Comparing Intermolecular Bonding and Melting Points

1. Ionic Compounds: These form when there is a large difference in electronegativity (typically 1.8 or higher), causing one atom to completely transfer electrons to another. This results in a metal forming a positive cation and a nonmetal forming a negative anion.
2. Polar Covalent Molecules: These form when atoms share electrons unequally (electronegativity difference between 0.5 and 1.7). This creates "partial charges," where one side of the molecule is slightly negative ($\delta-$) and the other is slightly positive ($\delta+$).
3. Nonpolar Covalent Molecules: These form when electrons are shared equally (electronegativity difference of 0.4 or lower). This usually occurs between two identical atoms or atoms with very similar electronegativity.

Step 2: Identify the Specific Intermolecular Forces

Once you know the substance type, you can identify the specific force responsible for holding the particles together.

Ion-Ion Forces

In ionic compounds, the attraction is between the positive cations and negative anions. Because these are full electrical charges, the ion-ion forces are extremely strong.

Hydrogen Bonding

This is a special, particularly strong type of dipole-dipole attraction found in certain polar covalent molecules. It occurs specifically when a hydrogen atom is bonded directly to a highly electronegative atom: Fluorine (F), Oxygen (O), or Nitrogen (N).

Dipole-Dipole Forces

In most other polar covalent molecules, the permanent partial positive and negative ends of the molecules attract one another. These are moderately strong attractions.

Dispersion Forces

Every molecule experiences dispersion forces (also known as London dispersion forces). These are caused by the constant motion of electrons, which can create temporary, random dipoles. In nonpolar covalent substances, these are the only forces present, making them the weakest type of attraction.

Step 3: Rank the Forces to Predict Melting Points

The final step is to use the hierarchy of force strength to rank the substances. The rule is simple: Stronger attractions require more thermal energy to overcome, resulting in higher melting points.

The general hierarchy from strongest to weakest is: $\text{Ion-ion forces} > \text{Hydrogen bonding} > \text{Dipole-dipole forces} > \text{Dispersion forces}$

Worked Example 1: Comparing $NaCl$, $H_2O$, and $N_2$

Let's rank the melting points of sodium chloride ($NaCl$), water ($H_2O$), and nitrogen gas ($N_2$) from lowest to highest.

• $N_2$ (Nitrogen): This is a nonpolar covalent molecule (two identical atoms). It relies on dispersion forces, which are the weakest.
• $H_2O$ (Water): This is a polar covalent molecule where Hydrogen is bonded to Oxygen. This allows for hydrogen bonding, which is much stronger than dispersion forces.
• $NaCl$ (Sodium Chloride): This is an ionic compound (metal + nonmetal). It is held together by ion-ion forces, which are the strongest.

Final Ranking (Lowest to Highest Melting Point): $N_2 < H_2O < NaCl$

Worked Example 2: Comparing $CH_4$, $HBr$, and $CaO$

• $CH_4$ (Methane): Nonpolar covalent $\rightarrow$ Dispersion forces (Weakest).
• $HBr$ (Hydrogen bromide): Polar covalent $\rightarrow$ Dipole-dipole forces (Moderate).
• $CaO$ (Calcium oxide): Ionic compound $\rightarrow$ Ion-ion forces (Strongest).

Final Ranking (Lowest to Highest Melting Point): $CH_4 < HBr < CaO$

Summary Table for Quick Reference

By following these three steps—identifying the substance, determining the force, and ranking the strength—you can confidently predict the physical properties of almost any substance you encounter in your studies.