Hydrogen bonding is a bonding type consisting of dipole and dispersion forces. A hydrogen bond is the attractive force between a hydrogen atom attached to a molecule and an atom of a different molecule.
According to the Pauling scale of electro-negativities of the elements, it can be viewed that the three most electronegative elements in the periodical table are nitrogen, oxygen and fluorine. These are also called heteroatoms. The heteroatoms have a partial negative charge while hydrogen has a partial positive charge.
Hydrogen bonding is generally stronger than most bonds bar covalent bonds.
Bonding within water molecules occur between oxygen and hydrogen. These covalent bonds mean that the electrons are shared between the oxygen and hydrogen atoms to create a complete valence shell. However, electrons tend to spend more time around the oxygen atom due to its higher electro-negativity. This creates a partial positive charge around the hydrogen atoms, and a partial negative charge around the oxygen atom. When other water molecules are present, the negatively charged end of one molecule will be attracted to the positively charged end of the other molecule, as shown below:
Hydrogen bonding is not a type of intramolecular force. Eg. Ionic, metallic and covalent bonding, it is an intermolecular force. An intramolecular force is one that exists between the atoms in a molecule. With water, it is the covalent bond present between the oxygen and two hydrogen atoms being intramolecular and the force that exists between neighboring molecules being intermolecular. Eg. Dispersion forces, dipole-dipole bonds. It is also known that intramolecular forces are much stronger than intermolecular forces. With relation to strength, the covalent bond in water is approximately 25 times the strength of the intermolecular hydrogen bond caused by di-pole attraction.
One type of intermolecular force is dipole-dipole bonding. Dipole-dipole interactions are the forces that occur between two molecules with permanent dipoles. These work in a similar manner to ionic interactions, but are weaker because only partial charges are involved. Most dipole-dipole interactions make the melting point and boiling points of substances very high. This is a property of hydrogen bonding, with this evidence it is apparent that hydrogen bonding is a type of dipole-dipole interaction.
Dipole-dipole bonds also refer to the unequal share of electrons, therefore making one end of the molecule slightly positive, and one end slightly negative. The word dipole simply means; Di – two, pole – magnetic pole. Therefore, it means two magnetic poles.
This can be shown in further detail in the diagram below:
The positively charged end of one covalent compound comes into contact with the negatively charged end of another polar covalent compound.
Polar bonds are defined as ‘a covalent bond in which there is a separation of charge between one end and the other, in which one end is slightly negative and one end is slightly positive’. The hydrogen – oxygen bonds in water typify this. This is one of the reasons why dipole-dipole bonding can occur.
How does hydrogen bonding actually occur? It can be described in a series or diagrams:
The hydrogen is attached directly to one of the most electronegative elements, causing the hydrogen to acquire a significant amount of positive charge.
Each of the elements to which the hydrogen is attached is not only significantly negative, but also has at least one “active” lone pair
The hydrogen (delta positive) then attaches to a lone pair.
In ammonia, the hydrogens are all lacking electrons, because the more electronegative nitrogen pulls the electrons away from them. The result is that the hydrogens have a delta+ pole, while the lone pair of electrons is a significant delta- pole. It is this significant attraction between these poles on different molecules that cause hydrogen bonding.
The hydrogen bonding in water (H20) is the most significant, because both hydrogens are delta+ and both lone pairs are delta-. The result is that each water molecule can form four hydrogen bonds with other water molecules. The tetrahedral arrangement (diagram below) is a strong arrangement (like diamond and graphite). This means a lot of kinetic energy is needed to disrupt the structure. This means in other words, that water has an exceptionally high melting point for its molecular mass.
In conclusion, hydrogen bonding occurs between a highly delta positive hydrogen atom and a lone pair of electrons on a nitrogen, fluorine or oxygen atom. The hydrogen bonding that occurs is the strongest form of intermolecular, therefore dipole-dipole bonding. Also, due to its geometrical shape and strong bonds, water has a high melting point and also boiling point.
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