The Water Molecule

How does the structure of water contribute to its unique properties?

Water is one of the few compounds found in a liquid state over most of the Earth's surface. Like other molecules, water (H₂O) is neutral. The positive charges on its 10 protons balance out the negative charges on its 10 electrons. However, there is more to the story.

Polarity With 8 protons, water's oxygen nucleus attracts electrons more strongly than the single protons of water's two hydrogen nuclei. As a result, water's shared electrons are more likely to be found near the oxygen nucleus. Because the oxygen nucleus is at one end of the molecule, as shown in Figure 2–6, water has a partial negative charge on one end, and a partial positive charge on the other.

A molecule in which the charges are unevenly distributed is said to be “polar,” because the molecule is a bit like a magnet with two poles. The partial charges on a polar molecule are written in parentheses, (–) or (+), to show that they are weaker than the charges on ions such as Na⁺ and Cl^–.

Hydrogen Bonding Because of their partial positive and negative charges, polar molecules such as water can attract each other. The attraction between a hydrogen atom with a partial positive charge and another atom with a partial negative charge is known as a hydrogen bond. The most common partially negative atoms involved in hydrogen bonding are oxygen, nitrogen, and fluorine.

FIGURE 2–6 A Water Molecule A water molecule is polar because there is an uneven distribution of electrons between the oxygen and hydrogen atoms. The negative pole is near the oxygen atom and the positive pole is between the hydrogen atoms.