Final answer:
Polarizability is the capacity of an atom or molecule's electron cloud to deform in response to an electric field, resulting in a dipole moment. Larger atoms and molecules are generally more polarizable, which leads to stronger London dispersion forces and higher melting and boiling points.
Step-by-step explanation:
Polarizability is a measure of the ability of an atom or molecule's electron cloud to be distorted by an electric field, resulting in a dipole moment. This tendency varies with the size of the atom or molecule; larger atoms have more loosely held electrons and are, therefore, more polarizable. For instance, when comparing the polarizability of the halogens, we see an increasing order from top to bottom in the periodic table: F<Cl<Br<I. A higher polarizability implies that the substance can have greater dispersion forces, which relates to higher melting and boiling points due to the stronger intermolecular forces.
Polarization occurs as positive and negative charges shift to opposite ends of an atom or molecule. A polar molecule is one where this charge separation is permanent due to an unequal distribution of electrons.
In summary, substances with larger atomic or molecular sizes and higher polarizability will have stronger London dispersion forces, contributing to increases in melting and boiling points for similar materials.