Intermolecular Forces

What Holds A Liquid Or Solid Together? In ionic compounds, all atoms are equally bonded together in the crystal What about covalent?

Two Types of Covalent Compounds Covalent Network Crystal - all atoms covalently bonded together equally - very strong and hard - ex: diamond, graphite

Covalent Molecular Substance - individual molecules attracted to one another through intramolecular forces - not anywhere near as strong as networks - ex: water

Intermolecular Forces Force of attraction between molecules in a covalent molecular substance Three types: Dipole – Dipole forces Hydrogen Bond London Dispersion forces

Dipole- Dipole Between polar molecules Positive pole of one ion attracted to the negative pole of another

Hydrogen Bonding Attraction between hydrogen atom in one molecule and an unshared pair of electrons on the atom of another molecule

London Dispersion or Van der Walls Force Hold non-polar molecules together Caused by motion of electrons

Strength of Interaction Substances held together by ionic bonds or covalent network bonds are very rigidly connected: mostly solid at room temp and melt at very high temps Those held by dipole-dipole or hydrogen bonds are fairly strongly connected: liquid at room temp or solids that melt at low temps Those held by london dispersion forces are weakly connected: gases at room temp; need to be very cold to become liquid

Ionic vs. Covalent Ionic Covalent Molecular Melting Point High Low   Ionic Covalent Molecular Melting Point High Low Vaporize No Maybe Crystalline Yes, hard & brittle Maybe, soft Dissolve in Water Yes If polar covalent Conduct electricity when liquid Conduct electricity when dissolve in water

Example Substance A melts at 600oC, dissolves well in water, and does not conduct electricity when solid but does when dissolved in water. Is the substance held together by ionic or covalent bonds?

Example Identify the interparticle forces present in each substance a) HI b) SO2 c) Ne d) CF4 e) CH3OH

Stronger the intermolecular forces, - the more solid in nature the substance - higher the melting and boiling points - higher the viscosity and surface tension - lower the vapor pressure

Metallic Bond Bond that exists between metal atoms

Electron Sea Model Metal atoms give up valence electrons and form + ions The released electrons move freely around the + metal ions

Properties of Metallic Bonds Good conductors of electricity – free electrons Malleable and ductile – not in rigid position so ions can be shaped and drawn into wires Conduct heat because of free electrons Lusterous – absorb and emit light in regular pattern due to free electrons

Alloy Mixtures of metals

Interstitial Alloy One metal in the mixture is a much smaller atom than the other Increases the overall density of the metal Tends to decrease malleability and ductility Steel is an example: carbon between iron

Substitutional Alloy Atoms of similar radius mixed Density is an average of the pure metal density Stays malleable and ductile Brass is an example: Copper and Zinc