- Slides: 40
What’s coming up? ? ? • • • Oct 25 Oct 27 Oct 29 Nov 1 Nov 3, 5 Nov 8, 10 Nov 12 Nov 15 Nov 17 Nov 19 The atmosphere, part 1 Midterm … No lecture The atmosphere, part 2 Light, blackbodies, Bohr Postulates of QM, p-in-a-box Hydrogen and multi – e atoms Multi-electron atoms Periodic properties Valence-bond; Lewis structures Ch. 8 • Nov 22 VSEPR Ch. 11 • • • Hybrid orbitals; VSEPR MO theory bonding wrapup Review for exam Ch. 11, 12 Nov 24 Nov 26 Nov 29 Dec 1 Dec 2 Ch. 8 Ch. 9, 10 Ch. 11
Electron cloud probability distributions for different types of bond
Dipole moment will align molecules in an electric field
Nitrate anion NO 3 Put a pair between each atom O O N O nitrogen does not have noble gas structure!!! form a double bond by sharing a pair from one of the oxygen atoms……….
FORM A DOUBLE BOND BETWEEN O AND N - Here is one O O N O Here is another! - - O O N O
Experiment shows all three bonds are the same. All bond lengths 128 pm O N All bond angles 120 0 O O Any one of the structures suggests one is different! O O N Double Bond Single Bond O Should be different! So…….
RESONANCE We use a double headed arrow between the structures. . O O N N N O O The electrons involved are said to be DELOCALIZED over the structure. The blended structure is a RESONANCE HYBRID O
Elements in rows 3 and following can exceed the octet rule: When it is necessary to exceed the octet rule the extra electrons go on the central third row element. F SF 6 F S … 12 S F I 3 - F F F I I I Central I … 10
FREE RADICALS Molecules which have unpaired electrons. NO 2 Is a free radical Total number of valence electrons = 5+6+6 = 17 O N O Form double bond to get N close to octet O N O O RESONANCE N O
PREDICTING THE SHAPES OF MOLECULES from the Lewis electron dot structure using the principle that electron pairs stay as far apart as possible. Electron pairs BOND PAIRS LONE PAIRS H O H
VALENCE SHELL ELECTRON PAIR REPULSION: VSEPR Based on the idea that all electron pairs repel each other. The bonding and lone pairs push apart as far as possible……. . This means that atoms bound to a central atom are as far apart as possible……. we can find the molecular shape! Lets see how it works…. . .
My currents interests are in the field of Molecular Geometry. I have Ronald J. Gillespie Professor Emeritus B. Sc. , Ph. D. , D. Sc. (London), F. R. S. C. (U. K. ), F. C. I. C. Molecular Geometry Contact Information Curriculum Vitae long been interested in further developing the VSEPR model and at the same time trying to understand why certain molecules appear to be exceptions to the model. Partly in collaboration with my colleague Richard Bader, I have been making use of the analysis of calculated electron density distributions to better understand the VSEPR model and molecular geometry in general. We have shown that the Laplacian of the electron density provides evidence for the localized lone pairs of the VSEPR model and we have developed the Lennard-Jones function which also provides evidence for lone pairs on a different basis. One of the largest classes of exceptions to the VSEPR model are certain molecules of the transition metals. We have shown that the deviations of the geometry of these molecules from the VSEPR model can be related to the distortion of the metal atom core from a spherical shape which we have been able to study by means of the Laplacian electron density. This investigation is continuing. Recently I have shown that the intramolecular distance between two given ligands is remarkably constant over a wide variety of molecules which led me to suggest that interligand interactions are much more important in determining geometry than has previously generally been supposed. This observation has led me to develop the ligand close packing (LCP) model.
Se. O 2 SO 2 O Se O Resonance (like SO 2) LEWIS STRUCTURE S O O O Experiment shows that both S-O bonds are equivalent. We say that the real SO 2 molecule is a hybrid of the two resonance forms.
