RLO Transcript: Hydrogen Bonding
This is a transcript of the Reusable Learning Object (RLO) entitled "Hydrogen Bonding", online at:

http://www.nottingham.ac.uk/nursing/sonet/rlos/science/hydrogen_bonding/
Introduction
You may be familiar with forces or 'bonds' that hold atoms together within (intra-) molecules (molecular), examples of these are ionic or covalent bonds. There are also bonds or intermolecular forces that operate between molecules holding them together into liquid or solid structures. The hydrogen bond is an example of an intermolecular bond. Hydrogen bonds are much weaker than the types of bonds found within molecules. This doesn’t make them any less important – hydrogen bonds have a very important role in determining the properties of chemical and biological substances. Intramolecular forces such as ionic and covalent bonds determine the chemical properties of materials. When they are broken, the chemical properties change and the substance becomes a different molecule. When hydrogen bonds are broken it is usually only the physical properties of the substance that are altered. Try the next activity to illustrate this point.
Polar Molecules
In order to explain what a hydrogen bond is we must first understand what a polar molecule is. When atoms share electrons within a molecule to form a covalent bond, if the atoms are roughly the same size (e.g. carbon and oxygen in carbon monoxide or CO) then the electrons are evenly distributed amongst the atoms. If the atoms are differently sized then the electrons are pulled towards the larger, stronger atom since electrons are negatively charged and are subject to attraction from positive charges around them. 

Atoms within molecules have different strengths of positive attraction for electrons.  The larger atoms have a stronger attraction to pull the negatively charged electrons towards them. This means that one side or pole of the molecule will have a larger negative charge and the other pole a larger positive charge. The molecule is then described as a ‘polar’ molecule. 

Water molecules are polar – the oxygen atom is 16 times larger and stronger than the hydrogen atom. The oxygen pole of the H2O molecule is therefore slightly negative and the hydrogen atom's pole is slightly positive because the oxygen atom has a greater pull on the shared electrons. This uneven distribution of charges also accounts for the shape of the water molecule.

The positive and negatively charged ends of a polar molecule will attract the opposite charges of the other molecules within the substance. In water the molecules will arrange themselves according to these forces of attraction. 
Hydrogen bonds 
In the diagram you can see that a dashed line has appeared showing where the individual water molecules are attracted to each other according to their opposite electrical charges. This bond between hydrogen and oxygen atoms of two different water molecules is called a hydrogen bond. Hydrogen bonds can form in this way in substances where oxygen and hydrogen or nitrogen and hydrogen groups are present. You can see a hydrogen bond between nitrogen and hydrogen atoms in the substance ammonia.

Molecules containing covalent bonds and hydrogen form the strongest polar forces because hydrogen is such a small atom; almost everything is bigger and so able to have a disproportionately large share of the electrons in the bond.

So although all polar molecules are capable of this sort of electrical attraction, it is only in these particular cases that the attraction is given the name hydrogen bond. This is because the forces of attraction here are unusually strong and warrant special attention.
Water
Water has a high melting point and a high boiling point compared to other liquids – this is because the hydrogen bonds holding the molecules together are stronger than for other substances and it takes a lot of heat to break up the clusters of water molecules.

Why do ice cubes float in a glass of water? When water freezes each of its molecules is surrounded by 4 others linked by hydrogen bonds. Water expands when it becomes ice because of the open space that exists between the arrangements of molecules – this means that it is less dense than water and therefore ice will float on the denser water.

Surface tension is another property of water that can be explained by hydrogen bonds. Look at the molecules of water in the beaker shown. The surface tension of water is high because the hydrogen bonds are tending to pull the water molecules at the surface in towards the centre of the beaker – click on the figure to see this. 
Proteins and protein building
Hydrogen bonds play a very important role in many other biological molecules. The molecule messenger RNA (mRNA) carries the code that instructs cells in which order to arrange amino acids (the building blocks of proteins) to produce a specific protein. mRNA consists of strands of ribose sugar molecules, phosphate groups and a sequence of bases. The four bases uracil, cytosine, adenine and guanine are shaped in such a way as to allow them to pair up with another base by hydrogen bonding. The formation of these base pairs is a crucial stage in the synthesis of proteins.

While RNA is responsible for ensuring that a particular protein is produced at a particular time when the body needs it, a permanent copy of all the codes for all proteins in the nucleus is in the DNA molecule (deoxyribonucleic acid). DNA is similar in composition to mRNA but is a double strand coiled into a helix shape. As we saw with mRNA, the bases in DNA also pair up to form hydrogen bonds that hold the double helix shape together. The hydrogen bonds are a bit like the rungs on a spiral staircase.
Resources

Title & URL 	Description	
Acids, Alkalis and Bases: An Introduction (RLO)	This RLO gently introduces students to the pH scale, the concept of indirect measurement and molecule dissocation into ions. It explains why pure water is netural (ph 7), why acids have a low pH value, and why alkalis and bases have a high pH value.	
Acids, Alkalis and Bases: Further Application (RLO)	This RLO explains further the concept of acid, base and alkali dissociation in water and the effect this has on a solution's pH. This RLO also explains the terms buffers, denaturation, neutralisation and the formation of salt.	
Atomic Bonding (RLO)	This RLO examines ionic, covalent and mixed bonding in atoms.	
Atomic Structure (RLO)	Introducing basic atomic structure, properties, chemistry and notation. 	
Elements that make up the human body (RLO)	Introducing the periodic table of elements, and identifying the major elements involved in the human body and their roles.	
A closer look at water	A short but instructive animation illustrating the polarity of water and hydrogen bonding. Produced by Northland Community & Technical College, Minnesota, USA. 	
Wikipedia: Hydrogen Bond	A detailed entry on hydrogen bonding, and its role in water and proteins, on the public domain encyclopaedia Wikipedia	

Document updated: 16/11/2007