Question

Draw the net dipole for a water molecule using standard convention.

Short answer

The net dipole of a water molecule points from the hydrogen atoms towards the oxygen atom.

Step-by-step solution

Understanding the Water Molecule Structure

A water molecule, H₂O, consists of two hydrogen atoms covalently bonded to one oxygen atom. The molecule has a bent shape due to the two lone pairs on the oxygen atom, which create an angle of approximately 104.5° between the hydrogen-oxygen-hydrogen atoms.

Electronegativity and Bond Dipoles

Oxygen is more electronegative than hydrogen, which means it has a stronger ability to attract electrons. This difference in electronegativity causes the electrons in the O-H bonds to be pulled more towards the oxygen, creating two bond dipoles with the negative end (represented by the arrowhead) pointing towards the oxygen.

Drawing the Bond Dipoles

Represent each O-H bond dipole with an arrow pointing from the hydrogen towards the oxygen. The line end of the arrow starts at the hydrogen atom, and the arrowhead points towards the oxygen atom. This illustrates the direction of electron density.

Determining the Net Dipole

To find the net dipole moment, consider the vector sum of the bond dipoles. Since the water molecule is bent, the bond dipoles do not cancel each other out completely, and their vector sum points upwards and towards the oxygen, creating a net dipole moment.

Drawing the Net Dipole

Draw a single arrow representing the net dipole moment. Place the tail at the geometric center of the molecule and the head pointing towards the oxygen atom, following the direction determined in Step 4. This arrow should be larger than the individual bond dipoles to represent the overall polarity of the molecule.

Key concepts

Bond Polarity

In chemistry, bond polarity is a key concept that refers to the unequal sharing of electrons between two atoms in a covalent bond. This unequal sharing occurs because different atoms have different affinities for attracting electrons, giving rise to regions of partial positive and negative charges.
In a polar covalent bond:

• The atom with a greater electron affinity becomes slightly negative.
• The other atom, with lesser electron affinity, becomes slightly positive.

This separation of charge leads to what is known as a dipole moment, where the direction of the dipole can be represented by an arrow. The tail of the arrow starts at the positive end, and the arrowhead points to the negative end, indicating the direction in which the electrons are more concentrated. In water molecule bonds, this means electrons gravitate more towards the oxygen, making water a prime example of a molecule with polar bonds.

Electronegativity

Electronegativity is the measure of an atom's ability to attract and hold onto electrons in a chemical bond. It is a vital factor in determining bond polarity. The higher the electronegativity, the greater an atom's power to attract electrons.
For example,

• Oxygen has an electronegativity of about 3.44 on the Pauling scale.
• Hydrogen's electronegativity is roughly 2.20.

This substantial difference means oxygen attracts the shared electrons in a water molecule more effectively than hydrogen, creating partial negative and positive charges. Electronegativity not only predicts how atoms will interact but also helps in understanding the molecular structure and properties of compounds. In the case of water, this results in an overall dipole moment, making it a polar molecule.

Dipole Moment

The dipole moment is a quantitative measure of the polarity of a molecule and is crucial in understanding molecular interactions and reactions. It depends on two factors:

• The amount of charge difference.
• The distance between the charges.

A molecule with a large difference in electronegativity and a peculiar shape will typically exhibit a strong dipole moment. In the water molecule, this is seen in the way the oxygen and hydrogen atoms are bonded. The dipole moment is represented with an arrow pointing towards the region with more electron density (oxygen in water), signifying the overall polarity of the molecule. This characteristic affects water's ability to dissolve substances and influences how it interacts with other molecules.

Water Molecule

The water molecule, known for its formula H₂O, is a prime example of polar covalent bonding due to its unique molecular structure. Consisting of two hydrogen atoms and one oxygen atom, the molecule forms a bent shape. This is because the lone pairs on the oxygen repel the bonded pairs slightly, resulting in an angle of approximately 104.5° between the hydrogen atoms.
The consequences of this configuration are significant:

• Water exhibits a net dipole moment, leading to its well-known polar nature.
• Its ability to create hydrogen bonds results in high surface tension and boiling point.
• These properties make water an excellent solvent for polar substances.

The geometry and polarity of water are fundamental to many biological processes and environmental phenomena.

Bent Molecular Shape

The bent molecular shape of water is an outcome of the electron pair repulsion theory. The oxygen in water has two lone pairs along with two hydrogen atoms which contribute to this shape. Lone pairs occupy more space than bonded pairs, resulting in a repulsion that pushes the hydrogen-oxygen-hydrogen bonds closer together, forming an angle of 104.5°.
Understanding the bent shape is crucial because:

• It prevents the cancelation of bond dipoles, contributing to the molecule's overall polarity.
• It is integral to the molecule's ability to form hydrogen bonds.
• It influences how water interacts with other polar substances and ions.

The bent shape of the water molecule is essential for its characteristic chemical and physical properties, influencing both its role in nature and use in diverse applications.