Bonding Bringing the atoms together
More than one atom Until now, we have been consumed with describing individual atoms of elements. However, isolating individual atoms in most elements is an arduous task given the scale. Most matter is composed of many, many atoms. Matter Pure substances* Elements Compounds Mixtures Homogeneous Heterogeneous *Pure is very hard to achieve in quantity
Pure substances – typically more than one atom Image by E.B. Watson So what is holding the polyatomic elements together?
Covalent bonds – the sharing of electrons 1s 1 to 1s 2 ([He]) 1s 1 & [He] 2s 2 2p 5 to [Ne] [He] 2s 2 2p 5 to [Ne]
Covalent bonding Two hydrogen atoms in close proximity can share their electrons so that each takes on an electronic structure similar to He – a noble gas. The diatomic H-H system: Images by E.B. Watson
The Covalent Bond Model Electron Swapping Best with elements of nearly identical electronegativity. e.g. H 2 Hydrogen is 1s 1 Another proton and electron would make He, a much more stable configuration- so when you bring two hydrogens together they share their electrons so each has 2 (sort of).
This electron sharing is a very strong bond. This tends to by asymmetrical, and therefor difficult to pack into repeating lattices H 2 O 2 Cl 2 F 2 Image from Gill, 1996
Image from Pauling, 1970
Diamond’s excited state Image modified from Zoltai and Stout, 1984
Sharing an electron in 2p Sharing an electron in two vacancies with 1s in 2p
Electron distribution Image after Zoltai and Stout, 1984
Double bonding Two electron pairs are shared Triple bonding Three electron pairs are shared
Coordinate covalent bonds – the shared electron is donated by atom. Most carry an overall negative charge (CO 3 ) 2- , (OH) - , (HCO 3 ) - , (PO 4 ) 3-
For atoms with appropriate electron structure, electron swapping is quite easy when matter is condensed Metallic bonding lithium (Li) 1s 2 2s 1 The 2s electrons are delocalized – move around Image by E.B. Watson
Image by E.B. Watson
Covalently bonded atoms have no charge, but most examples will orient themselves in an electric field Couloumb’s law So there must be an ionic character to the bond
Ionic bonding Ionization - losing or gaining valence electrons Can only attract so far – “solid spheres”
Ionic and covalent bonding are endmembers there is a range of between these two extremes. How do we know if the bonding is dominated by one form or another. Electronegativity! Measuring electronegativity E AB > � E(A 2 ) + � E(B 2 ) E AB = � E(A 2 ) + � E(B 2 ) + � � is excess energy due to ionic attraction (+ and -) � = 23(X A - X B ) ---X is electronegativity --
Periodicity of electronegativity
� = 23|X A - X B | F, X = 4 H, X = 2.1 Ionic-covalent character makes molecules dipolar
Silcates vs. Sulfides Silcates Si-O Sulfides M-S Fe 1.65 Fe-S:|1.8-2.5| = 0.7 S 2.5 Si 1.8 Si-O: |1.8-3.5| = 1.7 O 3.5
Ionic compounds A compound containing just two elements "-ide“. The metal is named first: NaCl sodium chloride Multiple valence state Fe 3+ or Fe 2+ . Higher valence state with the suffix "- ic" and the lower with "-ous": Fe 2 O 3 ferric oxide FeO ferrous oxide Today we tend to use a more explicit approach Fe 2 O 3 iron(III) oxide FeO iron(II) oxide
More than two elements with polyatomic anions such as hydroxide OH - carbonate (CO 3 ) 2- nitrate (NO 3 ) - phosphate (PO 3 ) 3- The compound names follow pretty logically: KOH potassium hydroxide CaCO 3 calcium carbonate Polyatomic anions can have more than one configuration, e.g. nitrate (NO 3 ) - nitrite (NO 2 ) -
Covalent compounds CO 2 carbon dioxide (Greek "di" for 2) CO carbon monoxide (Greek "mono" for 1) CCl 4 carbon tetrachloride (Greek "tetra" for 4) Empirical Formulas Always used in ionic compounds NaCl CaF 2 CH 2 O
Molecular Formulas Used for largely covalent compounds While CH 2 O accurately describes the ratio of elements in glucose, it fails to characterize the whole molecule
Structural formulas show the geometry and bonding
You are already familiar with atomic mass… Recall that the atomic mass of an element is the mass of the individual isotopes proportionally distributed. Molecular mass is obtained by summing the atomic mass for the number of atoms of each element. CaF 2 at. mass tot. mass 1 atom Ca 40.1 au 40.1 au 2 atoms F 19.0 au 38.0 au m.m. 78.1 au
Avogadro's number Mg + � O 2 � MgO A mole is the number of atoms or molecules (6.02 x 10 23 ) needed to make a mass (g) equivalent to the atomic or molecular mass.
