Additional Aspects of Molecular Bonding & Structure Chapters 8 and 9 BLB 12 th

8.2 Ionic Bonding Energetics of Ionic Bond Formation Na(s) + ½ Cl 2 (g) → NaCl(s)ΔH f ° = −410.9 kJ ΔH = 147 kJ/mol Lattice energy – energy required to completely separate the ions in one mole of an ionic compound Na + (g) + Cl‾(g) → NaCl(s)ΔH lattice = −788 kJ Lattice energy ↑ as ion charges ↑ and size ↓`

Hess’s Law for lattice energy (Born-Haber cycle)

8.2 Ionic Bonding Electron Configurations of Ions (also 7.4) Atoms will gain or lose electrons to achieve a noble gas configuration. Transition metal ions: s electron(s) are lost first.

8.3 Covalent Bonding Atoms share electrons.

8.4 Bond Polarity Unequal sharing of electrons in a covalent bond Electronegativity – the ability of an atom in a molecule (bonded) to attract electrons to itself

8.4 Bond Polarity Nonpolar covalent bond – electrons shared equally Polar covalent bond – electrons shared unequally due to different electronegativity values –Greater electronegativity difference, more polar the bond (higher dipole moment) Dipole moment – measured magnitude of a dipole. –Predict direction –No calculations

The greater the difference in electronegativity, the more polar the bond.

Electronegativity Electronegativity difference < 0.5 nonpolar polar > 2.0 ionic Rough guidelines only. See p Examples: C–H N–O Na–Cl Cl–Cl

9.3 Molecular Polarity Depends upon the polarities of the bonds and the molecular geometry of the molecule Bond dipole moments are vector quantities. A molecular dipole moment is the vector sum of its bond dipoles. A molecule can be nonpolar, that is, have a net dipole moment of zero, even if bond dipole(s) exist.

CO 2 a nonpolar molecule H 2 O a polar molecule

9.3 Molecular Polarity Examples KrF 2 SO 2 ICl 3 XeO 4

8.5 Drawing Lewis Structures Formal Charges (p. 307) There may be more than one valid Lewis Structure for a given molecule. Formal charges are used to determine the most reasonable structure. Calculate a formal charge (FC) for each atom: FC = (# valence e¯) − (# e¯ belonging to atom)

8.5 Drawing Lewis Structures Formal Charges Best structure? The one with the formal charges closest to zero and where the most negative charges reside on the most electronegative atoms. For H, Be, and B, formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.

8.6 Resonance Structures Lewis structures and the VSEPR theory are means by which we try to mimic or predict the experimentally determined properties of molecules. When a combination of single and multiple bonds are used, it implies that the bond lengths are unequal. They’re not!

Resonance Structures The measured bond lengths are an average of the representative structures; somewhere between a single and double bond length.

Resonance The organic compound benzene, C 6 H 6, has two resonance structures. It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.

8.8 Strengths of Covalent Bonds Bond enthalpy – energy required to break a bond Enthalpy of reaction – estimate from difference of the bonds broken minus the bonds formed

413 kJ kJ328 kJ kJ −104 kJ

9.4 Orbital Overlap & 9.5 Hybrid Orbitals Electrons exist in orbitals. Valence Bond Theory – bonding model where orbitals overlap to form bonds Hybridization combines orbitals into hybrid orbital sets that match experimentally determined geometried. σ bond – one area of overlap π bond – two areas of overlap

Orbital Overlap σ bonds – single area of overlap

Orbital Overlap in the formation of H 2

sp Hybrid Orbitals

sp 2 Hybrid Orbitals

sp 3 Hybrid Orbitals

Hybrid Orbital Overlap

9.6 Multiple Bonds

C 2 H 4 – the whole picture

C 2 H 2 – the whole picture