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

2 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, 4 th Ed.; Prentice Hall, McMurry, John; Fay, Robert. Chemistry, 4th Ed.; Pearson Education,

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4 Ionization Energy of Atoms

5 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

6 Ionic Bond

7 Covalent Bonding Potential energy (kJ/mol) Electron Density for the H 2 molecule An electron density plot for the H 2 molecule shows that the shared electrons occupy a volume equally distributed over BOTH H atoms.

8 Polar Covalent Molecule% IonicMolecule% Ionic H2H2 0 CsF70 CO2 LiCl73 NO3 LiH76 HI6 KBr78 ClF11 NaCl79 HBr12 KCl82 HCl18 KF82 HF41 LiF84 NaF88 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

9 What is an Atom? Protons and neutrons make up the heavy, positive core, the NUCLEUS, which occupies a small volume of the atom.

10 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.

11 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.

12 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 ψ 2. Wavefunctions: ψ

13 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.

14 radial probability distribution wave function electron density

15 p orbitals: 3 p orbitals l = 1, m l = +1, 0 -1

16 d orbitals: Five d orbitals l = 2, m l = +2, +1, 0 - 1, -2

17 Atomic Orbital Hybridization: sp

18 Atomic Orbital Hybridization: sp 2

19 Atomic Orbital Hybridization: sp 3

20 Covalent Bonding σ - 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.

21 σ – bond H 2

22 σ – bond H 2

23 σ – bond p-orbital end-to-end F 2

24 σ – bond mixed HCl CH 4

25 Lone Electron Pairs

26 π - bond π-orbital bonding side-to-side

27 Double Bonds: Examples Allene Ethene Double bonds consist of 1 σ and 1 π bonds in these molecules. Double Bond == Bond Order 2

28 Triple Bonds Ethyne (acetylene) Triple bonds consist of 1 σ and 2 π bonds. Triple Bond == Bond Order 3

29 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. Molecular Orbital Theory: orbitals are delocalized over the entire molecule. Which is more correct?

30 Delocalization – Resonance Structures Benzene OR

31 Benzene The six MOs of benzene showing lobe phases and relative energies.

32 Visualization Surfaces Electron Density Surface Electrostatic Potential Map Electrostatic Potential Maps of Hydrogen Halides

33 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 1 st 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 1 st 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.