BOND............HYDROGEN BOND -- THE ULTIMATUM BOND

The next thing which I thought to be interesting to tell about water is, THE BONDING OF WATER MOLECULES.

To understand the causes of the unusual properties of water, a basic understanding of chemical bonding and the structure of the water molecule is necessary. The ability of elements (basic building blocks of all matter at the atomic level) to combine and form compounds, depends on the ability of their atoms to give up or acquire electrons (negatively charged subatomic particles that orbit the nucleus of the atom). Elements that tend to give up electrons become positively charged ions (cations); and elements that tend to acquire electrons become negatively charged ions (anions).

The power of an element to combine with other elements to form compounds is termed the valence of the element. Valence is a positive or negative whole number (based on the number of electrons gained or lost), and for inorganic compounds, the algebraic sum of the valence numbers of the combining elements is zero. For example, sodium readily gives up one of its electrons to become a cation with a valence of +1, and chloride tends to attract one electron to form an anion with a valence of –1. The oppositely charged ions attract forming a molecule of sodium chloride [(+1) + (-1) = 0]. The electrostatic attraction of oppositely charged ions to form a compound is termed ionic bonding. Both of the elements that combine to make water, hydrogen and oxygen, exist separately in molecules containing two atoms each (H2 and O2). The two atoms are held together by sharing an electron pair in a chemical bond termed a covalent bond. Covalent bonds are much stronger than the ionic bond. The two atoms held together by the covalent bond make a molecule that is much more stable than the individual atoms. The chemical bonds in the water molecule are covalent bonds since the hydrogen atoms combine with the oxygen atom in shared electron pairs. It is the unique distribution of the electrons in the resulting chemical bond that causes the hydrogen atoms to bond with the oxygen atom at a bond angle of 104.5o.

The oxygen atom exerts a relatively strong pull on the shared electron pair causing the hydrogen atoms to become electropositive regions and the oxygen atom to become an electronegative region. Because the positive and negative regions are not evenly distributed around a center point, the water molecule is termed a polar molecule. The polar nature of the water molecule causes it to become electrostatically attractive to other water molecules as well as other ions in solution and contact surfaces with electrostatic sites. The electropositive hydrogen atoms on the water molecule will be attracted to the electronegative oxygen atoms of adjacent water molecules. This “bridging” phenomenon is called hydrogen bonding. Hydrogen bonding is only about 10 percent of the strength of the covalent bond, but it is responsible for most of the unusual properties of water (high freezing and boiling points, high heat capacity, high heats of fusion and evaporation, solvency, and high surface tension).

Hydrogen bonding is responsible for maintaining the integrity of the water molecule during chemical reactions. While other compounds undergo chemical changes (ionization), the water itself will maintain its chemical integrity. In pure water, a relatively small number of molecules will ionize into hydrogen and hydroxyl ions. Thus, pure water is a relatively poor conductor of electrical current. The specific resistance of theoretically pure water is 18.3 megohm-cm, while most potable water supplies have a resistivity of less than 10,000 ohm-cm. Therefore, the purity of water can be readily measured with a conductivity or resistivity meter.

Hydrogen bonding is the reason for the lower density of ice relative to water. At freezing, the water molecules arrange themselves along the directional lines of the hydrogen bonds causing water to expand and become less dense. For this reason, ice floats on water. Increased pressure lowers the melting point of water. The pressure applied to ice by the blade of an ice skate melts the ice providing a layer of water for the ice skater to gracefully glide along the ice. Even at extremely cold temperatures, high pressure will weaken the crystal lattice; this is the reason that huge ice masses such as glaciers will gradually flow. The polar nature of the water molecule causes the molecule to align in an electric or magnetic field. The electronegative oxygen atom aligns toward the positive pole, and the electropositive hydrogen atoms align toward the negative pole.

Water has an exceptionally large dipole moment (1.87 x 10-18 e.s.u.) relative to most other inorganic compounds. Dipole moment is the product of the distance between the charges multiplied by the magnitude of the charge in electrostatic units (e.s.u.).

LETS HAVE SOME WATER NOW...................