Covalent Bonding

Covalent bonds exist when the electrons are shared equally between the pair of atoms. This is typified by the homonuclear diatomics such as Br₂. In general covalent bonds are formed when the electronegativity difference (Δχ) between the two atoms is < 0.4. This effect can be measured experimentally, or calculated using advanced Quantum Chemistry software. The effect can be seen, as shown below. The Jmol image is the Electrostatic Potential Energy (ESP) mapped onto the Electron Density surface. The electron density surface is the boundary within which is contained 95% of the electron density of the molecule. It basically corresponds to the Van der Waals surface of the molecule. The ESP is color coded onto this surface. The coloring indicates areas of high electron density (red) - medium (green) - to low electron density (blue).

	The image clearly shows the presence of a σ bond between the atoms (the electron density lies along the internuclear axis). The green color indicates that the electrons are shared equally between the two atoms. (Note: the areas of blue arise due to the sp3 hybridization of the Br atoms.		A second example is Ethane. The Δχ difference between two C atoms is 0, and between a C and a H is only 0.35 meaning that both types of bonds are covalent. Again, the green color indicates a uniform electron density in the molecule implying that the electrons are shared equally between bonding atom pairs.
Bromine		Ethane