How many lone pairs does the water molecule, H2O, have? a. none b. one c. two d. three
The Correct Answer and Explanation is:
The correct answer is c. two.
Water (H₂O) is a polar molecule where the oxygen atom is bonded to two hydrogen atoms. To understand how many lone pairs are present on the oxygen atom in water, let’s first examine its electron configuration and molecular structure.
Electron Configuration:
Oxygen has 8 electrons. In its ground state, its electron configuration is 1s² 2s² 2p⁴. The oxygen atom needs two more electrons to complete its octet, which it gets by forming covalent bonds with the two hydrogen atoms. This means the oxygen atom forms two bonds, one with each hydrogen atom.
Lone Pairs on Oxygen:
Oxygen, in total, has 6 valence electrons (since it’s in Group 16 of the periodic table). Two of these electrons are involved in bonding with hydrogen atoms, so the remaining four electrons are not involved in bonding. These remaining electrons are organized into two lone pairs (each lone pair consists of two electrons).
Bonding and Geometry:
In H₂O, the oxygen atom forms two single bonds with two hydrogen atoms. The molecule adopts a bent or V-shaped geometry, due to the lone pairs of electrons on the oxygen. The lone pairs cause repulsion, pushing the hydrogen atoms closer together, resulting in a bond angle of approximately 104.5°.
Polarity and Other Considerations:
Water’s lone pairs play a significant role in its polarity. The presence of these lone pairs on oxygen leads to a net dipole moment, making water a polar molecule. The oxygen atom, being more electronegative than hydrogen, pulls the shared electrons closer to itself, creating partial negative charges on the oxygen and partial positive charges on the hydrogen atoms.
Thus, the water molecule has two lone pairs on the oxygen atom, making option c the correct answer.
