What is the role of valence electrons in the formation of chemical bond?
The valence, the outermost electrons, form the bond between 2 nuclei.
So what is a chemical bond? A chemical bond is a region of high electron density between 2 positively charged atomic nuclei. Electrostatic attraction between the electron cloud and the nuclei negate nucleus/nucleus repulsion, and a net attractive force results.
When we are introduced to chemical bonding (back in Year 10), we are told that electrons are TRANSFERRED between nuclei for ionic bonding, and electrons are SHARED for covalent bonding. What I have said above, is a slightly more sophisticated version. If you look at (calculated) maps of electron density in a covalent bond, you always see a high electron density BETWEEN the nuclei: this is the covalent bond, the glue that binds the nuclei together.