Final answer:
Permanent dipole-dipole forces arise from the attractive interaction between the oppositely charged ends of polar molecules, such as in a molecule of hydrogen chloride (HCl). These forces are weaker than ionic bonds and result from the permanent partial charges within the molecules.
Step-by-step explanation:
Permanent dipole-dipole forces arise in substances where molecules are polar. In a polar molecule, there is an uneven distribution of electrons between the atoms, which leads to a permanent separation of charge within the molecule, creating a dipole—a molecule with a positive and a negative end. For instance, hydrogen chloride (HCl) has a partially positive hydrogen atom and a partially negative chlorine atom. When many such polar molecules are present, they align themselves such that the positive end of one molecule is close to the negative end of a neighboring molecule. This attraction between oppositely charged regions is the foundation of dipole-dipole forces.
These forces are weaker than ionic bonds, which occur when electrons are outright transferred from one atom to another, leading to the formation of ions that attract each other with full charges. However, in dipole-dipole interactions, the involved particles possess only partial charges but are enough to make these forces significant in determining the physical properties of polar substances.