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Structure of water

Water cycles through the ground, the atmosphere and the bodies of living things. It is also a link between biotic and abiotic molecules. Water is needed by every living thing on Earth. Understanding the chemical structure of water will help you understand why it acts the way it does, whether in everyday situations such as drinking a glass of water, or on a less familiar scale, as seen in Video 1. The insect relies on the water remaining as drops, otherwise it would drown through its spiracles. But what properties of water molecules cause the drop to retain its form?

Water has always been essential to life

Although there are competing hypotheses about how the first cells originated (see subtopic A2.1), all agree that water was essential. Evidence suggests that liquid water has existed on Earth for more than four billion years. Water was required for:

Effective interaction of the naturally formed molecules of life, allowing growth and copying of these molecules.

Formation of compartments, which allowed the development of internal chemistry different from the surrounding environment.

Whatever the specifics may have been, living things have never stopped needing water, which remains abundant on Earth as a solid, liquid and gas. Living things are made up of one or more cells, and each cell contains cytoplasm, a water-based fluid. The chemical reactions that cells perform, known as metabolism, can only occur when the starting materials are in the watery environment of the cell (see subtopic C1.1). The plasma membrane, the barrier between the inside and outside of a cell, only functions in a watery environment (see subtopic B2.1).

Water was essential to the development of the first cells and remains essential as the medium, or facilitator, in which life’s processes take place due to its solvent properties, which are covered in the next section.

The chemical structure of water

A water molecule is formed from one oxygen atom covalently bonded to two hydrogen atoms. However, the pair of electrons in each covalent bond is not shared equally between the oxygen and a hydrogen atom. This is because of a property called electronegativity, which measures how strongly the nucleus of an atom attracts the electrons it shares with another atom.

Electronegativity is influenced by:

The number of protons in the nucleus. Protons are positively charged and therefore attract the negatively charged electrons.

The number of orbital levels in the atom. More orbital levels will decrease the pull on the shared electrons because they will be further away from the nucleus, and because the other negatively charged electrons will shield them from the positive charge of the nucleus.

Oxygen is more electronegative than hydrogen. Therefore, overall, the oxygen atom attracts the shared electrons more strongly than does the hydrogen atom. The uneven sharing of electrons is a polar covalent bond.

A water molecule is a polar molecule. As the shared electrons spend more time with the oxygen atom, the oxygen side of the atom has a partial negative charge, making it the negative pole. The hydrogen side has a partial positive charge, making it the positive pole. Note the geometry of the water molecule: the hydrogen atoms are at an angle (104.5°), not in a straight line. This is because the pairs of electrons repel each other, so they move as far apart as possible in three dimensions.

When drawing water, the element symbol is used to represent the atoms and a solid line is used to show a covalent bond. The superscript + and − symbols show the charge of the atom, and the small delta (δ) shows that the charge is partial. For example, the oxygen in water would be Oδ-. The correct drawing for water is shown in Figure 1.

A diagram showing the polarity of water. It is described in the above two paragraphs.

Figure 1. The polarity of water.

Bonding between water molecules

The partially positive hydrogen side of one water molecule is attracted to the partially negative charge on the oxygen side of other water molecules. An intermolecular force, known as a hydrogen bond, holds the molecules together (Figure 2). Because the charges are partial, the bond is weak and is indicated by a dashed line in drawings. In water, hydrogen bonds break and reform rapidly between different molecules, whereas the polar covalent bonds break much less frequently.

Four water molecules are shown, attached via dotted lines labelled as hydrogen bonds. The water molecules are made up of one central red circle attached to two outer grey circles via solid grey lines labelled as covalent bonds.

Figure 2. Hydrogen bonds between water molecules.

The structure of water leads to formation of chemical bonds: polar covalent bonds within water molecules and hydrogen bonds between them.

Cohesion and its consequences

Cohesion occurs when ‘like’ molecules are mutually attracted. Water is highly cohesive because of the hydrogen bonds between molecules.

Cohesion pulls water molecules together. The force of cohesion may counteract gravity, for example when water is pulled up into a domed droplet shape instead of being pulled flat. Vascular plants use the cohesion of water to transport water up thin tubes called xylem, reaching to the top of the plant, sometimes tens of metres above ground (Figure 3).

In the leaves of a tree, water evaporates from the surface of inner cell walls into air spaces in the leaf. As water molecules evaporate, they are replaced by water from within the cells. As more water molecules move from the xylem into the leaf, tension, a force to pull water against gravity, is transmitted down a continuous column of water molecules all the way to the root.

Due to the hydrogen bonds, water resists increase of its surface area, whether by gravity or by disruption from other objects. Some insects use the unusual strength of water’s surface tension to create a habitat. Recall the ant drinking a droplet of water in Figure 2. Water striders (Gerris lacustris, also called pond skaters) live on the surface of relatively calm and unpolluted bodies of water – see Video 2. Calm water is needed to provide a stable and undisrupted surface; however, many pollutants, including detergents, interfere with surface tension and cause the water striders to sink.