5.2: The Water Molecule

To understand the unusual properties of water, we need to understand the structure of atoms and molecules, particularly the water molecule.

Atoms and Electrons

Atoms consist of negatively charged electrons orbiting in shells that surround a nucleus. The nucleus contains neutrons that have no charge and protons that are positively charged. The electrical charge of a proton is equal and opposite to that of an electron, and the number of protons in each atom is equal to the number of electrons. Hence, atoms are electrically neutral.

Each electron shell is capable of holding only a certain number of electrons. For example, the innermost shell holds a maximum of two electrons, and the second shell holds a maximum of eight. Elements that have their outermost shell filled are noble gases (helium, neon, argon, krypton, xenon), which are inert (that is, they do not react chemically with most atoms).

The atoms of most elements have an outermost electron shell that is not completely filled with the maximum number of electrons. However, it is energetically favorable for each shell to be completely filled. Therefore, atoms of some elements, such as chlorine or oxygen, that have a nearly full outer electron shell, have a tendency to gain electrons in order to fill the shell. Atoms of other elements, such as sodium, that have outer electron shells less than half full, have a tendency to lose one or more electrons, producing an empty outer shell.

Chemical Bonds between Atoms

To fill their outer electron shells, two or more atoms of different elements can combine to create a molecule of a chemical compound. This can happen in two ways. First, one atom can donate one or more electrons to an atom of another element that has an incomplete outer shell. For example, a sodium atom can lose an electron, and a chlorine atom can gain this electron. Both now have completed outer electron shells. Sodium having lost an electron becomes a positively charged sodium ion, and chlorine becomes a negatively charged chloride ion. Because the two ions have opposite electrical charges, they are electrostatically attracted to each other and are bonded together by this attraction (Fig. 5-2). This type of bond is called an ionic bond.

Figure 5-2. The sodium and chlorine atoms of common salt are held together in a salt crystal by ionic bonds. The ionic bond is an electrostatic attraction between the positively charged sodium ion (Na⁺) and the negatively charged chlorine ion (Cl^–).

The second way in which atoms of different elements can share electrons is called a covalent bond. One or more outer-shell electrons spend part of their time in the outer shell of each of the two atoms that are bonded. For example, each of two hydrogen atoms in the water molecule can share its single electron with an oxygen atom. To describe such electron sharing simply, each hydrogen electron orbits both its own hydrogen atom nucleus and within the outer shell of the oxygen atom electron cloud. At the same time, two electrons from the outer shell of the oxygen atom orbit within their own shell and around the hydrogen atoms (Fig. 5-3a). In this way, the oxygen has the extra electrons it needs to fill its outer electron shell for part of the time, and each of the two hydrogen atoms has two electrons to fill its outer electron shell for part of the time. Because the outer shell of each atom in the molecule is full at least part of the time, the bonded atoms form a stable molecule. Most covalent bonds are stronger than most ionic bonds. Consequently, atoms in a covalently bonded molecule usually are more difficult to break apart than those that are bonded by ionic bonds.

Figure 5-3. The water molecule consists of two hydrogen atoms and one oxygen atom. (a) The oxygen atom has six electrons in its outer shell. It lacks two electrons that would be needed to complete this shell. Each hydrogen atom has one electron and needs one more to complete its single shell. In a water molecule, the electrons from the hydrogen atoms and the six electrons from the outer shell of the oxygen atom are shared and orbit around both the oxygen and the hydrogen atoms. This is a covalent bond in which electron clouds from the bonded atoms partially overlap. (b) Both hydrogen atoms in a water molecule are on one side of the oxygen atom and are separated by an angle of 105°. The hydrogen electrons orbit partially around the oxygen atom, leaving the positive nucleus of each hydrogen atom partially “exposed” and giving this side of the water molecule a slight net positive charge. Similarly, the electrons orbiting the opposite side of the oxygen atom give that side of the molecule a slight net negative charge. Hence, water is a polar molecule. (c) Hydrogen bonds are formed between water molecules when the positive side of one molecule is attracted to the negative side of another.

