5.4D: A Bent Triatomic - \(H_2 O\)

Water is a triatomic species, with one oxygen atom for every two hydrogen atoms per molecule. Through a combination of polarized bonds between the oxygen and hydrogen atoms, and a bent molecular geometry that gives the molecule an overall dipole moment, water posesses several unique chemical and physical properties that are unlike any other species of its size.

Molecular Structure

The classic model for the water molecule (H₂O) is a central oxygen atom bonded to two hydrogen atoms on either side. Unlike other triatomic species that have a linear shape, the bonds on a water molecule are tilted at a slight angle, due to the presence of lone-pair electrons on the oxygen atom. Through VSEPR theory, it is shown that H₂O has a steric number of 4, with two lone pairs and two bond pairs, making its molecular geometry "bent". As a result, the angle between the two oxygen-hydrogen bonds are approximately 104.5°, which is slightly smaller than the angle typically found in tetrahadral-shaped molecules (see above image). This is due to the particularly strong repulsions by the lone-pair electrons on the oxygen atom, which pushes the hydrogens closer together than usual. Each of the oxygen-hydrogen bonds measure approximately 95.84pm in length. This bent geometry due to the presence of lone pair electrons makes water different from typical linear triatomic species, both on an atomic and a macroscopic level.

Physical and Chemical Properties

Through its combination of bent geometry and polarized bonds, H₂O has unique physical properties that are uncharacteristic of most other small molecules. As a result of oxygen being more eletronegative than hydrogen, the oxygen-hydrogen bonds have an unequal sharing of electrons, with a majority of the negative charge going to the oxygen atom, and leaving the positive chage to the hydrogens. With the bent structure of H₂O, this creates a dipole on the overall molecule, with the oxygen end being negatively charged, and the hydrogen end being positively charged. As a polar molecule with hydrogen atoms, H₂O can undergo hydrogen bonding with neighboring H₂O atoms, in which the oxygen atoms are weakly bonded to hydrogen atoms from other molecules, due to the differences in electrical charge (see image above). This intermolecular attraction makes water a relatively stable substance, and gives it physical properties such as a relatively high boiling point (100°C) and melting point (0°C). Compare this to other molecules of this size, but are nonpolar, such as methane (CH₄), with a boiling point of -161.6 °C and melting point of -182.5 °C.

References

1. Housecroft, Catherine E. & Sharpe, Alan G. (2008). Inorganic Chemisty (3rd ed.). United Kingdom: Pearson Education Limited.
2. Petrucci, Ralph H., Harwood, William S., Herring, Geoffrey F., & Madura, Jeffry D. (2007). General Chemistry: Principles & Modern Applications (9th ed.). New Jersey: Pearson Education, Inc.

Outside Links

• http://en.Wikipedia.org/wiki/Water
• http://en.Wikipedia.org/wiki/Water_model

Problems

1) Would you expect BeH₂ to also be a bent triatomic molecule?
- No, because Be lacks the presence of lone pair electrons, which would give the molecule a linear geometry.

2) What makes water highly unusual in its solid phase, compared to its liquid phase?
- Unlike most substances, wherein the solid state is denser than the liquid state, water is less dense in the solid state, meaning its volume expands when freezing.

3) Would you expect ethanol to mix with water? Why or why not?
- Yes, because both substances are polar, the two liquids are miscible, and will mix to form a single homogeneous solution.