G. N. Lewis: A covalent bond is formed when two atoms _____________________.

H + H ---> H-H

In terms of orbitals:

(H + H)

(H-H)

We have _____________________ of orbitals.

There is a build up of electron density between the nuclei so the electrons are attracted to both nuclei and there is a net stabilization.

The electron also moves over a larger volume so the ground state energy is ____________.

Principle:

Another view:

Isosurfaces for H₂

solid (electon density = 0.08) ca. covalent radius

mesh (electron density = 0.002) ca. Van der Waals radius

Molecular Potential Energy Curve

At long distances there is a net ______________ and at short distances there is a net _______________.

At R_e, the equilibrium distance, there is a _______________.

At that point the molecule is at a minimum energy.

Principle:

Polar Bonds

If there is a difference in electronegativity between atoms, there will be an uneven distribution of electrons between the atoms and the bond will be polar. This means the bond has a negative and a positive end. This can significantly affect chemical reactivity: all other things being equal, a polar bond will be more reactive than a nonpolar bond.

Polar bonds may also make the molecule as a whole polar resulting in a dipole. This will affect the physical properties of the molecule.

Example:

HF

Difference in electronegativity = 4.0 - 2.2 = 1.8

Rough rules of thumb:

D(c).........bond type

0 - 0.5......nonpolar

0.5 - 1.7...polar covalent

> 1.7........ionic

Electron Density contour plot for HF

Electron Density Isosurfaces for HF

Ionic or Covalent?

Electrostatic Potential Mapped onto Electron Density Isosurface (0.002)

red is negative potential energy for a proton

blue is postive potential energy for a proton

Which end would be protonated?

Let's look at LiH

D(c) = 2.2 - 1.0 = 1.2

covalent?

Lithium Hydride, LiH

Electron Density contour plot

Isosurfaces for LiH

What kind of bonding is this?

Electrostatic Potential Mapped onto Electron Density Isosurface (0.002)

Which end would be protonated? (Reaction with water)