results when a polar molecule has a center of positive charge separate from a center of negative charge

15 C. Bond Polarity and Dipole Moments Water molecule dipole moment

16 C. Bond Polarity and Dipole Moments The polarity of water affects its properties Permits ionic compounds to dissolve in it Causes water to remain liquid at higher temperature (boiling point is elevated) Hydrogen Bonds

17 Like Dissolves Like Polar compounds dissolve in polar liquids Non-polar compounds dissolve in nonpolar liquids Like Dissolves Like Video

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19 Objectives 1. To learn about stable electron configurations 2. To learn to predict the formulas of ionic compounds 3. To learn about the structures of ionic compounds 4. To understand factors governing ionic size

20 A. Stable Electron Configurations and Charges on Ions Atoms in stable compounds often have a noble gas electron configuration Metals lose electrons to reach noble gas configuration Nonmetals gain electrons to reach noble gas configuration How is salt created from sodium and chlorine atoms?

21 A. Stable Electron Configurations and Charges on Ions In each case what does the resulting ion have the same electron configuration as?

22 A. Stable Electron Configurations and Charges on Ions

23 A. Stable Electron Configurations and Charges on Ions Predicting Formulas of Ionic compounds Chemical compounds are always electrically neutral

24 A. Stable Electron Configurations and Charges on Ions Predicting Formulas of Ionic compounds Chemical compounds are always electrically neutral Use the Periodic Table to predict the formulas and show the electron configuration of the ions in: Sodium Fluoride Calcium Bromide Aluminum Oxide Barium Telluride

25 B. Ionic Bonding and Structures of Ionic Compounds Structures of Ionic Compounds Ions are packed together to maximize the attractions between ions X-Ray Crystallography

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27 B. Ionic Bonding and Structures of Ionic Compounds Structures of Ionic Compounds Cations are always smaller than the parent atom Anions are always larger than the parent atom Radii in picometers (10-12 m)

28 B. Ionic Bonding and Structures of Ionic Compounds Ionic Compounds Containing Polyatomic Ions Polyatomic ions generally act as simple ions The covalent bonds hold the polyatomic ion together so it behaves as a unit. It has an overall charge like a simple ion

29 A. Writing Lewis Structures In writing Lewis structures we include only the valence electrons Most important requirement Atoms achieve noble gas electron configuration (octet rule, duet rule)

30 A. Writing Lewis Structures Bonding pairs are shared between 2 atoms Unshared pairs (lone pairs) are not shared and not involved in bonding

31 A. Writing Lewis Structures

32 A. Writing Lewis Structures In order to achieve a stable inert gas configuration different atoms typically form certain numbers of bonds. How many bonds would be formed by? C H O Cl N Br These numbers are sometimes called the valency of the element

33 More Complex Molecules Draw the Lewis Structure for: CH 3 CHOH CH 2 NH 2 How many bonds does each atom tend to make? What is the molecular structure? How many valence electrons? Distribute the electrons to satisfy each atom

34 B. Lewis Structures of Molecules with Multiple Bonds Single bond covalent bond in which 1 pair of electrons is shared by 2 atoms e.g. Ethane, C 2 H 6 Double bond covalent bond in which 2 pairs of electrons are shared by 2 atoms e.g. Ethylene, C 2 H 4 Triple bond covalent bond in which 3 pairs of electrons are shared by 2 atoms e.g. Acetylene, C 2 H 2

35 B. Lewis Structures of Molecules with Multiple Bonds A molecule shows resonance when more than one Lewis structure can be drawn for the molecule When you can write resonance structures this generally indicates stability of the molecule Draw resonance structures for SO 2 and C 6 H 6

36 Lewis Structures Check List Count the valence electrons Arrange atoms into a likely molecule (think about valencies) Put in initial bonds Arrange extra electrons in multiple bonds and/or lone pairs Are all the valence electrons used? Does each atom have a noble gas electron configuration?

37 How Do Lewis Structures Explain Polyatomics? A polyatomic ion is a charged molecule. It has covalent bonds +/- extra electrons Can Lewis structures explain the common occurrence of polyatomics?

38 B. Lewis Structures of Polyatomic Ions Consider the Lewis structure for the cyanide ion, CN - Number of valence electrons from C and N is 4+5 = 9 Plus one from the negative charge = 10 Apply the octet rule to the various options :C N: -1 charge Draw the line structures, including lone pairs, for carbonate, sulfate, hydroxide, nitrate and ammonium ions. Show also the resonance structures where relevant

39 B. Lewis Structures of Molecules with Multiple Bonds Some Exceptions to the Octet Rule Boron incomplete octet Molecules containing odd numbers of electrons NO and NO 2

40 Objectives 1. To understand molecular structure and bond angles 2. To learn to predict molecular geometry from the number of electron pairs 3. To learn to apply the VSEPR model to molecules with double bonds

41 Hemoglobin is the ironcontaining oxygentransport metalloprotein in the red cells of the blood in mammals and other animals. Hemoglobin transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the oxygen load. Hemoglobin

42 Draw possible structures that you can think of for the molecules below. Think in three dimensions. A 2, AB, AB 2, AB 3, AB 4, AB 5, AB 6

43 Structures for some simple molecules

44 A. Molecular Structure Three dimensional arrangement of the atoms in a molecule Water, H 2 O - bent

45 A. Molecular Structure Linear structure atoms in a line Carbon dioxide, CO 2

46 A. Molecular Structure Trigonal planar atoms in a triangle Boron Trifluoride, BF 3

47 A. Molecular Structure Tetrahedral structure Methane, CH 4

48 B. The VSEPR Model Valence shell electron pair repulsion (VSEPR) model Molecular structure is determined by minimizing repulsions between electron pairs Pairs of electrons in bonds and lone pairs influence the molecular structure 2 balloons 3 balloons 4 balloons 5 balloons 6 balloons

49 B. The VSEPR Model Two Pairs of Electrons BeCl o - linear

50 B. The VSEPR Model Three Pairs of Electrons BF o trigonal planar

51 B. The VSEPR Model Four Pairs of Electrons CH o tetrahedral

52 B. The VSEPR Model Predicting Molecular Structure using the VSEPR model 1. Draw the Lewis structure 2. Count the pairs of electrons and arrange to minimize repulsions 3. Determine the positions of the atoms 4. Name the molecular structure according to the position of the atoms not the lone electron pairs e.g. NH 3 has four pairs of electrons surrounding the N atom which form a tetrahedron. The molecule is named however as a trigonal pyramid

53 B. The VSEPR Model Predicting molecular shapes H 2 O molecular shapes molecular shapes 2

54 C. Molecules with Double Bonds When using VSEPR model to predict molecular geometry of a molecule a double bond is counted as the same as a single electron pair ethylene H-C-H angle is 120 o