Intermolecular Forces.docx.docx

Inter-molecular Forces Dispersion forces        .

Electrons are not always evenly distributed around the nucleus. Dispersion forces are weak attractive forces caused by the uneven distribution of electrons around the nucleus and between neighbouring atoms. One side of the atom can temporarily become more positive than the other side due to the motion of the outershell electrons. Dipoles (uneven distribution of charge) are, therefore, created in atoms due to these asymmetric charge distributions. These temporary dipoles continuously break down and reform. The creation of temporary dipoles in one atom can induce dipoles in adjoining atoms. Generally, the larger (molecular mass) an atom or molecule, the larger the dispersion forces between it and its neighbours. THEY ARE THE ONLY FORCES BETWEEN NON-POLAR MOLECULES.

Dipole–dipole forces   

Molecules with permanent dipoles (polar molecules) attract one another. The negative end of one dipole attracts the positive end of the neighbouring dipole. Dipole–dipole forces are stronger than dispersion forces in molecules of similar molecular mass.

Hydrogen bonding  Hydrogen bonding is the attraction between a partially positive hydrogen atom in one molecule and a non-bonding electron pair in a fluorine, oxygen or nitrogen atom bonded to hydrogen atoms in a neighbouring molecule.  It has been suggested that the hydrogen bond has about 10% covalent character. This helps to account for the strength of this bond compared with other intermolecular forces.  Hydrogen bonding raises the melting and boiling points of covalent molecular substances much more than dispersion or dipole–dipole forces.  Hydrogen bonding is an important force between the chains of natural polymers such as proteins and DNA.