A molecule that has a charge cloud that is easily distorted is said to be very polarizable and will have large dispersion forces one with a charge cloud that is difficult to distort is not very polarizable and will have small dispersion forces. The measure of how easy or difficult it is for another electrostatic charge (for example, a nearby ion or polar molecule) to distort a molecule’s charge distribution (its electron cloud) is known as polarizability. Thus, they are less tightly held and can more easily form the temporary dipoles that produce the attraction. In a larger atom, the valence electrons are, on average, farther from the nuclei than in a smaller atom. The increase in melting and boiling points with increasing atomic/molecular size may be rationalized by considering how the strength of dispersion forces is affected by the electronic structure of the atoms or molecules in the substance. Melting and Boiling Points of the Halogens Trends in observed melting and boiling points for the halogens clearly demonstrate this effect, as seen in. F 2 and Cl 2 are gases at room temperature (reflecting weaker attractive forces) Br 2 is a liquid, and I 2 is a solid (reflecting stronger attractive forces). Larger and heavier atoms and molecules exhibit stronger dispersion forces than do smaller and lighter atoms and molecules. The forces are relatively weak, however, and become significant only when the molecules are very close. illustrates how changes in physical state may be induced by changing the temperature, hence, the average KE, of a given substance.ĭispersion forces result from the formation of temporary dipoles, as illustrated here for two nonpolar diatomic molecules.ĭispersion forces that develop between atoms in different molecules can attract the two molecules to each other. These forces serve to hold particles close together, whereas the particles’ KE provides the energy required to overcome the attractive forces and thus increase the distance between particles. IMFs are the various forces of attraction that may exist between the atoms and molecules of a substance due to electrostatic phenomena, as will be detailed in this module. The phase in which a substance exists depends on the relative extents of its intermolecular forces (IMFs) and the kinetic energies (KE) of its molecules. The differences in the properties of a solid, liquid, or gas reflect the strengths of the attractive forces between the atoms, molecules, or ions that make up each phase. Particles in a solid vibrate about fixed positions and do not generally move in relation to one another in a liquid, they move past each other but remain in essentially constant contact in a gas, they move independently of one another except when they collide.Particles in a solid are tightly packed together and often arranged in a regular pattern in a liquid, they are close together with no regular arrangement in a gas, they are far apart with no regular arrangement.Note that we will use the popular phrase “intermolecular attraction” to refer to attractive forces between the particles of a substance, regardless of whether these particles are molecules, atoms, or ions.Ĭonsider these two aspects of the molecular-level environments in solid, liquid, and gaseous matter: In the following description, the term particle will be used to refer to an atom, molecule, or ion. Explain the relation between the intermolecular forces present within a substance and the temperatures associated with changes in its physical stateĪs was the case for gaseous substances, the kinetic molecular theory may be used to explain the behavior of solids and liquids.Identify the types of intermolecular forces experienced by specific molecules based on their structures.Describe the types of intermolecular forces possible between atoms or molecules in condensed phases (dispersion forces, dipole-dipole attractions, and hydrogen bonding).By the end of this section, you will be able to:
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