Understanding intermolecular attractions is easier when you track charge signs. This page shows how partial charges ($\delta^+$, $\delta^-$) determine the geometry and relative strength of dipole–dipole versus ion–dipole interactions.

Core idea: attraction depends on charge signs

Partial charges in polar bonds

A polar molecule has regions of $\delta^+$ and $\delta^-$. Intermolecular attractions are strongest when opposite charge regions align closely and directly.

Electrostatic basis (qualitative)

This proportionality explains why interactions strengthen with larger charge magnitudes and shorter distances; it also highlights why replacing a partial charge with a full ionic charge usually increases attraction.

The figure shows how the sign of interacting charges controls the direction of the electrostatic force. Like charges repel and unlike charges attract, with the interaction occurring along the line connecting the charges and depending on the separation distance $r$. This same sign-based reasoning is what you apply when you line up $\delta^+$ with $\delta^-$ (or ions with the opposite end of a dipole). Source

Dipole–dipole interactions using partial charges

What is interacting?

To compare dipole–dipole interactions, focus on:

• Signs: $\delta^+$ must face $\delta^-$ for attraction; like signs produce repulsion.

• Alignment (orientation): molecules can rotate, so attractions are strongest when dipoles line up “head-to-tail.”

• Dipole magnitude: larger bond polarity and molecular geometry that produces a larger net dipole increases the possible attraction.

How to reason with partial charges

• Identify the most positive H/atom (often bonded to an electronegative atom) as likely $\delta^+$.

• Identify the most electronegative atoms as likely $\delta^-$.

• Predict the lowest-energy arrangement: $\delta^+$ of one molecule nearest $\delta^-$ of another.

• Recognise that a molecule can experience both attractive and repulsive contacts depending on how it approaches a neighbour.

Ion–dipole interactions using partial charges

What is interacting?

An ion has a full charge (e.g., $+1$, $-1$, $+2$), while the polar molecule provides partial charges. Using signs:

• A cation is attracted to the molecule’s $\delta^-$ end.

This hydration schematic depicts an ion–dipole interaction between a cation and surrounding water molecules. The water molecules orient so the oxygen end (electron-rich, $\delta^-$) points toward the positive ion, while the hydrogen ends ($\delta^+$) point away. The image emphasizes that ion charge strongly organizes nearby dipoles into a preferred orientation. Source

• An anion is attracted to the molecule’s $\delta^+$ end.

How to compare ion–dipole vs dipole–dipole with partial charges

When comparing, keep the distance and general size in mind, then prioritise charge magnitude:

• Ion–dipole often stronger because $|q_{\text{ion}}|$ is typically larger than $|\delta|$ on an atom in a dipole.

• A $2+$ or $2-$ ion generally interacts more strongly than a $1+$ or $1-$ ion with the same dipole (greater $|q_{\text{ion}}|$).

• Orientation still matters, but the ion’s electric field strongly biases the polar molecule to rotate so the correct $\delta$ end points toward the ion.

Practical comparison workflow (what AP expects you to do)

Step-by-step qualitative comparison

• Label each polar molecule with $\delta^+$ / $\delta^-$ at the relevant atoms.

• For dipole–dipole:

  • Check whether a plausible approach places $\delta^+$ near $\delta^-$ (attraction) or $\delta^+$ near $\delta^+$ / $\delta^-$ near $\delta^-$ (repulsion).

  • Consider whether the molecular shape allows good alignment (stronger) or forces poorer alignment (weaker).

• For ion–dipole:

  • Pair cation → $\delta^-$ and anion → $\delta^+$.

  • Compare likely strength by considering full vs partial charge and how close the ion can approach the charged end.

Language to use in explanations

• “Attraction occurs between the oppositely charged regions (ion with $\delta$ end, or $\delta^+$ with $\delta^-$).”

• “This interaction is stronger/weaker because the interacting charges are full vs partial, and because distance/orientation affects Coulombic attraction.”