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Introduction to Chemical Mixtures

Author(s): David R. Caprette, PhD

Structure of Water

A molecule carries no net electric charge when there are as many negatively charged orbiting electrons as there are positively charged protons in its atomic nuclei. Different parts (poles) of an electrically neutral molecule can nevertheless carry a partial positive or negative charge that can attract or repel other charged structures. Such molecules are called polar molecules.

A water molecule consists of two hydrogen atoms and one oxygen atom and is electrically neutral. Water molecules are highly polar, however. Oxygen has a greater affinity for electrons than does hydrogen, so on average, the outer electrons are closer to the oxygen atom than to either of the two hydrogen atoms. The result is that the oxygen atom carries a partial negative charge and the hydrogen atoms each carry a partial positive charge.

Because opposite charges attract each other, the hydrogen atoms of a water molecule tend to "stick" to oxygen atoms on nearby water molecules. This property is called hydrogen bonding. Hydrogen bonding keeps water from spreading out when it is placed on a surface, that is, it gives water the property of surface tension. This structure of water is so stable that when something else is tossed into water it tends to separate out (that is, the water molecules separate themselves from the intruding molecules) unless the new substance contributes something "special" to the structure of water. In scientific terms, it takes free energy to disrupt the molecular structure of water or of any solvent. Therefore, if a substance is to go into solution spontaneously, it must make the structure of water even more stable.