Intra- and Intermolecular Forces

Whether a particular group of bonded molecules takes the form of asolid, liquid, or gas depends not only on the bonds that exist withineach individual molecule, but also on the presence and type of bondsbetween molecules. Hark back to the different types of bonds wereviewed in the last chapter: ionic, covalent, and metallic. Of these,ionic bonds tend to be the strongest, and this means that substancesthat contain ionic bonds are solids at room temperature. Substancesthat are primarily made up of covalent bonds, which are weaker, can besolid or liquid, and their state will depend on the presence and typeof intermolecular forces.
    The two main types of intermolecularforces that exist between molecules are dipole-dipole forces (includinghydrogen bonds) and London dispersion forces.

    Dipole-Dipole Forces

    Dipole-dipole attractions take place whentwo or more neutral, polar molecules are oriented such that theirpositive (+) and negative (-) ends are close to each other.

_7O|Px]    Because of the attraction between unlikecharges, this is a fairly strong type of intermolecular force, andmolecules held together by dipole-dipole forces tend to be in the solidor liquid state. Also, for molecules that are about the same size andweight, the strength of the dipole-dipole forces increases as thedegree of polarity increases. In other words, the more polar a moleculeis, the stronger the dipole-dipole forces it will form with itself andother molecules.

    One very important and unique case of the dipole-dipole attraction is known as hydrogen bonding.Hydrogen bonds are not true bonds: they’re just strong attractiveforces between the hydrogen on one molecule and a highlyelectronegative atom on a nearby molecule.

    Hydrogen bonds most commonly form betweenhydrogen atoms and fluorine, oxygen, or nitrogen. This type ofintermolecular force is responsible for water’s unique characteristics,such as its high specific heat and boiling point temperature—but moreabout that later.

    London Dispersion Forces—Weak Intermolecular Forces

    London forces are relatively weak forces ofattraction that exist between nonpolar molecules and noble gas atoms,like argon (a noble gas) and octane (a hydrocarbon; C8H18). These types of attractive forces are caused by a phenomenon known as instantaneous dipole formation.In this process, electron distribution in the individual moleculessuddenly becomes asymmetrical, and the newly formed dipoles are nowattracted to one another.

    The ease with which the electron cloud of an atom can be distorted to become asymmetrical is called the molecule’s polarizability.Think of this as a probability issue. The greater the number ofelectrons an electron has, the farther they will be from the nucleus,and the greater the chance for them to shift positions within themolecule. This means that larger nonpolar molecules tend to havestronger London dispersion forces. This is evident when you look at thediatomic elements in group 7, the halogens. All of these diatomicelements are nonpolar, covalently bonded molecules. Now, going down thegroup, fluorine and chlorine are gases, bromine is a liquid, and iodineis a solid! For nonpolar molecules, the farther you go down the group,the stronger the London dispersion forces.