The Structure of Water

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Figure 1: Water Molecule1

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    The water  molecule is comprised of two hydrogen (H) atoms and one oxygen atom. The oxygen atom has 8 electrons, and each H has 1 electron.  The H atoms bond to the oxygen by sharing a pair of electrons in what is called a covalent bond.  In each pair of shared electrons, one electron is contributed by the H (black) and one from the oxygen (red).

Water is a molecule (H2O) that contains two hydrogen atoms each sharing a pair of electrons with an oxygen atom (see Figure 1). When atoms share electrons in this way, a covalent bond is created. These bonds are essential to living organisms.

In water molecules, oxygen and hydrogen atoms share electrons unequally. Electrons, which always carry a negative charge, are drawn more strongly to oxygen atoms. Because the shared electrons spend more time circling the oxygen nucleus and less time circling the nuclei of the hydrogen atoms, the water molecule becomes polarized with distinct negative (oxygen) and positive (hydrogen) ends. This property is referred to as molecular polarity.

Closer Look

This paper published at the E.R. Johnson Research Foundation, University of Pennsylvania presents a more in depth description of water structure and properties. from the Stockholm Resilience Centre to lean more about resilience.

Since positive and negative charges attract each other, polar water molecules align when they approach each other—the positive hydrogen end of one molecule is attracted to the negative oxygen end of a second molecule. This attraction is called a hydrogen bond (see Figure 2). Water molecules are loosely bound together by weak hydrogen bonds, which gives water its liquid property. If hydrogen bonds were stronger, water would be a rigid, rather than a fluid substance.

water hydrogen bonds

Figure 2: Hydrogen Bonding.1

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    There are 5 water molecules, each consisting of one oxygen atom (red ball) and two hydrogen atoms (white balls). The bonds holding the H2O molecules together are weak hydrogen bonds. These weak bonds give water its fluid structure.

A drop of water contains trillions of water molecules held together by weak hydrogen bonds. The tendency of water molecules to bond is called cohesion. The fact that water molecules cohere using weak hydrogen bonds is what gives water its fluid consistency.

Water’s polarity also makes it adhesive, which means that it will be attracted to other types of molecules with positive and negative charges. Think of how trees transport water from the soil up into their high branches without any kind of pump. Water molecules enter the root and adhere to the molecules that make up the walls of the plant’s conductive tissues, called the xylem cells, which are shaped like tiny drinking straws. Water evaporates into the air at the opposite end of the xylem—the leaf—causing an upward, cohesive pull on the entire column of water and replacing the water that was vaporized.

Looking Ahead

Later in this chapter, you will learn beautifully flowing, life-giving fluidity of water makes possible what many religions of the world see as the spiritual significance of water.

Cohesion and adhesion are some of water’s most remarkable qualities. They are strong enough to work against the force of gravity, allowing water to move to the top of a tree hundreds of feet high. This movement is called capillary action and is a key way plants transport fluids through their bodies.