Bonding Bringing the atoms together More than one atom Until now, - - PowerPoint PPT Presentation
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
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.
Pure substances*
Elements Compounds
Mixtures
Homogeneous Heterogeneous
*Pure is very hard to achieve in quantity
So what is holding the polyatomic elements together?
Image by E.B. Watson
Covalent bonds – the sharing of electrons 1s1 to 1s2 ([He]) 1s1 & [He] 2s2 2p5 to [Ne] [He] 2s2 2p5 to [Ne]
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
Electron Swapping
This electron sharing is a very strong bond. This tends to by asymmetrical, and therefor difficult to pack into repeating lattices H2 O2 Cl2 F2
Image from Gill, 1996
Image from Pauling, 1970
Image modified from Zoltai and Stout, 1984
Sharing an electron in 2p Sharing an electron in two vacancies with 1s in 2p
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 (CO3)2-, (OH)-, (HCO3)-, (PO4)3-
lithium (Li) 1s22s1 The 2s electrons are delocalized – move around For atoms with appropriate electron structure, electron swapping is quite easy when matter is condensed
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
Ionization - losing
electrons Can only attract so far – “solid spheres”
Periodicity of electronegativity
Ionic-covalent character makes molecules dipolar
A compound containing just two elements "-ide“. The metal is named first: NaCl sodium chloride Multiple valence state Fe3+ or Fe2+. Higher valence state with the suffix "- ic" and the lower with "-ous": Fe2O3 ferric oxide FeO ferrous oxide Today we tend to use a more explicit approach Fe2O3 iron(III) oxide FeO iron(II) oxide
More than two elements with polyatomic anions such as hydroxide OH- carbonate (CO3)2- nitrate (NO3)- phosphate (PO3)3- The compound names follow pretty logically: KOH potassium hydroxide CaCO3 calcium carbonate Polyatomic anions can have more than one configuration, e.g. nitrate (NO3)- nitrite (NO2)-
CO2 carbon dioxide (Greek "di" for 2) CO carbon monoxide (Greek "mono" for 1) CCl4 carbon tetrachloride (Greek "tetra" for 4) Empirical Formulas Always used in ionic compounds NaCl CaF2 CH2O Covalent compounds
Used for largely covalent compounds While CH2O accurately describes the ratio
characterize the whole molecule Molecular Formulas
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. CaF2
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 + O2 MgO A mole is the number of atoms or molecules (6.02 x 1023) needed to make a mass (g) equivalent to the atomic or molecular mass.
Image from Gill, 1996
Balance between the
atoms and the inter- electron repulsions Attraction F = kc q1 q2 r-2 Repulsion F is elastic rebound
Images by E.B. Watson
Image from Gill, 1996
NaCl vs. NaF F:1s2 2s2 2p5 Cl:1s2 2s2 2p6 3s2 3p5 Both are strongly electronegative So what differences might you expect?
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- = CaF2
T Vibration 25oC 0.025 nm 600oC 0.060 nm
Image from Blackburn and Dennen, 1994
Colors here denote different layers of atoms
Image from Klein and Hurlbut, 1985
Two ways to model NaCl: left, atoms of Na and Cl shown, right, atoms of Cl shown with coordination polyhedra (octahedra)
Six-fold symmetrical arrangement of tetrahedra all joined by common oxygen atoms.
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