Atoms, Electrons, Chemical Bonding, and Orbitals

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Atoms, Electrons, Chemical Bonding, and Orbitals
- Science Honors Program - Computer Modeling and
Visualization in Chemistry

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This presentation is for educational, non-profit
purposes only. Please do not post or distribute
this presentation to anyone outside of this
course. Many of the graphics in this
presentation are from Bruice, Paula Yurkanis.
Organic Chemistry, 4th Ed. Prentice Hall,
2003. http//wps.prenhall.com/wps/media/objects/72
4/741576/ McMurry, John Fay, Robert. Chemistry,
4th Ed. Pearson Education, 2003. http//wps.prenh
all.com/wps/media/objects/602/616516/
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Ionization Energy of Atoms
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Electronegativity
Ionization Energy (IE) amount of energy
necessary to remove an electron from a neutral
atom.Electron Affinity (EA) amount of energy
released when an atom absorbs an extra
electron.Electronegativity Average of above
two properties for an isolated atom.
What type of bond will form between two atoms?1.
Ionic 2. Covalent 3. Polar covalent
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Ionic Bond
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Covalent Bonding
An electron density plot for the H2 molecule
shows that the shared electrons occupy a volume
equally distributed over BOTH H atoms.
Potential energy (kJ/mol)
Electron Density for the H2 molecule
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Polar Covalent
Percent Ionic Character of Diatomic Molecules
Molecule Ionic Ionic Molecule Ionic Ionic
H2 0   CsF 70  
CO 2   LiCl 73  
NO 3   LiH 76  
HI 6   KBr 78  
ClF 11   NaCl 79  
HBr 12   KCl 82  
HCl 18   KF 82  
HF 41   LiF 84  
      NaF 88  
HF has a polar covalent bond. It has a dipole
moment.
The difference in electronegativity between the
two bonded atoms determines the ionic character
(or covalent character) of the bond.
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What is an Atom?
Protons and neutrons make up the heavy, positive
core, the NUCLEUS, which occupies a small volume
of the atom.
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Photoelectric Effect
Photoelectric Effect the ejection of electrons
from the surface of a substance by light the
energy of the electrons depends upon the
wavelength of light, not the intensity.
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Atomic Orbitals Wave-particle duality.
Traveling waves vs. Standing Waves. Atomic and
Molecular Orbitals are 3-D STANDING WAVES that
have stationary states.
Example of 1-D guitar string standing wave.
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Atomic Orbitals How do electrons move around
the nucleus?
Density of shading represents the probability of
finding an electron at any point. The graph shows
how probability varies with distance.
Wavefunctions ?
Since electrons are particles that have wavelike
properties, we cannot expect them to behave like
point-like objects moving along precise
trajectories. Erwin Schrödinger Replace the
precise trajectory of particles by a wavefunction
(?), a mathematical function that varies with
position Max Born physical interpretation of
wavefunctions. Probability of finding a particle
in a region is proportional to ?2.
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s Orbitals
Wavefunctions of s orbitals of higher energy have
more complicated radial variation with nodes.
Boundary surface encloses surface with a gt 90
probability of finding electron
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electron density
wave function
radial probability distribution
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p orbitals 3 p orbitals l 1,
ml 1, 0 -1
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d orbitals Five d orbitals l 2, ml
2, 1, 0 - 1, -2
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Atomic Orbital Hybridization sp
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Atomic Orbital Hybridization sp2
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Atomic Orbital Hybridization sp3
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Covalent Bonding
s - bond e- density in the bond is distributed
symmetrically about the bond axis. p - bond e-
density in the molecular orbital has a nodal
plane that contains the bond axis.
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s bondH2
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s bond H2
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s bondp-orbitalend-to-endF2
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s bondmixedHCl CH4
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Lone Electron Pairs
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p - bond
p-orbital bonding side-to-side
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Double Bonds Examples
Double bonds consist of 1 s and 1 p bonds in
these molecules. Double Bond Bond Order 2
Allene
Ethene
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Triple Bonds
Triple bonds consist of 1 s and 2 p
bonds. Triple Bond Bond Order 3
Ethyne (acetylene)
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Valence Bond Theory vs. Molecular Orbital Theory
For Polyatomic Molecules Valence Bond Theory
Similar to drawing Lewis structures. Orbitals
for bonds are localized between the two bonded
atoms, or as a lone pair of electrons on one
atom. The electrons in the lone pair or bond do
NOT spread out over the entire molecule. Molecula
r Orbital Theory orbitals are delocalized over
the entire molecule. Which is more correct?
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Delocalization Resonance Structures
Benzene
OR
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Benzene
The six MOs of benzene showing lobe phases and
relative energies.
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Visualization Surfaces
Electron Density Surface Electrostatic Potential
Map
Electrostatic Potential Maps of Hydrogen Halides
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Exercise
Atoms. Do a 1 electron atom such as H, or
He, then do multi-electron atom Do quantum
calculation. Visualize orbitals. What is
the 1st IE of the molecule? Pick a binary
compound. Verify with me if molecule makes
sense. Do quantum calculation. Draw
electron density surface Draw electrostatic
potential map. Is molecule polar? What is
electronegativity difference between atoms?
Find bond length. Bond order? Visualize the
orbitals. Which ones are bonding,
anti-bonding? Which are sigma, which are pi?
What is the 1st IE of the molecule? Do same as
above for a polyatomic molecule, such as benzene
Not too big, or well wait for ever for the
calculation. Verify with me if molecule makes
sense.