IB Topic 4: Bonding. pure carbon is covalently bonded in three of different forms (allotropes) 1....

Allotropes of Carbon and Intermolecular Forces

IB Topic 4: Bonding

Carbon allotropes pure carbon is covalently bonded in

three of different forms (allotropes)1. graphite2. diamond3. fullerenes

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Graphite possesses a layer structure the layers of carbon atoms are

arranged in an repeating fashion

Diamond more compact structure and dense

than graphite one of the hardest materials known

Fullerenes highly stable chemically most famous is the “buckeyball”

› discovers awarded Nobel Prize in 1996› over 1000 fullerene compounds have been made

composed of carbon atoms that form a hollow, cage-like structure› interesting feature of fullerenes is their ability to

enclose other atoms

Intermolecular Forces forces that occur between molecules much weaker than intramolecular (within

the molecule) forces› it takes 464 kJ/mol to break the H-O bonds within

a water molecule and only 19 kJ/mol to break the bonds between water molecules

the strength of the intermolecular forces determines the physical properties of the substance› melting, boiling, reacting, solubility, conductivity,

volatility this will be covered in next PowerPoint

van der Waals forces

van der Walls YouTube (:20) also also known as London Dispersion

Forces even nonpolar molecules have forces that

hold them together the distribution of electrons around an

individual atom, at a given instant in time, may not be perfectly symmetrical› this can produce a temporary, instantaneous

dipoles (polar molecule)› this can then induce a nearby molecule to be

polar and therefore a very weak attraction between the two molecules and so on, and so on…

Sticky secret. Tiny hairs on geckos' feet help maximize

contact with surfaces, allowing van der Waals forces to go to work.

Dipole-Dipole Forces attractive forces between the positive end

of one polar molecule and the negative end of another polar molecule

must be in close proximity for the dipole-dipole forces to be significant

stronger than van der Waal's forces

Hydrogen Bonding

YouTube Hydrogen Bonding (1:40) YouTube Hydrogen Bonding Video (:58) a specific type of dipole-dipole type

interactions stronger than other dipole-dipole and/or

dispersion forces the hydrogen in a molecule (e.g. H-F, H-

O or H-N) is bonded to a small, highly electronegative element (usually an F, O or N atom) on another molecule