Video Transcript
Water is one of the simplest polar solvents, and it contains just one electronegative
oxygen atom that is covalently bonded to two hydrogen atoms. Which of the following diagrams shows how charge is distributed in a water
molecule?
To answer this question, we need to understand some key terms. We are told that water is a polar solvent. A polar solvent is made up of polar molecules. And polar molecules contain polar bonds, which are created as a result of differences
in electronegativity between atoms in a molecule. Electronegativity is the ability or tendency of an atom to attract, or pull, a
bonding pair of electrons towards itself. We are also told that each water molecule contains an electronegative oxygen
atom.
A simple way to think about this is that the oxygen atom attracts each bonding pair
of electrons more strongly than do the hydrogen atoms. And so each bonding pair of electrons ends up closer to the oxygen atom than either
of the hydrogen atoms.
Since the bonding pair of electrons is closer to the oxygen atom, a high proportion
of the electron density surrounds the oxygen atom, relative to the hydrogen
atoms. This creates a partial negative charge at the oxygen end of the covalent bonds and a
partial positive charge at the hydrogen ends.
We use the Greek letter 𝛿 to indicate a partial charge. This unequal charge distribution across these oxygen–hydrogen bonds makes these bonds
polar. And since the molecule is bent and contains two ends, or we could say two poles,
which differ in charge, we say the water molecule is polar too.
Now we are ready to look at the answer options again. If we compare our diagram to the answer choices, we see that the diagram which shows
how charge is distributed in a water molecule is answer (B).
