Why is water a polar molecule?

Water is a polar molecule because it has two polar OH bonds whose dipole moments do not cancel each other. These dipole moments point toward oxygen and add up to give the molecule a net dipole moment.

This polarity is responsible for many of the characteristic properties of water, including some of its chemical reactivity, its melting and boiling points, and its ability to act as a universal solvent for ionic and polar solutes, among others.

In other words, the polarity of water, like that of any other molecule, is a direct consequence of the polarity of its bonds, as well as of molecular geometry. Understanding these two concepts and how they apply to the water molecule will give a more complete idea about the polarity of molecules.

What is a polar bond?

A polar bond is a type of covalent bond in which one of the two atoms is more electronegative than the other, so the electron density of the bond attracts more strongly. The consequence of this is that the electrons are not shared equally. The more electronegative atom acquires a partial negative charge (identified by δ-), while the other acquires a partial positive charge (identified by δ+).

Both partial charges are of equal magnitude and opposite sign, making polar bonds electric dipoles .

Whether or not two atoms form a polar covalent bond depends on the difference between their electronegativities. If the difference is too large, the bond will be ionic, but if it is very small or zero, it will be a pure covalent bond. Finally, the bond will be polar covalent if the difference is intermediate. The limits for each case are presented in the following table:

dipole moment

Polar bonds are characterized by the dipole moment. This is a vector denoted by the Greek letter μ (mu) pointing along the bond in the direction of the more electronegative atom. The magnitude of this vector is given by the product of the magnitude of the separated charge, which is proportional to the difference in electronegativities, and the distance between the two charges, that is, the bond length.

The dipole moment is essential to understanding why water is polar, since the total polarity of a molecule comes from the vector sum of all its dipole moments.

molecular geometry

The geometry of a molecule indicates the way its atoms are distributed around a central atom. For example, in water, the central atom is oxygen, so the molecular geometry indicates how the two hydrogen atoms are oriented around the oxygen.

There are different ways to determine molecular geometry. The simplest is through the theory of valence electron pair repulsion, which states that the pairs of electrons that surround the central atom (whether bonding or lone pairs of electrons) will be oriented to be as far as possible from each other. other.

After determining how the electrons are distributed around the central atom, the geometry is determined by looking at where the bonds point (not taking into account the lone pairs of electrons).

Having understood these two concepts, let us now analyze the water molecule, its bonds and its geometry:

The OH bonds in water are polar bonds.

Water has two hydrogen atoms bonded to one oxygen atom. The electronegativity difference between oxygen and hydrogen is 1.24, making it a fairly polar bond (see table above). The figure above illustrates the dipole moment of this bond. Note should be taken of the fact that the vector is often drawn to the side of the link for easy viewing; however, it actually coincides with the OH bond, pointing from the hydrogen nucleus toward the oxygen nucleus.

The water molecule has angular geometry

^{In the water molecule, the oxygen atom is sp 3} hybridized and is surrounded by four pairs of electrons (the two hydrogen bonding pairs and two unshared pairs). The valence electron pair repulsion theory states that four pairs of electrons will point toward the ends of a regular tetrahedron. In other words, the two hydrogen atoms will point towards two of the four corners of a tetrahedron, making the water molecule an angular molecule.

The angle between the two bonds should be a tetrahedral angle of 109.5º, but the two lone pairs of electrons repel the bonding electrons more strongly, narrowing the angle slightly. The result is that the two OH bonds in water form an angle of 104.45º as shown in the figure above.

Polar bonds + angular geometry = polar molecule

It is important to recognize the fact that having polar bonds does not ensure that a molecule is polar. In fact, carbon dioxide has two polar bonds, but their dipole moments cancel each other out. For this reason, the molecule is nonpolar.

This does not happen with the water molecule, since it is not linear but angular. Now that we have a clear picture of the characteristics of the water molecule, we can move on to determining the net dipole moment of the molecule. This is done by drawing both dipole moments on top of the molecule, and then carrying out the vector addition:

The addition can be carried out graphically, using the parallelogram method, as shown on the right side of the previous figure. As can be seen, both dipole moments produce a net dipole moment pointing towards the oxygen passing through the center of the molecule.

Ultimately, this net dipole moment is the reason why water is a polar molecule.