Se. O 2 O Se O ELECTRON PAIR GEOMETRY THREE ELECTRON PAIRS AROUND THE SELENIUM ATOM. Se VSEPR treats double bonds like a single bond TRIGONAL PLANAR Now place the oxygen atoms
Se. O 2 O Se O Electron Pair Geometry is trigonal planar ADD OXYGENS Se Se O Se. O 2 IS V-SHAPED, OR BENT O
H CH 4 H C H There are four H electron pairs around the carbon atom.
The best arrangement for four electron pairs: 109. 5° TETRAHEDRAL C 4 electron pairs tetrahedral electron pair geometry Put on the H-atoms…….
There is a better arrangement for four electron pairs: TETRAHEDRAL H 109. 5° C C H H H 4 electron pairs tetrahedral EPG The shape of CH 4 is tetrahedral. NOW LOOK AT AMMONIA
NH 3 H N H H There are four electron pairs around the nitrogen atom. The electron pair geometry around the nitrogen is tetrahedral: PUT ON THE 3 H ATOMS N
NH 3 H N H H There are four electron pairs around the nitrogen atom. The electron pair geometry around the nitrogen is tetrahedral: PUT ON THE 3 H ATOMS N N H H H The shape of NH 3 is trigonal pyramidal.
H 2 O H There are four electron pairs around the oxygen atom. The electron pair geometry around the oxygen is tetrahedral: PUT ON THE 2 H-ATOMS O O H The shape of H 2 O is V-shaped or bent. H
VALENCE BOND THEORY A covalent bond is formed by an overlap of two valence atomic orbitals that share an electron pair. The better the overlap the stronger the bond The orbitals need to point along the bonds Lets look at methane
METHANE: a tetrahedral molecule H CH 4 C What orbitals are used? H Hydrogen atoms bond using their 1 s orbitals. Carbon needs four orbitals to bond with. [He] 2 s 22 p 2 Try 2 s, 2 px , 2 py and 2 pz
The electronic configuration of carbon is: [He] 2 s 22 p 2 The orbital diagram is: [He] The Lewis dot structure is . . C. . Promote one of the 2 s electrons
PROMOTE AN ELECTRON [He] 2 s 22 p 2 excited state The Lewis dot structure is still [He] 2 s 12 p 3 C Four unpaired electrons We can use these to form chemical bonds
A covalent bond is formed by an overlap of two valence atomic orbitals that share an electron pair. Bonds formed with s orbitals will be different to bonds formed with p orbitals. Experiment shows that all four bonds are identical. The three p orbitals are mutually perpendicular, suggesting 90° bond angles. Experiment shows that methane has 109. 5° bond angles. We get round this by combining the orbitals
We need four orbitals pointing to the vertices of a tetrahedron…. We remember that orbitals are just algebraic functions and so we can combine them H C H H H Combining orbitals is called HYBRIDIZATION
COMBINING ORBITALS TO FORM HYBRIDS HYBRIDIZATION : the combination of two or more “native” atomic orbitals on an atom to produce “hybrid” orbitals RULE: the number of atomic orbitals that are combined must equal the number which are formed All resulting hybrid orbitals are identical.
HYBRIDIZATION Combine one s and one p a sp- hybrid + + ADD the orbitals 2 s+ 2 p
The positive part adds to positive part CONSTRUCTIVE INTERFERENCE 2 s+ 2 p + + The positive part cancels negative part DESTRUCTIVE INTERFERENCE
Combine one s and one p to give an sp- hybrid 2 s+ 2 p + REMEMBER IF WE MIX TWO WE MUST GET TWO BACK The other combination is s - p
The positive part adds to positive part 2 s- 2 p CONSTRUCTIVE INTERFERENCE + + The positive part cancels negative part DESTRUCTIVE INTERFERENCE
2 s- 2 p + We get two equivalent sp orbitals ORIENTED AT 1800
sp-HYBRIDIZATION The s and p orbitals The two sp-hybrids Directed at 1800