What are Intermolecular forces? Intermolecular forces are weak forces of attraction between some covalent molecules. These attractions are responsible for determining whether a molecular compound is a gas, a liquid or a solid at room temperature. Intermolecular attractions are weaker than metallic, ionic or covalent bonds. The order of bond strength is the following: Metallic > Ionic > Covalent > Intermolecular

Two Types of Intermolecular Forces Hydrogen Bonds (Stronger) The Bond between the Hydrogen of one molecule to either Nitrogen, Oxygen or Fluorine of another molecule Van Der Waals (Weaker) Dipole Interactions (stronger than dispersion, but weaker than hydrogen bonds) Dispersions Forces (weakest of all) The order of strength for intermolecular compounds is hydrogen bonding > dipole interactions > dispersion forces

Hydrogen Bonds To fully understand hydrogen bonds, you need to understand what causes a molecule to become polar: Polarity is when two atoms in a covalent molecule have a different affinity for electrons When one atom has a higher electron affinity than another, it causes the majority of the electrons in the molecule to be closer to that atom, making it more electronegative Hydrogen can form a hydrogen bond with 3 elements: N, O and F Hydrogen bonds are formed between the covalently bonded hydrogen of one molecule and an unshared electron pair of another molecule

Hydrogen Bonds Hydrogen bonds are the reason why water has surface tension and DNA can be split open to replicate

Van Der Waals Dipole Interactions The attraction of two poles in a polar compound Similar to hydrogen bonds, just not as strong because of two reasons: the electronegativity difference is lower it is between two poles NOT an unshared electron pair Dispersions Forces (weakest of all) Caused by the movement of electrons in a non-polar molecule Electrons momentarily move to one side of a molecule, which causes the neighboring molecule’s electrons to momentarily move to the opposite side as well

How to determine the melting point of a compound The stronger the force is between the molecules of a covalent compound or the atoms of an ionic or metallic compound, the higher the melting and boiling point will be for that compound…. The melting point of a compound will also tell you what state that compound will be in at room temperature (solid, liquid or gas)

The melting point of metallic compounds Atoms in a metallic bond have the strongest attractive force and therefore the highest melting point WHY: metallic cations are all tightly packed together with the valence electrons keeping the nuclei close together since the electrons are shared Note: Their strong attraction to each other is also why the metal can be easily reshaped without breaking

The melting point of ionic compounds The attractive force between cations and anions in an ionic compound is greater than a covalent compound, but less than a metallic compound It is the difference in charge that makes them form a crystalline structure

The melting point of covalent compounds Since polar covalent compounds form hydrogen bonds and dipole interactions, they have a stronger attractive force than the non-polar ionic compounds and therefore have a higher melting point Polar Non Polar