Van der Waals Force and the Hydrogen Bond

Molecules are electrically neutral. However, there is a weak attractive force between all molecules called the van der Waals force. It is caused by the attraction of the protons of one atom to the electrons of another. An additional attractive force or bond, called the hydrogen bond, is present between the molecules of water. In other chemical compounds, the hydrogen bond is either absent or weaker than it is in water. The strength of the hydrogen bond is what gives water most of its anomalous properties (Table 5-2).

Table 5-2. Anomalous Properties of Water and Their Importance

Property	Special Characteristic of Water	Importance
Heat capacity	Higher than that of any solid or liquid other than ammonia.	Water moderates climate in coastal regions. Currents transport large amounts of heat. Ocean water temperatures are relatively invariable in comparison with terrestrial temperatures.
Latent heat of fusion	Higher than that of any substance other than ammonia.	When ice forms, most of the energy lost is released to the atmosphere, and ice absorbs large amounts of heat in melting. Ice therefore acts as a thermostat to keep high-latitude water and atmosphere near the freezing point all year.
Latent heat of vaporization	Higher than that of any other substance.	Heat is transported from the low-latitude ocean by evaporation and atmospheric circulation and released to the atmosphere through precipitation at higher latitudes.
Thermal expansion	Pure water has a density maximum at 4°C. The temperature at which the maximum occurs decreases as salinity increases. There is no maximum at salinity >24.7.	Freshwater and low-salinity seawater stay unfrozen under ice in winter in lakes and estuaries.
Solvent property	Can dissolve more substances than any other liquid.	Water dissolves minerals from rocks and transports them to the oceans. Water is the medium in which the chemical reactions that support life occur.
Surface tension	Higher than that of any other liquid.	Surface tension controls the formation of droplets in the atmosphere and bubbles in the water. Some organisms use surface tension to anchor themselves to or walk on the surface.
Physical states	The only substance present as a gas, liquid, and solid within the temperature range at the Earth’s surface.	Water vapor evaporated from the ocean helps transport heat from warm low latitudes to cold high latitudes. Liquid water also contributes to that transport through ocean currents. In polar regions, the presence of both ice and water moderates climate.

Like other molecules, the water molecule is electrically neutral. However, the arrangement of atoms and electrons in the water molecule is such that the side of the molecule away from the hydrogen atoms has a small net negative charge, whereas the two areas where the hydrogen atoms are located have a small net positive charge (Fig. 5-3b). Molecules that behave as though they have a positive and a negative side are called “polar molecules.” The reasons for the polarity are complicated, but they can be understood through a simplified model (Fig. 5-3a). The six electrons in the outer shell of the oxygen atom are arranged in pairs. The two hydrogen atoms are covalently bonded to the electrons in one of these pairs. This leaves two pairs of unshared orbiting electrons on the side of the oxygen atom opposite the hydrogen atoms, causing this side of the oxygen to have a negative charge bias. On the hydrogen side of the molecule, hydrogen electrons are shared with, and actually spend more time on, the oxygen atom, leaving the positively charged nucleus of each hydrogen atom “exposed.” Hence, this side of the molecule has a positive charge.

Because the water molecule is polar, the negatively charged side of one molecule is attracted to the positively charged side of an adjacent molecule. This attractive force is the hydrogen bond (Fig. 5-3c). It is relatively strong, but not as strong as ionic or covalent bonds. The relative strengths of bonds between atoms and molecules are listed in Table 5-3. The relative strength of a bond is an indication of how much energy is needed to break that bond. Substantially more energy is needed to break hydrogen bonds than is needed to counter van der Waals forces between molecules. The relatively high strength of the hydrogen bond is responsible for the anomalous properties of water.

Table 5-3. Relative Strengths of Bonds between Atoms and Molecules

Type of Bond	Approximate Relative Strength
Van der Waals force (between molecules)	1
Hydrogen bond (between molecules)	10
Ionic bond (between atoms)	100
Covalent bond (between atoms)	Usually >1000