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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/724/741576/

McMurry, John; Fay, Robert. Chemistry, 4th Ed.; Pearson Education, 2003.http://wps.prenhall.com/wps/media/objects/602/616516/

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Ionization Energy of Atoms

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ElectronegativityIonization 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

Pot

entia

l ene

rgy

(kJ/

mol

)Electron Density for the H2 molecule

An electron density plot for the H2 molecule shows that the shared electrons occupy a volume equally distributed over BOTH H atoms.

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Polar CovalentMolecule % Ionic Molecule % 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.

Percent Ionic Character of Diatomic Molecules

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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 EffectPhotoelectric 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 WAVESthat 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.

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 (ψ), a mathematical function that varies with position

Max Born: physical interpretation of wavefunctions. Probability of finding a particle in a region is proportional to ψ2.

Wavefunctions: ψ

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s Orbitals

Boundary surface encloses surface with a > 90% probability of finding electron

Wavefunctions of s orbitals of higher energy have more complicated radial variation with nodes.

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radial probability distribution

wave function

electron density

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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 Bondingσ - bond: e- density in the bond is distributed symmetrically about

the bond axis.π - bond: e- density in the molecular orbital has a nodal plane that

contains the bond axis.

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σ – bondH2

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σ – bond H2

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σ – bondp-orbitalend-to-endF2

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σ – bondmixedHCl CH4

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Lone Electron Pairs

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π - bond

π-orbital bonding side-to-side

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Double Bonds: Examples

Allene

Ethene

Double bonds consist of 1 σ and 1 π bonds in these molecules.

Double Bond == Bond Order 2

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Triple Bonds

Ethyne (acetylene)

Triple bonds consist of 1 σ and 2 π bonds.

Triple Bond == Bond Order 3

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Valence Bond Theory vs. Molecular Orbital TheoryFor 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.

Molecular Orbital Theory: orbitals are delocalized over the entire molecule.

Which is more correct?

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Delocalization – Resonance StructuresBenzene

OR

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BenzeneThe six MOs of benzene showing lobe phases and relative energies.

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Visualization SurfacesElectron Density Surface

Electrostatic Potential Map

Electrostatic Potential Maps of Hydrogen Halides

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ExerciseAtoms. 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 we’ll wait for ever for the calculation. Verify with me if molecule makes sense.