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An Introduction to Intermolecular Forces 1 – Van der Waals’ Forces The electrons in atoms are moving around. At any moment in time they are unlikely to be evenly spread. This gives the atom or molecule a temporary dipole. This dipole moment will induce a temporary dipole in a neighboring atom by attracting/repelling its electron charge cloud. 2 – Dipole-dipole forces Polar molecules (molecules with permanent dipoles) will attract other molecules with permanent dipoles. 3 – H-bonds 3 – H-bonds These are a special case of dipole-dipole forces. They occur when H is bonded to N, O or F (which are very electronegative). As H only has the two electrons in the covalent bond, if they are pulled away from the H atom, the H nucleus is exposed. The “H-bond” is the attraction between a lone pair on the N, O or F to the + H on a neighbouring molecule. Drawing H bonds from lone pair on N, O, F to + on H, e.g. NH3 H H N H H H N H Drawing H bonds from lone pair on N, O, F to + on H, e.g. H2O H O H H O H Drawing H bonds from lone pair on N, O, F to + on H, e.g. HF H F H F Also in: alcohols (O-H bonds) carboxylic acids (O-H bonds) amines (N-H bonds) proteins (N-H bonds) In each pair of molecules, which molecule has the highest boiling point and why? 1) Br2 & I2 2) Br2 & HBr 3) H2O & H2S 4) CH4 & C4H10 5) CH3OCH3 & CH3CH2OH 6) (CH3)3N & (CH3)2NH 7) CCl4 & CHCl3