Water (H2O) is a polar molecule and a polar solvent. What does this mean? When a molecule is polar, it means its positive and negative electrical charges are unevenly distributed so part of the molecule is partially positive, while part is partially negative. In diagrams, the lowercase letter delta (δ) shows the charge distribution in a polar molecule.
The positive charge comes from the protons in the atomic nucleus, while the negative charges comes from the electrons. Each hydrogen atom in a water molecule has one electron that spends most of its time between the hydrogen and oxygen nucleus, leaving the hydrogen nucleus more exposed than if the electron wasn’t part of a chemical bond. The hydrogen atoms carry a partial positive charge. Meanwhile, the oxygen atom has two unbound electron pairs that are as far as possible from each other and the chemical bonds, giving the oxygen atom a partial negative charge.
To understand why a water molecule is polar, while similar-seeming molecules (e.g., carbon dioxide or CO2) are not polar, you need to understand the roles of electronegativity and molecular geometry in polarity.
Electronegativity and the Polarity of Water
Atoms with different electronegativity values form polar bonds. If the electronegativity difference is large enough (e.g., between a metal and a nonmetal), a highly polar ionic bond forms. Slight differences between atoms (e.g., two different nonmetals) lead to polar covalent bond formation. Electrons participating in a polar covalent bond spend more time closer to one atom than the other, leading to partial positive and negative charges around the atoms. So, a molecule like carbon monoxide (CO) is polar. The carbon atom has a partial positive charge, while the oxygen atom has a partial negative charge.
Molecular Geometry and the Polarity of Water
But, molecular geometry also plays a part in molecule polarity. Although the covalent bonds between carbon and oxygen are polar in carbon dioxide (CO2), the molecule is not polar. This is because carbon dioxide is a linear molecule and the partial positive and negative charges effectively cancel each other out. In other words, its net dipole moment is zero.
Unlike carbon dioxide, water is not a linear molecule. Water has a bent geometry, with 104.5°. The bent shape means the positive and negative charges aren’t evenly distributed and don’t cancel each other out. Water has a net dipole moment.
The reason water has a bent geometry is because the oxygen atom has two lone electron pairs. The electronic structure of oxygen is 1s2 2s2 2p4. Each hydrogen atom contributes one electron to fill the valence shell and give oxygen 1s2 2s2 2p6, but this means four of the electrons (2 pairs) in the 2p shell aren’t participating in a chemical bond. The electron pairs have the same negative electrical charge, so they repel each other. They are also repelled by the chemical bonds between the hydrogen and oxygen atoms, but not by the same amount. At the same time, the hydrogen atoms repel each other. The balancing act between the repulsion leads to a tetrahedral geometry. But, the electron pairs are an invisible component of the geometry, so what we see is a bent molecule.
Why Water Is a Polar Solvent
The shape and polarity of the water molecule affects its interaction with other water molecules and with other compounds. The reason water is a polar solvent is because it attracts either a positive or negative electrical charge of a solute. The oxygen atom’s partial negative charge attracts hydrogen atoms from other water molecules and positive regions from other molecules. Meanwhile, hydrogen’s partial positive charge attracts oxygen atoms from other water molecules and negative regions of other molecules.
The attraction between oxygen and hydrogen atoms of neighboring water molecules leads to hydrogen bond formation. Hydrogen bonds aren’t as strong as covalent bonds and not all water molecules in a sample participate in them. At any given time, about 20% of water molecules are free to interact with other chemical species. This interaction is called dissolving or hydration. It is a key property of water that gives water the name “universal solvent.” While water dissolves more substances than any other solvent, it isn’t really “universal” because it only dissolves polar solutes.
Remember, although water is polar, it’s also electrically neutral. The partial positive and negative charges may be unequally separated, but they still cancel each other out. Each water molecule contains 10 protons and 10 neutrons, but a net charge of 0.
References
- Huheey, J.E.; Keiter, E.A.; Keiter, R.L. (1993). Inorganic Chemistry: Principles of Structure and Reactivity (4th ed.). HarperCollins, New York.
- Jensen, William B. (2009). “The Origin of the “Delta” Symbol for Fractional Charges”. J. Chem. Educ. 86 (5): 545. doi:10.1021/ed086p545
- Pauling, L. (1960). The Nature of the Chemical Bond (3rd ed.). Oxford University Press. ISBN 0801403332.