Facts About Water Molecules: Structure & Properties

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  • 0:00 Plenty of Water
  • 0:56 Water Properties & Structure
  • 3:56 Hydrogen Bonding
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Lesson Transcript
Instructor: Gail Marsella
Water is more unique than you might think, despite the huge quantities available on our planet. This lesson explores the structure of water and how it contributes to the properties we find so useful.

Plenty of Water

Water, water, everywhere. Whole oceans of it cover most of the earth. If we piled all the water in the world up on the United States, it would cover the entire country to a depth of over a hundred miles, an amount approximately 332,500,000 cubic miles in volume. Water vapor in the atmosphere keeps the planet much warmer than it would otherwise be. The human body is more than half water, more for babies, less for adults, and virtually every biochemical reaction necessary for life runs in water. We use the power in swiftly moving water to produce electricity for cities; this is known as hydropower. We grow plants, wash dishes, and put out fires with water. We ski on it, cook with it, and drink it. We even complain about it - particularly when it snows or rains too much - but we need it.

Water Properties & Structure

We think of water as common and familiar, but chemically it is one strange molecule. Most chemicals shrink and sink when they freeze, but water expands and floats. Most lightweight molecules are gases at room temperature, but water is a liquid. It has an unusually high boiling point, 100 degrees Celsius or 212 degrees Fahrenheit, and freezing point, 0 degrees Celsius or 32 degrees Fahrenheit. Water also dissolves a large number of different substances, from solid table salt to liquid alcohol, and to a lesser extent gases such as oxygen and carbon dioxide.

All of these properties result from water's structure. An atom is the smallest piece of an element, and it can bond with other atoms to make a molecule, which in turn is the smallest piece of a compound. Water is a compound, and each of its molecules has two hydrogen atoms bonded to a single oxygen atom.

Notice the bent shape of the water molecule. All atoms have negatively charged particles called electrons circling around a positively charged nucleus. The electrons arrange themselves in shells and are most stable if paired, and the electron pairs are as far apart as possible. In water, only the outermost electrons, those in the outer shell, participate in bonding. An oxygen atom alone has six electrons in its outer shell, and each hydrogen atom initially has one. When the hydrogen atoms share their electrons with two of the oxygen atom's outermost shell electrons, two bonds form, one each between a hydrogen atom and the oxygen.

Now the oxygen atom has eight electrons in its outer shell - its own six electrons plus the two from the hydrogen atoms. This makes two bonding pairs and two non-bonding pairs. The four pairs of electrons around oxygen all take up space, and the most stable arrangement is at the corners of a tetrahedron, a pyramid shape with oxygen at the center and the four electron pairs pointing toward the four corners. As a result, the water molecule looks bent, with the hydrogen atoms on one side of the oxygen atom and the unpaired electrons on the other side.

Additionally, oxygen draws electrons to itself very strongly; a chemist would say it is highly electronegative. The hydrogen atoms, which are much less electronegative than oxygen, lose some electron density to the oxygen, so the electric charge inside the molecule becomes lopsided. The oxygen side of water is more negative and the hydrogen side more positive, forming a dipole, which means the molecule is positive on one side and negative on the other, even though the molecule overall is neutral. Most molecules have weak partial charges like this, which are written as delta plus and delta minus, but only in water and a few other molecules are the partial charges this strong and this fixed. We say that a molecule with a permanent dipole is polar.

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