Question

If water were a linear molecule, (a) would it still be polar, and (b) would the water molecules still be able to form hydrogen bonds with one another?

Short answer

(a) It would not be polar. (b) Hydrogen bonding would be significantly reduced.

Step-by-step solution

Understanding Molecular Polarity

Molecular polarity depends on the geometry of the molecule and the electronegativity of the atoms involved. A molecule with a symmetric distribution of charge is nonpolar. Water (H₂O) typically has a bent shape with an angle of about 104.5°, making it polar because the oxygen atom is more electronegative than the hydrogen atoms, creating a net dipole moment.

Assessing Polarity in a Linear Configuration

If water were a linear molecule, the spatial arrangement of the atoms would be such that the dipole moments of the O-H bonds would cancel out due to symmetry. Specifically, in a linear shape, the oxygen would be in the center with the two hydrogens on opposite sides (H-O-H). The dipole moments in opposite directions would cancel, resulting in a nonpolar molecule.

Evaluating Hydrogen Bonding Potential

Hydrogen bonding occurs when a hydrogen atom, bonded to a very electronegative atom like oxygen, experiences an attraction to another electronegative atom with lone pairs, such as oxygen from another water molecule. While the strength of hydrogen bonds depends on the dipole moment, even a nonpolar linear molecule could, theoretically, exhibit some hydrogen bonding if polar bonds existed. However, the significant decrease in polarity would drastically reduce the ability of water molecules to form hydrogen bonds as effectively as in their bent configuration.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It plays a critical role in determining the physical and chemical properties of substances, including molecular polarity. The shape of a molecule results from the arrangement of its electron pairs around the central atom, which are repelled by each other due to their negative charge.
In the case of water (H₂O), it adopts a bent or angular geometry with an approximate bond angle of 104.5°. This bent shape is crucial for its polar nature, as it allows an unequal distribution of electrical charge within the molecule, contributing to its many unique properties, such as high surface tension and solvent capabilities. If water were to be linear, the polarity characteristics would change drastically due to altered spatial arrangements of its atoms.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges within a molecule. It is a vector quantity with both a direction and magnitude and is a key factor in determining the polarity of a molecule. A molecule is polar if it has a net dipole moment, indicating an uneven charge distribution.
In water, the dipole moment arises from the more electronegative oxygen atom pulling the electron density away from the hydrogen atoms. This creates a partial negative charge on the oxygen and a partial positive charge on the hydrogens, resulting in a strong dipole moment. However, in a linear shape, the dipole moments of the O-H bonds would cancel each other out, leading to a nonpolar molecule. A nonpolar molecule does not have a net dipole moment as the charges are distributed symmetrically.

Hydrogen Bonding

Hydrogen bonding is an intermolecular force that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen or nitrogen, is attracted to another electronegative atom nearby. This type of bonding is strong compared to other van der Waals forces but weaker than covalent or ionic bonds. Hydrogen bonds play a vital role in the unique properties of water, including its high boiling point and surface tension.
For water in its usual bent shape, hydrogen bonding is efficient due to the molecule's polarity, which facilitates strong intermolecular attractions. If water were linear, the reduced or absent dipole moment would lead to weak or nonexistent hydrogen bonds, significantly altering water's physical properties and its ability to form such bonds effectively with neighboring molecules.

Electronegativity

Electronegativity is an atom's ability to attract and hold onto electrons within a chemical bond. It is a fundamental property that influences a wide range of chemical behaviors, including bond formation and polarity. In the context of water, the oxygen atom has a high electronegativity compared to hydrogen, which causes the shared electrons in the O-H bonds to be more attracted towards the oxygen atom.

• The difference in electronegativity between oxygen and hydrogen is a primary reason for water's polar nature.
• The more significant the difference in electronegativity between two atoms, the more polar the bond is likely to be.

When considering a linear configuration for water, the role of electronegativity in creating polarity would be minimized, as the symmetrical shape would negate any dipole moment caused by differences in electronegativity.