Chemical Bonding Types p.318-319 Ionic bond – ionic compound and electron transfer Structure: Metal + Non-metal or Cation+ Anion e.g. CsF, KBr, NH4NO3 (DIY: assign opposite charges) Strong attractive force in solid state, result in high m.p., b.p. Such attraction is subject to change in the presence of water: some readily dissociate in water to form independent ions (see solubility rule p.178) – strong electrolytes! 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin (B) Covalent Bond –Electron pair(s) shared equally between two identical atoms (compare to e-transfer) e.g. H2, Halogens,O2, N2, , forming molecules, and the result is more stable than two separate atoms H2 : Both 1s orbitals are filled to form covalent bond Each electron is simultaneously attracted by 2 adjacent nuclei. A strong bond, requires bond energy to break it apart and regenerate atoms diamond is strong for this reason (network covalent-bonding)! The result molecule is said to be non-polar Q: draw orbital diagram for halogen atoms to see how the diatomic molecules are formed 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin (C) Polar covalent bond Involves two types of non-metal atoms Electron pair unequally shared in between, but not to the extent to cause electron transfer, e.g. HF(g), HCl(g) electron probability is higher near F- or Cl- atom Such bond carries dipole moment or polarity is said to be polar Polarity increases m.p. & b.p., and water-loving feature of a substance! Note: the difference between HCl(g) and HCl(aq) Q: Identify which bond of the following would be most likely polar covalent- Mg-O, Cu-Cl, N-N , C-O , I – I , P-H , N- H 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Electronegativity Ability of an atom to attract bonding electrons to itself is called ~ Relate E.N. in Fig 11.3 p.320 to unfilled orbitals; higher values indicate “electron-pulling” characters; identify the trend of E.N. going down a column and across a period Elements with highest E.N.: F > O > N = Cl Lowest three elements ? Group 8 not included in the E.N. table Use absolute difference in E.N. values to determine bond polarity, the larger the difference makes the bond more polar. Self-check 11.1 p.321 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Difference is zero  pure covalent (non-polar) bond. difference approaches 2.0  almost ionic Water molecules are polar, mutually attract each other. A powerful solvent. See charge distribution. Can surround and attract both cations/anions, note the position of the partially charged ends. Fig. 11.5-7, p.322-3 In water molecules, all atoms have achieved inert gas electron configuration  true for all stable compounds! P.324-5 Relative sizes of ions compared to their parent atoms: F-atom < F- true for all anions Li-atom > Li+ true for all cations Fig. 11.9 p.328 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Lewis Structures AKA “Lewis dots” or “Lewis model”, particularly important to covalent molecules (non-metal compounds), powerful tool in predicting molecular structures & writing reaction mechanisms! Steps (p.331): Obtain the sum of valence electrons (chapter 10) from all the atoms Start with one pair of electrons (or one line) to link between atoms 3) Apply Duet Rule for H-atom (maximum capacity of 1s orbital) Apply Octet Rule to arrange remaining electrons for each second-row atoms, need multiple bonds sometimes Note: learn to recognize bonding pairs and Lone pairs (non-bonding, unshared electrons) in all examples; also resonance structures 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Exceptions Octet Rule failed in these cases: 1) Odd number valence electrons : e.g. some N- containing compounds 2) Electron-deficient atoms: e.g. Be, B 3) O2 molecule Fig.11.10 p.338 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Molecular Structure Molecular geometry (3-dimensional arrangement of the atoms in molecule) is important to biological effect, catalysis etc. VSEPR Model: Valence Shell Electron Pair Repulsion Each electron pair is one repulsion unit, whether bonding or non-bonding pair, they don’t want to come close to each other On central atom: bonding pair & lone pair must keep as far as possible (lone pairs take more space compared to bonding pairs) Structure around a given atom is determined by minimizing repulsion between electron pairs Do in class: Draw all examples p.339-344 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin

Chem 2 Chapter 11 Jean Lee Lin Rules for predicting molecular structure using VSEPR model, Read p.344 box (1) Bond angle after All electrons pairs are kept as far as possible (2) Molecular structure is named based on the position of atoms not e-pairs, because Non-bonding pairs are Not considered in structure-names! (Compare NH3 vs. CH4) (3) Each double bond, triple bond is considered one repulsion unit Review Table 11.4 p.345 To avoid common mistakes: must include non-bonding electron pairs first to determine bond angle, then only consider atoms in naming the structure! Do in class: Self check 11.5 p.344, see also lab section 1/17/2019 Chem 2 Chapter 11 Jean Lee Lin