Dipole+interactions

=Dipole - dipole forces=

When two atoms with a difference in electronegativity are bonded together, a dipole forms. This is because when one atom is more electronegative than its bonded atom, the electrons that used in bonding the atoms together shift slightly towards the one with more electronegativity, causing the molecule to have an imbalance in its charges. For example, in a HCl bond, the Hydrogen is slightly delta positive compared to Chlorine because Chlorine has a stronger electronegativity, therefore more electrons are attracted to the chlorine, making the charge slightly negative.



**Figure 1.** Permanent dipoles in HCl cause forces of dipole - dipole attraction between them
Dipole - dipole forces occur because of the **electrostatic attraction** between molecules with **permanent dipoles**, as a result of the difference in **electronegativity** of the bonded atoms. Figure 1 shows an example of an dipole - dipole attraction. On one end the molecule has a partial positive charge (H), and on one end has a partial negative charge (Cl). Since we know that opposite charges attract, therefore molecules will be attracted to neighbouring molecules with opposite charges. In the case of HCl, there will be a dipole - dipole attraction (marked by the dashed lines) between hydrogen and chlorine because hydrogen is positively charged and chlorine is negatively charged.

This type of forces are always stronger than van der Waal's forces. Compared with non polar compounds, polar compounds have higher **melting and boiling points** (assuming they have the same molecular mass)

The strength of dipole-dipole attraction vary depending on difference between two molecules' electronegativity (polarity). Therefore, interemolecular force caused by dipole-dipole attraction will decrease in HCl>HBr>HI this order. This is because difference of electronegativity between H and Cl is greater than that of H and I.