CHAPTER 2: BONDING AND PROPERTIES Properties depend on - arrangement of atoms - interactions between atoms Study Bonding: • What promotes bonding? What types of bonds are there? • What properties are inferred from bonding?

Atomic Structure (Freshman Chem.) atom – electrons – 9.11 x 10-31 kg  protons neutrons nucleus of atom: “size of a point on a page” -! فراغ atomic number = No. of protons in nucleus of atom = No. of electrons of neutral species  A [=] atomic mass unit = amu = 1/12 mass of 12C   Atomic wt = wt of 6.023 x 1023 molecules or atoms   Unit 1 amu/atom = 1g/mol C 12.011 H 1.008 etc. } 1.67 x 10-27 kg

Atomic Structure Valence electrons: in the outer-shell determine all of the following properties Chemical: reactivity Electrical: conductivity Thermal: conductivity Optical: transmittance

Electronic Structure - Review Electrons have wavelike and particulate properties. This means that electrons are in orbitals defined by a probability. Each orbital at discrete energy level 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 (direction in space) 1, 3, 5, 7 (-l to +l) ms = spin ½, -½

Electron Energy States Electrons... • have discrete energy states • tend to occupy lowest available energy state. 1s 2s 2p K-shell n = 1 L-shell n = 2 3s 3p M-shell n = 3 3d 4s 4p 4d Energy N-shell n = 4

SURVEY OF ELEMENTS - Review • Most elements: Electron configuration not stable. Electron configuration (stable) ... 1s 2 2s 2p 6 3s 3p 3d 10 4s 4p Atomic # 18 36 Element 1 Hydrogen Helium 3 Lithium 4 Beryllium 5 Boron Carbon Neon 11 Sodium 12 Magnesium 13 Aluminum Argon Krypton • Why? Valence (outer) shell usually not filled completely.

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 Primary Bonding: Ionic, Covalent and Metallic example: C (atomic number = 6) 1s2 2s2 2p2 valence electrons

Electronic Configurations ex: Fe - atomic # = 26 1s2 2s2 2p6 3s2 3p6 3d 6 4s2 valence electrons 1s 2s 2p K-shell n = 1 L-shell n = 2 3s 3p M-shell n = 3 3d 4s 4p 4d Energy N-shell n = 4

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

Electronegativity • Ranges from 0.7 to 4.0, • Large values: tendency to acquire electrons. Smaller electronegativity Larger electronegativity

Ionic bond – metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities   ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 [Ne] 3s2  1s 2 2s 2p 6 (stable) 10 Neon Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6 [Ne] [Ne]

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

Examples: Ionic Bonding • Predominant bonding in Ceramics NaCl MgO Give up electrons Acquire electrons CaF 2 CsCl

Covalent Bonding similar electronegativity  share electrons bonds determined by valence – s & p orbitals dominate bonding Example: CH4 shared electrons from carbon atom from hydrogen atoms H C CH 4 C: has 4 valence e -, needs 4 more H: has 1 valence e -, needs 1 more Electronegativities are comparable.

Primary Bonding Metallic Bond -- delocalized as electron cloud Ionic-Covalent Mixed Bonding % ionic character =   where XA & XB are Pauling electronegativities %) 100 ( x Ex: MgO XMg = 1.3 XO = 3.5

SECONDARY BONDING + - Arises from interaction between dipoles • Fluctuating dipoles asymmetric electron clouds + - secondary bonding H 2 ex: liquid H • Permanent dipoles-molecule induced + - -general case: secondary bonding Adapted from Fig. 2.14, Callister 7e. Cl Cl -ex: liquid HCl secondary H H bonding secondary bonding -ex: polymer secondary bonding

Summary: Bonding Type Bond Energy Comments Ionic Large! Nondirectional (ceramics) Covalent Variable Directional (semiconductors, ceramics polymer chains) large-Diamond small-Bismuth Metallic Variable large-Tungsten Nondirectional (metals) small-Mercury Secondary smallest Directional inter-chain (polymer) inter-molecular

Interactions Between Atoms Far Atoms: Attraction Close Atoms: Repulsion + - Spacing (r) Atomic Radius (R) + - + - r Dynamic Equilibrium + - Equilibrium spacing = ro ro = 2 R

Bonding Curve

Bonding Curve Energy – minimum energy most stable r A B EN = EA + ER = Energy balance of attractive and repulsive terms r A n B EN = EA + ER = - ro Attractive energy EA Net energy EN Repulsive energy ER Interatomic separation r

Bonding - Estimations r A B E = certain f(r) e.g. E = Eo = Estimation of Bonding Energy (Eo) and Equilibrium Spacing (ro) Given: Bonding occurs at energy minimum At minimum: Derivative dE/dr = g(r) = 0 Solve g(r) = 0 for r ro Substitute r=ro in the Energy Eqn: Get Eo = E (r=ro) Atomic radius R = ro/2 r A n B E = certain f(r) e.g. E = - Eo = “bond energy” Energy r o For many atoms, ro is approximately 0.3 nm.

Properties From Bonding: Tm • Bond length, r • Melting Temperature, Tm r o Energy r • Bond energy, Eo Eo = “bond energy” Energy r o unstretched length smaller Tm larger Tm Tm is larger if Eo is larger.

Properties From Bonding : a • Coefficient of thermal expansion, a D L length, o unheated, T 1 heated, T 2 Coefficient of thermal expansion D L = a ( T - T ) 2 1 L o • a ~ symmetry at ro r o Smaller a Larger a Energy unstretched length Eo a is larger if Eo is smaller.

Summary: Primary Bonds Ceramics Large bond energy large Tm large E small a (Ionic & covalent bonding): Metals Variable bond energy moderate Tm moderate E moderate a (Metallic bonding): Polymers Directional Properties Secondary bonding dominates small Tm small E large a (Covalent & Secondary): secondary bonding Bonding in Solids > Liquids > Gases

ANNOUNCEMENTS Reading: Focus on Atomic Bonding in Solids Core Problems: 2.7, 2.8, 2.12, 2.13, 2.17, 2.19, 2.21 (Callister 3rd Addition) Quiz: Next Class

Quiz Rank materials A, B and C according to Increasing melting temperature > > Decreasing bonding energy > > Increasing Atomic Radius > > Increasing coefficient of thermal expansion > > Energy Atomic Spacing r A B c

Quiz Rank materials A, B and C according to Increasing melting temperature > > Decreasing bonding energy > > Increasing Atomic Radius > > Increasing coefficient of thermal expansion > > Energy A Atomic Spacing r B c