CHAPTER 2: BONDING AND PROPERTIES

Transcription

1 CHAPTER 2: BONDING AND PROPERTIES ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What properties are inferred from bonding? Chapter 2-1

2 Atomic Structure (Freshman Chem.) atom electrons 9.11 x kg protons } neutrons 1.67 x kg atomic number = # of protons in nucleus of atom = # of electrons of neutral species A [=] atomic mass unit = amu = 1/12 mass of 12 C Atomic wt = wt of x molecules or atoms 1 amu/atom = 1g/mol C H etc. Chapter 2-2

3 Atomic Structure Valence electrons determine all of the following properties 1) Chemical 2) Electrical 3) Thermal 4) Optical Chapter 2-3

4 Electronic Structure Electrons have wavelike and particulate properties. This means that electrons are in orbitals defined by a probability. Each orbital at discrete energy level is determined by quantum numbers. Quantum # Designation n = principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.) l = subsidiary (orbitals) s, p, d, f (0, 1, 2, 3,, n-1) m l = magnetic 1, 3, 5, 7 (-l to +l) m s = spin ½, -½ Chapter 2-4

5 Electron Energy States Electrons... have discrete energy states tend to occupy lowest available energy state. 4d 4p N-shell n = 4 3d 4s Energy 3p M-shell n = 3 3s 2p 2s Adapted from Fig. 2.4, Callister & Rethwisch 3e. L-shell n = 2 1s K-shell n = 1 Chapter 2-5

6 SURVEY OF ELEMENTS Most elements: Electron configuration not stable. Element Hydrogen Helium Lithium Beryllium Boron Carbon... Neon Sodium Magnesium Aluminum... Argon... Krypton Atomic # Electron configuration 1s 1 1s 2 (stable) 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p s 2 2s 2 2p 6 (stable) 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 1s 2 2s 2 2p 6 3s 2 3p s 2 2s 2 2p 6 3s 2 3p 6 (stable)... 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 (stable) Adapted from Table 2.2, Callister & Rethwisch 3e. Why? Valence (outer) shell usually not filled completely. Chapter 2-6

7 Electron Configurations Valence electrons those in unfilled shells Filled shells more stable Valence electrons are most available for bonding and tend to control the chemical properties example: C (atomic number = 6) 1s 2 2s 2 2p 2 valence electrons Chapter 2-7

8 Electronic Configurations ex: Fe - atomic # = 26 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 4d 4p 3d 4s N-shell n = 4 valence electrons Energy 3p M-shell n = 3 3s 2p 2s Adapted from Fig. 2.4, Callister & Rethwisch 3e. L-shell n = 2 1s K-shell n = 1 Chapter 2-8

9 The Periodic Table Columns: Similar Valence Structure give up 1e - give up 2e - H Li Be Na Mg K Rb Ca Sr give up 3e - Sc Y Oaccept 2e- accept 1e - inert gases S Se Te F Cl Br I He Ne Ar Kr Xe Adapted from Fig. 2.6, Callister & Rethwisch 3e. Cs Ba Po At Rn Fr Ra Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. Chapter 2-9

10 Electronegativity Ranges from 0.7 to 4.0, Large values: tendency to acquire electrons. Smaller electronegativity Larger electronegativity Adapted from Fig. 2.7, Callister & Rethwisch 3e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Chapter 2-10

11 Ionic bond metal + nonmetal Dissimilar electronegativities donates accepts electrons electrons ex: MgO Mg 1s 2 2s 2 2p 6 3s 2 O 1s 2 2s 2 2p 4 [Ne] 3s 2 Mg 2+ 1s 2 2s 2 2p 6 O 2-1s 2 2s 2 2p 6 [Ne] [Ne] Chapter 2-11

12 Ionic Bonding Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Na (metal) unstable electron Cl (nonmetal) unstable Na (cation) stable + - Coulombic Attraction Cl (anion) stable Chapter 2-12

13 Ionic Bonding Energy minimum energy most stable Energy balance of attractive and repulsive terms E N = E A + E R = A r B n r Repulsive energy E R Interatomic separation r Net energy E N Adapted from Fig. 2.8(b), Callister & Rethwisch 3e. Attractive energy E A Chapter 2-13

14 Examples: Ionic Bonding Predominant bonding in Ceramics NaCl MgO CaF2 CsCl Give up electrons Acquire electrons Adapted from Fig. 2.7, Callister & Rethwisch 3e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Chapter 2-14

15 Covalent Bonding similar electronegativity share electrons bonds determined by valence s & p orbitals dominate bonding Example: CH 4 shared electrons C: has 4 valence e -, needs 4 more CH4 H from carbon atom H: has 1 valence e -, needs 1 more Electronegativities are comparable. H C H H shared electrons from hydrogen atoms Adapted from Fig. 2.10, Callister & Rethwisch 3e. Chapter 2-15

16 METALLIC BONDING Arises from a sea of donated valence (or free) electrons (1, 2, or 3 from each atom). Energy level Free (conduction) electrons Conduction band Fermi energy Band gap Free electron cloud Positive ions Valance band Valance electrons Density of states Primary bond for metals and their alloys Chapter 2-12

17 Primary Bonding Metallic Bond -- delocalized as electron cloud Ionic-Covalent Mixed Bonding % ionic character = 1 e (X A X B ) 4 x (100%) where X A & X B are Pauling electronegativities Ex: MgO X Mg = 1.3 X O = (3.5 % ionic character 1 e 4 1.3) 2 x (100%) 70.2% ionic Chapter 2-17

18 Arises from interaction between dipoles Fluctuating dipoles Permanent dipoles-molecule induced -general case: -ex: liquid HCl SECONDARY BONDING asymmetric electron clouds secondary bonding H Adapted from Fig. 2.13, Callister & Rethwisch 3e. H secondary bonding secondary H Cl H Cl bonding ex: liquid H2 H2 H2 H secondary bonding H Adapted from Fig. 2.14, Callister & Rethwisch 3e. -ex: polymer secondary bonding Chapter 2-18

19 Type Ionic Summary: Bonding Bond Energy Comments Large! Nondirectional (ceramics) Covalent Metallic Secondary Variable large-diamond small-bismuth Variable large-tungsten small-mercury smallest Directional (semiconductors, ceramics polymer chains) Nondirectional (metals) Directional inter-chain (polymer) inter-molecular Chapter 2-19

20 Properties From Bonding: T m Bond length, r r Melting Temperature, T m Energy Bond energy, E o Energy r o smaller T m r unstretched length r o r E o = bond energy larger T m T m is larger if E o is larger. Chapter 2-20

21 Properties From Bonding : Coefficient of thermal expansion, length, Lo coeff. thermal expansion unheated, T1 heated, T2 L L Lo = (T 2 -T 1 ) ~ symmetric at r o Energy E unstretched length r o smaller r is larger if E o is smaller. o E larger o Chapter 2-21

22 Summary: Primary Bonds Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Large bond energy large T m large E small Variable bond energy moderate T m moderate E moderate Polymers (Covalent & Secondary): Directional Properties Secondary bonding dominates small T m small E large Chapter 2-22