What is the attraction between the positive end of one water molecule and the negative end of another?

In his well-known poem "The Rime of the Ancient Mariner", Samuel Coleridge wrote "water, water everywhere, nor any drop to drink." Coleridge was talking about being out on the ocean, but not having any water because he had killed an albatross (apparently bringing bad luck to everyone on the ship). About \(75\%\) of the Earth's surface is water. The major constituent of the human body (over \(60\%\)) is water. This simple molecule plays important roles in all kinds of processes.

Water is a simple molecule consisting of one oxygen atom bonded to two different hydrogen atoms. Because of the higher electronegativity of the oxygen atom, the bonds are polar covalent (polar bonds). The oxygen atom attracts the shared electrons of the covalent bonds to a significantly greater extent than the hydrogen atoms. As a result, the oxygen atom acquires a partial negative charge \(\left( \delta - \right)\), while the hydrogen atoms each acquire a partial positive charge \(\left( \delta + \right)\). The molecule adopts a bent structure because of the two lone pairs of electrons on the oxygen atom. The \(\ce{H-O-H}\) bond angle is about \(105^\text{o}\), slightly smaller than the ideal \(109.5^\text{o}\) of an \(sp^3\) hybridized atomic orbital.

Figure \(\PageIndex{1}\): The water molecule, visualized three different ways: ball-and-stick model, space-filling model, and structural formula with partial charges.

The bent shape of the water molecule is critical because the polar \(\ce{O-H}\) bonds do not cancel one another and the molecule as a whole is polar. The figure below illustrates the net polarity of the water molecule. The oxygen is the negative end of the molecule, while the area between the hydrogen atoms is the positive end of the molecule.

Figure \(\PageIndex{2}\): Water is a polar molecule, as greater electron density is found around the more electronegative oxygen atom.

Polar molecules attract one another by dipole-dipole forces, as the positive end of one molecule is attracted to the negative end of the nearby molecule. In the case of water, the highly polar \(\ce{O-H}\) bonds results in very little electron density around the hydrogen atoms. Each hydrogen atom is strongly attracted to the lone-pair electrons on an adjacent oxygen atom. These are called hydrogen bonds and are stronger than conventional dipole-dipole forces.

Figure \(\PageIndex{3}\): A hydrogen bond is the attraction between a lone pair of electrons on the oxygen atom of one molecule and the electron-deficient hydrogen atom of a nearby molecule.

Because each oxygen atom has two lone pairs, it can make hydrogen bonds to the hydrogen atoms of two separate other molecules. The figure below shows the result—an approximately tetrahedral geometry around each oxygen atom, consisting of two covalent bonds and two hydrogen bonds.

Figure \(\PageIndex{4}\): As a result of two covalent bonds and two hydrogen bonds, the geometry around each oxygen atom is approximately tetrahedral.

Summary

Water is a molecular compound consisting of polar molecules that have a bent shape.
The oxygen atom acquires a partial negative charge, while the hydrogen atom acquires a partial positive charge.

LICENSED UNDER

Electrons are shared differently in ionic and covalent bonds. Covalent bonds can be non-polar or polar and react to electrostatic charges.

Ionic bonds, like those in table salt (NaCl), are due to electrostatic attractive forces between their positive (Na+) and negative charged (Cl-) ions. In unit two, we compared atoms to puppies and electrons to bones in our analogy of how bonding works. In ionic bonding, each puppy starts out with an electron bone, but one puppy acts like a thief and steals the other puppy’s bone (see Fig. 3-1a). Now one puppy has two electron bones and one puppy has none. Because the electron bones in our analogy have a negative charge, the puppy thief becomes negatively charged due to the additional bone. The puppy that lost its electron bone becomes positively charged. Because the puppy who lost his bone has the opposite charge of the thief puppy, the puppies are held together by electrostatic forces, just like sodium and chloride ions!

In covalent bonds, like chlorine gas (Cl2), both atoms share and hold tightly onto each other’s electrons. In our analogy, each puppy again starts out with an electron bone. However, instead of one puppy stealing the other’s bone, both puppies hold onto both bones (see Fig. 3-1b).

Some covalently bonded molecules, like chlorine gas (Cl2), equally share their electrons (like two equally strong puppies each holding both bones). Other covalently bonded molecules, like hydrogen fluoride gas (HF), do not share electrons equally. The fluorine atom acts as a slightly stronger puppy that pulls a bit harder on the shared electrons (see Fig. 3-1c). Even though the electrons in hydrogen fluoride are shared, the fluorine side of a water molecule pulls harder on the negatively charged shared electrons and becomes negatively charged. The hydrogen atom has a slightly positively charge because it cannot hold as tightly to the negative electron bones. Covalent molecules with this type of uneven charge distribution are polar. Molecules with polar covalent bonds have a positive and negative side.

Fig. 3-1: Bonding using a puppy analogy. In this analogy, each puppy represents an atom and each bone represents an electron.

Water is a Polar Covalent Molecule

Water (H2O), like hydrogen fluoride (HF), is a polar covalent molecule. When you look at a diagram of water (see Fig. 3-2), you can see that the two hydrogen atoms are not evenly distributed around the oxygen atom. The unequal sharing of electrons between the atoms and the unsymmetrical shape of the molecule means that a water molecule has two poles - a positive charge on the hydrogen pole (side) and a negative charge on the oxygen pole (side). We say that the water molecule is electrically polar.

Fig. 3-2: Different ways of representing the polar sharing of electrons in a water molecule. Each diagram shows the unsymmetrical shape of the water molecule. In (a) & (b), the polar covalent bonds are shown as lines. In part (c), the polar covalent bonds are shown as electron dots shared by the oxygen and hydrogen atoms. In part (d), the diagram shows the relative size of the atoms, and the bonds are represented by the touching of the atoms.

Activity

The polar covalent bonding of hydrogen and oxygen in water results in interesting behavior, suc

Molecule Orientation

Water is attracted by positive and by negative electrostatic forces because the liquid polar covalent water molecules are able to move around so they can orient themselves in the presence of an electrostatic force. (see Fig. 3-4).

These forces can be observed in the following video:

Although we cannot see the individual molecules, we can infer from our observations that in the presence of a negative charge, water molecules turn so that their positive hydrogen poles face a negatively charged object. The same would be true in the presence of a positively charged object; the water molecules turn so that the negative oxygen poles face the positive object. See Fig. 3-5 for an artist interpretation.

Symmetry and Asymmetry

Remember that in a polar molecule, one atom’s pull is stronger than the other’s. Polar covalent molecules exist whenever there is an asymmetry, or uneven distribution of electrons in a molecule. One or more of these asymmetric atoms pulls electrons more strongly than the other atoms. For example, the polar compound methyl alcohol has a negative pole made of carbon and hydrogen and a positive pole made of oxygen and hydrogen (see Fig. 3-6).

When molecules are symmetrical, however, the atoms pull equally on the electrons and the charge distribution is uniform. Symmetrical molecules are nonpolar. Because nonpolar molecules share their charges evenly, they do not react to electrostatic charges like water does. Covalent molecules made of only one type of atom, like hydrogen gas (H2), are nonpolar because the hydrogen atoms share their electrons equally. Molecules made of more than one type of covalently bonded nonmetal atoms, like carbon dioxide gas (CO2), remain nonpolar if they are symmetrical or if their atoms have relatively equal pull. Even large compounds like hexane gasoline (C6H14), is symmetrical and nonpolar. Electrostatic charges do not seem to have much, if any, effect on nonpolar compounds. See Fig. 3-6 for examples of polar and nonpolar molecules.