CHAPTER 2: BONDING AND PROPERTIES ISSUES TO ADDRESS... • 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 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 6.023 x 1023 molecules or atoms   1 amu/atom = 1g/mol C 12.011 H 1.008 etc. } 1.67 x 10-27 kg

Atomic Structure Valence electrons determine all of the following properties Chemical Electrical Thermal Optical

Electronic Structure Electrons have wavelike and particulate properties. This means that electrons in orbitals are 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 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 Adapted from Fig. 2.4, Callister 7e.

SURVEY OF ELEMENTS • Most elements: Electron configuration not 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 Adapted from Table 2.2, Callister 7e. • 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 example: C (atomic number = 6) 1s2 2s2 2p2 valence electrons

Electronic Configurations 26 1s2 2s2 2p6 3s2 3p6 3d 6 4s2 valence electrons ex: Fe - atomic # = 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 Adapted from Fig. 2.4, Callister 7e.

The Periodic Table • Columns: Similar Valence Structure inert gases give up 1e give up 2e accept 2e accept 1e give up 3e 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 Adapted from Fig. 2.6, Callister 7e. 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. Adapted from Fig. 2.7, Callister 7e. (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. 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  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)

Ionic Bonding 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 = + - Attractive energy EA Net energy EN Repulsive energy ER Interatomic separation r A & B are constants that depend on the specific ionic system, while n is typically = 8. Repulsion diminishes more quickly with distance due to the exponential factor n. The magnitude of this bonding energy and the shape of the energy-versus interatomic separation curve vary from material to material, and they both depend on the type of atomic bonding. Furthermore, a number of material properties depend on Eo, the curve shape, and bonding type. Adapted from Fig. 2.8(b), Callister 7e.

Examples: Ionic Bonding • Predominant bonding in Ceramics NaCl MgO Give up electrons Acquire electrons CaF 2 CsCl Adapted from Fig. 2.7, Callister 7e. (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.

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 Covalent bonds are directional, meaning that atoms so bonded prefer specific orientations relative to one another; this in turn gives molecules definite shapes, as in the angular (bent) structure of the H2O molecule. Electronegativities are comparable. Bonding is directional. Adapted from Fig. 2.10, Callister 7e.

Metallic Bonding Metallic Bond -- delocalized as electron cloud  

Mixed Bonding Ionic-Covalent Mixed Bonding % ionic character =   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 Adapted from Fig. 2.13, Callister 7e. • Permanent dipoles-molecule induced Adapted from Fig. 2.14, Callister 7e. H Cl secondary bonding + - -general case: -ex: liquid HCl secondary bonding -ex: polymer secondary bonding

IMPORTANCE OF SECONDARY BONDING In some cases the type of bond explains materials’ properties Gecko’s have very sticky feet that cling to any surface! There are a large number of microscopic hairs on their toe-pads. Which are self cleaning! Weak intermolecular forces are established between the hairs and any surface; resulting in adhesion. Self cleaning reversible adhesives!

http://webecoist. momtastic http://webecoist.momtastic.com/2014/12/17/climb-like-a-gecko-biomimicry-enables-extremely-sticky-adhesive/

ANAMOLOUS BEHAVIOUR OF WATER exhibits the anomalous and familiar expansion upon freezing—approximately 9 volume percent expansion. its single O atom can bond to two hydrogen atoms of other water molecules. 1 water molecule has 4 H-bonds; leading to a non-close packed or open structure, hence expansion on freezing. This structure is destroyed on melting so in liquid state the water molecules are more closely packed 1 molecule has 4.5 H-bonds leads to increase in density. A watering can that ruptured along a side panel-bottom panel seam. Water that was left in the can during a cold late-autumn night expanded as it froze and caused the rupture. The arrangement of water molecules in (a) solid ice, and (b) liquid water.

CONSEQUENCES OF ANAMOLOUS BEHAVIOUR OF WATER This phenomenon explains why icebergs float. • Why, in cold climates, it is necessary to add antifreeze to an automobile’s cooling system (to keep the engine block from cracking) • Why freeze-thaw cycles break up the pavement in streets and cause potholes to form. • If there were no H-bonding water will boil at -800C! Life on earth as we know it will not be possible!

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 coeff. thermal expansion D L length, o unheated, T 1 heated, T 2 ( - 1 = a T 2 ) D L o • a ~ symmetry at ro r o smaller a larger a Energy unstretched length Eo a is larger if Eo is smaller.

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

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