Covalent and Metallic Bonding

Transcription

1 Covalent and Metallic Bonding Chemistry for Earth Scientists, DM Sherman University of Bristol Failures of Ionic Model: Ionic radius of S -2 NaCl structure CaS is highly ionic. Cell constant 5.70 Å Ca 2 radius = 1.14 Å* S -2 radius = 1.71 Å PbS (Galena) has cell constant of 5.94 Å Bond length = ½ x cell constant Pb 2 radius = 1.33 Å* S -2 radius = 1.64 Å * from bond lengths in oxides (Shannon, 1976) Page 1

2 Failure of Ionic Model: Structure of CaF 2 (Fluorite) Ionic Radius Ion IV VI VIII XII Ca F Using Pauling s first rule: R Ca (VI) /R F(IV) = 0.97 R Ca (VIII) /R F(IV) =1.07 R Ca (VIII) /R F(IV) =1.24 Predict Ca in 12-fold coordination Ionic radius of F - derived from one ionic compound does not transfer to another. Failure of Ionic Model: Sulfide Structures Triolite/Pyrrhotite (FeS) Cinnabar (HgS) Molybdenite (MoS 2 ) Mackinawite (FeS) Page 2

3 Chemical bonding: ionic vs. covalent When electrons are completely transferred between atoms to yield cations and anions, the atoms will be held together by ionic bonds. If atoms have similar electronegativities, they adopt closed-shell configurations by sharing electrons with each other; the atoms are held together by covalent bonds. Covalent Bonds Formed between atoms of similar electronegativity. Atoms are held together by sharing electrons. Sulfide minerals. Most organic compounds. In anhydrite (CaSO 4 ), the S-O bond in SO 4-2 ions is very covalent. The Ca-O bond is ionic. Page 3

4 Metallic Bond Extreme example of covalent bond: electrons are delocalized throughout the crystal. Formed between metal atoms of similar electronegativity and with weakly held electrons. Can form between metal atoms in sulfide minerals. Mackinawite (FeS) has metallic Fe-Fe bonds between shared FeS 4 tetrahedra. Chalcophiles, Lithophiles, Siderophiles.." Page 4

5 Chemical Bonding: Theoretical Pictures" Ionic Approximation ( Crystal Field Theory) Localized electrons MgO Molecular Orbital Theory (Covalent Bonds) FeO, SiO 2 Band Theory (Metallic Bonds) Delocalized electrons FeS Molecular Orbital Theory" Linear Combination of Atomic Orbitals (LCAO) The quantum states of a molecule are molecular orbitals (ψ i ). These are constructed by taking linear combinations of atomic orbitals (φ i ) to yield bonding molecular orbitals ψ = c 1 φ 1 c 2 φ 2 = and antibonding molecular orbitals ψ = c 1 φ 1 c 2 φ 2 - = Page 5

6 Molecular Orbital Theory: π vs σ bonds" σ-antibonding π-antibonding p y p x π-bonding σ-bonding Molecular Orbital Theory: Simple Diatomic Molecules" Page 6

7 Molecular Orbital Theory of Si-O bond" In quartz, the Si-O bond is partially covalent: electrons are shared by the O(2p) and Si(3p) orbitals. The core atomic orbitals (e.g., Si 1s, 2s, 2p and O 1s) are highly localized and do not participate in bonding; they retain their atomic character. Energy Si Atomic Levels 3p 3s σ* π* π σ σ* σ O Atomic Levels 2s 2p 2p 1s 2s 1s Molecular orbitals involving ligand p-orbitals and metal d-orbitals" σ-antibonding (e g symmetry) - π-antibonding (t 2g symmetry) σ-bonding (e g symmetry) π-bonding (t 2g symmetry) Page 7

8 Electronic Structures of Fe-Mn Oxides The d-orbitals can be viewed as antibonding molecular orbitals. The crystal-field splitting results from σ- vs. π- antibonding. Energy Bands in Solids Page 8

9 Insulators, Metals and Semiconductors Semiconduction in Sulfides Sulfide minerals tend to be opaque because of the small bandgap Galena (PbS) Sphalerite (ZnS) Pyrite (FeS 2 ) Chalcopyrite (CuFeS 2 ) Triolite (FeS) Molybdenite (MoS 2 ) Page 9

10 Semiconduction in Sulfides The semiconducting properties of sulfides can be exploited using geophysical measurements of outcrop electrical resistivity (conductivity). Mineral Formula Conductivity (S/m) Pyrite FeS 2 20 to 2 x 104 Chalcopyrite CuFeS 2 1 x 104 Pyrrhotite Fe 1-x S 1 x 105 Arsenopyrite FeAsS 3 x 103 Galena PbS 3 x 104 Copper Wire: Cu 5.9 x 107 The Octet Rule and Valence Shell Electron Pair Repulsion Theory This is a useful model for predicting the structures of covalent molecules. It is especially useful for molecules constructed of first-row atoms (e.g., N, C, O and F). In a molecule, each atom contributes the electrons in its outer shell (n-quantum number) as valence electrons. The total valence electrons are then grouped into electron pairs. The electron pairs are distributed in the molecule so that each atom is surrounded by 8 valence electrons (except for H, which only wants 2). Page 10

11 The Octet Rule and Valence Shell Electron Pair Repulsion Theory Example: the water molecule (H 2 O) Each H (1s 1 ) contributes 1 valence electron. The O atom (1s 2 2s 2 2p 4 ) contributes 6 valence electrons. The only way to distribute the valence electrons in the molecule is as follows: Note: each line represents a pair of electrons. Repulsion between electron pairs causes the bent geometry.. Summary Covalent bonds form when ions of high charge (e.g., Si 4, S 6 ) bond with anions (O -2 ). Bonds with S -2 tend to be covalent. Metallic bonds may form when transition metal cations are in close proximity. Molecular Orbital Theory is the complete picture of bonding that embraces ionic, covalent and metallic bonds. VSEPR can be used to predict the structures of covalent molecules. Page 11

12 Crystal Field Theory The d-orbitals: Recall the shapes of the atomic orbitals. The d-orbitals are unique: the xy,yz and xz orbitals do not have electron density along the x, y and z-axes. The z 2 and x 2 -y 2 orbitals, however, point along the x y and z-axes.. Crystal Field Theory Splitting of the d-orbital energies when surrounded by ligands: Δ tet = 4/9Δ oct Energy xy (2/5)Δ tet xz yz (3/5)Δ tet xy xz yz (3/5)Δ oct (2/5)Δ oct xy xz yz Page 12

13 Crystal Field Stabilization Energy (CFSE) d 3 configuration: (Mn 4 or Cr 3 ) Energy xy xz yz xy xz yz xy xz yz CFSE =(2/5)Δ tet CFSE = (6/5)Δ oct Octahedral site preference energy = (6/5-4/9*2/5)Δ oct = 1.02Δ oct Δ oct = kj/mole Crystal Field Stabilization Energy (CFSE) d 5 configuration: (Mn 2 or Fe 3 ) Energy CFSE = 0 xy xz yz xy xz yz CFSE = 0 xy xz yz Octahedral site preference energy = (6/5-4/9*2/5)Δ oct = 0Δ oct Page 13

14 Crystal Field Stabilization Energy (CFSE) d 8 configuration: (Ni 2 ) Energy xy xz yz xy xz yz xy xz yz CFSE =(2/5)Δ tet CFSE = (6/5)Δ oct Octahedral site preference energy = (6/5-4/9*2/5)Δ oct = 1.02Δ oct Δ oct = kj/mole Page 14