Chapter 2: Atomic Structure & Interatomic Bonding

Chapter 2: Atomic Structure & Interatomic Bonding

Chapter 2: Atomic Structure & Interatomic Bonding ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What properties are inferred from bonding? Chapter 2 - 1 Geckos toe pads Extremely large number of microscopically small hairs gecko-inspired bandage http://web.mit.edu/newsoffice/2008/adhesive-0218.html http://robotics.eecs.berkeley.edu/~ronf/Gecko/index.html Chapter 2 - Atomic Structure (Freshman Chem.) atom electrons 9.11 x 10-31 kg protons -27 1.67 x

10 kg neutrons } atomic number = # of protons in nucleus of atom = # of electrons of neutral species A [=] atomic mass unit = amu = 1/12 mass of 12C Atomic wt = wt of 6.022 x 1023 molecules or atoms 1 amu/atom = 1g/mol C H 12.011 1.008 etc. Chapter 2 - 3 Isotopes 2.2 Chromium has four naturally-occurring isotopes: 4.34% of 50Cr, with an atomic weight of 49.9460 amu, 83.79% of 52Cr, with an atomic weight of 51.9405 amu, 9.50% of 53Cr, with an atomic weight of 52.9407 amu, and 2.37% of 54Cr, with an atomic weight of 53.9389 amu.

On the basis of these data, confirm that the average atomic weight of Cr is 51.9963 amu. Chapter 2 - 4 Electric Charge Atomic Model Atomic Particle Electron Charge Mass 1.6 10-19 C 9.11 10-31 Kg Proton +1.6 10-19 C Neutron

0 1.673 10-27 Kg 1.675 10-27 Kg The coulomb unit is derived from the SI unit ampere for electric current i. Current is the rate dq/dt at which charge moves through a region. Chapter 2 - Atomic Structure Valence electrons determine all of the following properties 1)

2) 3) 4) Chemical Electrical Thermal Optical Chapter 2 - 6 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)

ml = magnetic 1, 3, 5, 7 (-l to +l) ms = spin , - Chapter 2 - 7 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 3s M-shell n = 3

Adapted from Fig. 2.4, Callister & Rethwisch 8e. 2p 2s L-shell n = 2 1s K-shell n = 1 Chapter 2 - 8 SURVEY OF ELEMENTS Most elements: Electron configuration not stable. Element Atomic # Hydrogen 1 Helium 2 Lithium 3 Beryllium 4

Boron 5 Carbon 6 ... Neon 10 Sodium 11 Magnesium 12 Aluminum 13 ... 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 2 ...

Argon ... Krypton 1s 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) 18 ... 36 Adapted from Table 2.2, Callister & Rethwisch 8e. 1s 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 1 ... Why? Valence (outer) shell usually not filled completely. Chapter 2 - 9

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) 1s2 2s2 2p2 valence electrons Chapter 2 - 10 Electronic Configurations ex: Fe - atomic # = 26 1s2 2s2 2p6 3s2 3p6 4d 4p 3d 6 4s2 N-shell n = 4 valence electrons 3d 4s

Energy 3p 3s M-shell n = 3 Adapted from Fig. 2.4, Callister & Rethwisch 8e. 2p 2s L-shell n = 2 1s K-shell n = 1 Chapter 2 - 11 give up 1egive up 2egive up 3e- Columns: Similar Valence Structure accept 2eaccept 1einert gases

The Periodic Table H He Li Be O F Ne Na Mg S Cl Ar K Ca Sc Rb Sr Y Cs Ba

Se Br Kr Te I Adapted from Fig. 2.6, Callister & Rethwisch 8e. Xe Po At Rn Fr Ra Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. Chapter 2 - 12

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 8e. (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 - 13 Ionic bond metal + donates electrons nonmetal accepts electrons Dissimilar electronegativities ex: MgO Mg

1s2 2s2 2p6 3s2 [Ne] 3s2 Mg2+ 1s2 2s2 2p6 [Ne] O 1s2 2s2 2p4 O2- 1s2 2s2 2p6 [Ne] Chapter 2 - 14 Ionic Bonding Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl

Na (metal) unstable Cl (nonmetal) unstable electron Na (cation) stable + Coulombic Attraction Cl (anion) stable Chapter 2 - 15 Bonding Forces and Energies The origin of an attractive force FA depends on the particular type of bonding that exists between the two atoms.

