Polarity of Water Molecules

The best example of this charge screening is the water molecule, represented as \({\text{H}}_{2}\text{O}\). Water is a strongly polar molecule. Its 10 electrons (8 from the oxygen atom and 2 from the two hydrogen atoms) tend to remain closer to the oxygen nucleus than the hydrogen nuclei. This creates two centers of equal and opposite charges—what is called a dipole, as illustrated in this figure. The magnitude of the dipole is called the dipole moment.

These two centers of charge will terminate some of the electric field lines coming from a free charge, as on a DNA molecule. This results in a reduction in the strength of the Coulomb interaction. One might say that screening makes the Coulomb force a short range force rather than long range.

Other ions of importance in biology that can reduce or screen Coulomb interactions are \({\text{Na}}^{+}\text{,}\) and \({\text{K}}^{+}\text{,}\) and \({\text{Cl}}^{–}\). These ions are located both inside and outside of living cells. The movement of these ions through cell membranes is crucial to the motion of nerve impulses through nerve axons.

Recent studies of electrostatics in biology seem to show that electric fields in cells can be extended over larger distances, in spite of screening, by “microtubules” within the cell. These microtubules are hollow tubes composed of proteins that guide the movement of chromosomes when cells divide, the motion of other organisms within the cell, and provide mechanisms for motion of some cells (as motors).

Molecular structure of two water molecules is shown. Oxygen atom of one water molecule has minus delta on it and the Hydrogen atom of other water molecule has a positive delta charge on it. The force on attraction between these two atoms is shown as dotted line.

This schematic shows water (\({\text{H}}_{2}\text{O}\)) as a polar molecule. Unequal sharing of electrons between the oxygen (O) and hydrogen (H) atoms leads to a net separation of positive and negative charge—forming a dipole. The symbols \({\delta }^{-}\) and \({\delta }^{+}\) indicate that the oxygen side of the \({\text{H}}_{2}\text{O}\) molecule tends to be more negative, while the hydrogen ends tend to be more positive. This leads to an attraction of opposite charges between molecules.

This lesson is part of:

Electric Charge and Electric Field

View Full Tutorial

Track Your Learning Progress

Sign in to unlock unlimited practice exams, tutorial practice quizzes, personalized weak area practice, AI study assistance with Lexi, and detailed performance analytics.