How to determine how many hydrogen bonds a molecule can form

INTERMOLECULAR FORCES:HYDROGEN BONDING

Introduction:

The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge.

To recognize the possibility of hydrogen bonding, examine the Lewis structure of the molecule. The electronegative atom must have one or more unshared electron pairs as in the case of oxygen and nitrogen, and has a negative partial charge. The hydrogen, which has a partial positive charge tries to find another atom of oxygen or nitrogen with excess electrons to share and is attracted to the partial negative charge. This forms the basis for the hydrogen bond.

In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.

In the graphic on the left, the hydrogen is partially positive and attracted to the partially negative charge on the oxygen. Because oxygen has two lone pairs, two different hydrogen bonds can be made to each oxygen.

This is a very specific bond as indicated. Some combinations which are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon.

Link to animation of Hydrogen Bonding in Water - Northland Community and Technical College

Hydrogen bonding is usually stronger than normal dipole forces between molecules. Of course hydrogen bonding is not nearly as strong as normal covalent bonds within a molecule - it is only about 1/10 as strong. This is still strong enough to have many important ramifications on the properties of water.

Comparison of Bond Lengths:

The graphic on the left shows a cluster of water molecules in the liquid state. Water is a polar molecule, with the oxygen (red) being the negative area and the hydrogen (white) being the more positive area. Opposite charges attract.

The bond lengths give some indication of the bond strength. A normal covalent bond is 0.96 Angstroms, while the hydrogen bond length is is 1.97 A.

Water Cluster - Chime in new window

Electrostatic Potential as an Indication of Polarity:

The molecular electrostatic potential is the potential energy of a proton at a particular location near a molecule.

Negative electrostatic potential corresponds to a attraction of the proton by the concentrated electron density in the molecules (from lone pairs, pi-bonds, etc.) (colored in shades of red).

Positive electrostatic potential corresponds to repulsion of the proton by the atomic nuclei in regions where low electron density exists and the nuclear charge is incompletely shielded(colored in shades of blue).

The polarity of the water molecule with the attraction of the positive and negative partial charges is the basis for the hydrogen bonding.

Polar Water - Chime in new window

Hydrogen bonds require a $\ce{\delta{+}}$ hydrogen atom capable of forming a hydrogen-bond, so $\ce{H2O}$ makes 2 H-bonds, and $\ce{NH3}$ has 3 H$\ce{\delta{+}}$ atoms capable of forming these bonds.

However hydrogen bonds also require an "active" lone pair capable of accepting a hydrogen-bond, where $\ce{H2O}$ can do 2 of these again, but $\ce{NH3}$ can only accept 1.

So, does water make 4 bonds or 2? Does ammonia make 1 bond or 2? The answer is both, but it depends on how you count it. If you look at an individual molecule in a cluster of water molecules it may appear to be surrounded and held in place by four H-bonds; it makes two with the hydrogen and accepts two with the lone pairs. That molecule is involved in 4 hydrogen bonds.

But if you take 100 water molecules and count how many hydrogen bonds there are between them, the answer will be about 200 because each molecules makes 2 bonds. If you consider each molecule making 4 bonds then you are double counting each bond being made and accepted. The same is true for ammonia, each molecule makes one hydrogen-bond, it also accepts one. You could say that an individual molecule is involved in two bonds, but if you count them all up each molecule only makes one each.

Because of these two different viewpoints, some texts will say water makes 4 and alcohols make 2 bonds, and some will say water makes 2 and alcohols make 1 bond. They are both right, but right in different ways. Whichever way you count them, you must be consistent. If this is related to an exam syllabus, the correct way is defined by that.

How many hydrogen bonds can molecules form?

Each hydrogen atom in the molecule can also form a hydrogen bond. So each water molecule can form a maximum of four hydrogen bonds.

How many hydrogen bonds can a molecule of water make?

Each water molecule can form two hydrogen bonds involving their hydrogen atoms plus two further hydrogen bonds utilizing the hydrogen atoms attached to neighboring water molecules. These four hydrogen bonds optimally arrange themselves tetrahedrally around each water molecule as found in ordinary ice (see right).