Image from Gill, 1996
Generally, directionality of bonds may be important in covalent species - bonds are generally weaker* Directionality less important for ionic bonds - radius ratios determine structures. * exception diamond
As atoms approach, there is a coulombic attraction E p (potential energy) = -e 2 /r But as ions approach, their electron distributions overlap, so there is a repulsion. E p =-e 2 /r + (be 2 )/r 12
Attraction F = k c q 1 q 2 r -2 Repulsion F is elastic rebound Balance between the overall attraction of atoms and the inter- electron repulsions Images by E.B. Watson
Image from Gill, 1996
NaCl vs. NaF F:1s 2 2s 2 2p 5 Cl:1s 2 2s 2 2p 6 3s 2 3p 5 Both are strongly electronegative So what differences might you expect?
As it tuns out, the ions are not hard spheres, but more squishy! There is some variation allowed in the range of “acceptable” radius ratios (cation/anion) in this configuration. Image from Blackburn and Dennen, 1994
Ionic bonding is a reduction of excess energy because of (+) or (-) charges. Ionically-bonded compounds are charge balanced controls stoichiometry, or ratio of atoms in the compound Na + + Cl - = NaCl Ca ++ + 2F - = CaF 2
Bonding The Ionic Bond Model Changing coordination
The atoms are not stationary. T Vibration 25 o C 0.025 nm 600 o C 0.060 nm Image from Blackburn and Dennen, 1994
Coordination
Coordination
Certain anion/cation ratios produce a predictable polyhedra in IONIC bonding.
No Yes Yes, but... Preferred
Symmetrically-packed structures • Atoms are spheres • Bonds must be either non-directional or highly symmetical Cubic packing Hexagonal packing
Molecular structures • Composed of atoms characterized by strongly directional bonding with low symmetry. • Result is clusters, chains, and layers of atoms that are strongly bonded internally, but weakly bonded to one another. • Examples: ice, organic molecules
Cubic Closest Packing Colors here denote different layers of atoms
Hexagonal Closest Packing
Symmetrically-packed structures can be monatomic or multiatomic. Symmetrical packing applies to the larger anions. Monoatomic - Native Elements (Au, Ag, S) Multiatomic - Most oxides and many silicates
Quartz: Two oxygens for every silicon. Charge of Si = +4, O = -2 We then say the stoichiometery is 1:2 Important: Si, Ti, Al, Fe, Mg, Ca, Na, K, P, O, S Today’s $10K question: how do we reconcile the overall stoichiometry (i.e. the chemical formula SiO 2 ) with the coordination of SiO 4 ?
The tetrahedra link together such that the oxygens are shared This illustrates the general principles of the architecture of crystals
Endmember types Polyhedra-frame structures • Bonding dominantly ionic • Anions group around cations to make coordination polyhedra Silicates (SiO 2 minerals) built of silicate tetrahedra which share corners with one another - this makes a framework. It imparts certain properties to the minerals: strong, hard, etc.
Halite (NaCl) Two ways to model NaCl: left, atoms of Na and Cl shown, right, atoms of Cl shown with coordination polyhedra (octahedra) Image from Klein and Hurlbut, 1985
Q How are U these related? A R T Z Six-fold symmetrical arrangement of tetrahedra all joined by common oxygen atoms.
Pauling's Rules Rule 1. Interatomic Distance . A coordination polyhedron of anions is formed about each cat-ion. The cation-anion distance being determined by the radius sum and the coordination number of the cation by the radius ratio. Rule 2. Electrostaic Valency Principle . In a stable coordination structure, the total strength of the valence bonds that reach an anion from all neigh-boring cations is equal to the charge of the anion. Rule 3. Sharing of Polyhedral Elements I . The existence of edges. and particularly of faces, common to two coordination polyhedra decreases the stability of ionic structures. Rule 4. Sharing of Polyhedral Elements II . In a crystal containing different cations, those with large valence and small coordination tend not to share polyhedral elements with each other. Rule 5. Principle of Parity . The number of essentially different kinds of constituents in a crystal structure tends to be small.
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