Repulsive forces arise from interactions between the negatively charged electron clouds for the two atoms and are important only at small values of r as the outer electron shells of the two atoms begin to overlap. 2.13 Calculate the force of attraction between a K+ and an O2ion the centers of which are separated by a distance of 1.5 nm. Chapter 2 - 16 Ionic Bonding Energy minimum energy most stable Energy balance of attractive and repulsive terms EN = EA + ER = - A r +

B rn Repulsive energy ER Interatomic separation r Net energy EN Adapted from Fig. 2.8(b), Callister & Rethwisch 8e. Attractive energy EA Chapter 2 - 17 Examples: Ionic Bonding Predominant bonding in Ceramics NaCl MgO CaF 2 CsCl Give up electrons Acquire electrons

Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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 - 18 Covalent Bonding similar electronegativity share electrons bonds determined by valence s & p orbitals dominate bonding Example: CH4 C: has 4 valence e-, needs 4 more CH 4 H: has 1 valence e-, needs 1 more H Electronegativities are comparable. H

C H shared electrons from carbon atom H shared electrons from hydrogen atoms Adapted from Fig. 2.10, Callister & Rethwisch 8e. Chapter 2 - 19 Metallic Bonding Metallic Bond: -Delocalized as electron cloud -Non-directional Chapter 2 - 20 Mixed Bonding Ionic-Covalent Mixed Bonding

% ionic character = (X A - X B )2 4 1e x (100%) where XA & XB are Pauling electronegativities Ex: MgO XMg = 1.2 XO = 3.5 % ionic character 1 - e

( 3 .5 - 1 . 2 ) 2 4 x (100%) 73.4% ionic Chapter 2 - 21 Pauling Electronegativities For TiO2, XTi = 1.5 and XO = 3.5, and therefore, 2 (0.25)(3.51.5) %IC = 1 - e 100 = 63.2%

Chapter 2 - 22 SECONDARY BONDING Arises from interaction between dipoles Fluctuating dipoles asymmetric electron clouds + - secondary bonding + - ex: liquid H 2 H2 H2 H H

H H secondary bonding Adapted from Fig. 2.13, Callister & Rethwisch 8e. Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer + - H Cl secon dary secondary bonding +

secondary bonding H Cl bondi ng - Adapted from Fig. 2.15, Callister & Rethwisch 8e. secondary bonding Chapter 2 - 23 Summary: Bonding Comments Type Bond Energy Ionic

Large! Nondirectional (ceramics) Covalent Variable large-Diamond small-Bismuth Directional (semiconductors, ceramics polymer chains) Metallic Variable large-Tungsten small-Mercury Nondirectional (metals) Secondary smallest

Directional inter-chain (polymer) inter-molecular Chapter 2 - 24 Properties From Bonding: Tm Bond length, r Melting Temperature, Tm Energy r Bond energy, Eo ro Energy r smaller Tm unstretched length ro

r Eo = bond energy larger Tm Tm is larger if Eo is larger. Chapter 2 - 25 Chapter 2 - 26 2.20 Make a plot of bonding energy versus melting temperature for the metals listed in Table 2.3. Using this plot, approximate the bonding energy for copper, which has a melting temperature of 1084C. Chapter 2 - 27 Properties From Bonding : a Coefficient of thermal expansion, a length, Lo coeff. thermal expansion unheated, T1

DL = a(T2 -T1) Lo DL heated, T2 a ~ symmetric at ro Energy unstretched length ro Eo Eo r a is larger if Eo is smaller. larger a smaller a Chapter 2 - 28 Summary: Primary Bonds

Ceramics (Ionic & covalent bonding): Metals large Tm large E small a Variable bond energy (Metallic bonding): Polymers (Covalent & Secondary): secon d Large bond energy ary bondi ng moderate Tm

moderate E moderate a Directional Properties Secondary bonding dominates small Tm small E large a Chapter 2 - 29 Water (Its Volume Expansion Upon Freezing) https://www.youtube.com/watch?v=4i5r65QGUpw Chapter 2 - 30 Unusual Expansion of Water Most substances contract upon cooling. But, water expands while cooling from 4 0C until it freezes. Chapter 2 -

ANNOUNCEMENTS Reading: Chapter-2 Self-Assessment Exercise for Chap-2 Homework for Chap-2: 1,4,7,8,14,19. Chapter 2 - 32

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