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Intermolecular forces


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Hydrogen bonding


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Water and other substances exhibit particularly strong dipole-dipole bonds between partially positively charged hydrogen atoms and other partially charged negative atoms. In covalent compounds involving hydrogen there can be the formation of a particularly strong inter-molecular bond called the hydrogen bond. This is particularly important in water. [Diagram goes here - download the original pdf to see it.] The permanent dipoles created by the different electronegativities of hydrogen and oxygen cause hydrogen atoms on one water molecule to be attracted to oxygen atoms on adjacent molecules. It is this hydrogen bonding that explains many of the distinctive properties of water. Most inter-molecular forces are very weak in comparison to the intra-molecular forces that hold molecules together. However, they hydrogen bond in water is approximately th of the strength of a normal bond, and this makes it particular strong for an inter-molecular bond. Ammonia is another substance that exhibits hydrogen bonding. [Diagram goes here - download the original pdf to see it.] Example (1) Discuss the polarity of the bonds in nitrogen and ammonia. Answer: Nitrogen is a diatomic molecule formed between two nitrogen atoms and involving a triple bond - that is, the sharing of three pairs of electrons. Both nitrogen atoms are essentially identical and have the same electronegativity. There is, consequently, no permanent tendency for one of the atoms to withdraw electron density from the other. The molecule will be non-polar. It may form instantaneous dipoles owing to the momentary uneven distribution of electrons within its orbits. Ammonia is formed between atoms of very different electronegativities - nitrogen is the third most electronegative element (with Pauling electronegativity of 3.1) and hydrogen has an intermediate electronegativity (Pauling electronegativity of 2.1). Although the bond is covalent, the stronger electric field of the nitrogen atom draws the electrons in the covalent bond more to the side of the oxygen atom - they spend, on average, more time with the oxygen atom. Consequently, the bond is polar. The assymetrical geometry of the ammonia molecule makes it into an overall permanent diploe. The polarity of the nitrogen-hydrogen bond is responsible for the formation of hydrogen bonds between adjacent ammonia molecules. Example (2): Make a diagram showing the hydrogen bonding between ethanol and water. Show any relevant dipoles. [Diagram goes here - download the original pdf to see it.] Example (3): Nitrogen and carbon monoxide are isoelectronic. (i) Explain what is meant by the term isoelectronic; (ii) construct dot and cross diagrams for nitrogen and carbon monoxide showing both the s and p subshells showing that they are isoelectronic. Answer: (i) Isoelectronic means having the same number of electrons and the same electronic configuration. Both carbon monoxide and nitrogen have 12 electrons in all and their configurations are identical as part (ii) shows. (ii) [Diagram goes here - download the original pdf to see it.]
Contents of
Intermolecular forces

1 Intra- and inter- molecular forces
2 Polar molecules
3 Dipole-dipole forces
4 Hydrogen bonding
5 Instantaneous dipole-induced dipole forces
6 Polar and non-polar substances
7 Exercise in Intermolecular Forces

Related articles: (1) Covalent Bonding, (2) Intermolecular forces