Van der Waals forces, also known as van der Waals interactions or London dispersion forces, are weak intermolecular forces that exist between atoms or molecules. These forces arise due to temporary fluctuations in electron distribution, resulting in temporary charges that induce dipole moments in neighboring atoms or molecules. Van der Waals forces are present in all molecules but are particularly significant in non-polar molecules.
There are three types of van der Waals forces:
London dispersion forces: These are the weakest type of van der Waals forces and occur between all atoms and molecules. They arise due to temporary fluctuations in electron distribution, creating instantaneous dipoles. These temporary dipoles induce dipoles in neighboring atoms or molecules, resulting in attractive forces.
Dipole-dipole interactions: These forces occur between polar molecules. Polar molecules have permanent dipole moments due to an uneven distribution of electron density. The positive end of one molecule attracts the negative end of another molecule, resulting in dipole-dipole interactions.
Hydrogen bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements such as oxygen, nitrogen, or fluorine. The hydrogen atom carries a partial positive charge, while the electronegative atom carries a partial negative charge. This creates a strong electrostatic attraction between the hydrogen atom of one molecule and the electronegative atom of another molecule.
Now, moving on to your second question, van der Waals forces become an ionic bond under certain conditions. An ionic bond is a type of chemical bond that forms between two ions of opposite charges. It occurs when one or more electrons are transferred from a metal atom (which tends to lose electrons) to a non-metal atom (which tends to gain electrons).
For van der Waals forces to become an ionic bond, the following conditions must be met:
Electron transfer: Van der Waals forces, which are based on temporary fluctuations in electron distribution, need to be overcome by a more permanent redistribution of electrons. In an ionic bond, one atom loses electrons to become a positively charged cation, while the other atom gains those electrons to become a negatively charged anion.
Electrostatic attraction: The positively charged cation and the negatively charged anion must be attracted to each other by electrostatic forces. The opposite charges hold the ions together, forming the ionic bond.
High electronegativity difference: Ionic bonds usually form between atoms with a large electronegativity difference. This ensures a strong transfer of electrons from one atom to another, resulting in distinct positive and negative charges on the ions involved.
Overall, the transition from van der Waals forces to an ionic bond requires a shift from temporary, weak interactions to a more permanent redistribution of electrons, resulting in the formation of charged ions that attract each other through electrostatic forces.