recap

2.2 recap

Some atoms form strong covalent bonds with other atoms by sharing one or more pairs of electrons. Unequal sharing of electrons produces polarity. Other atoms become ions by losing or gaining electrons, and they interact with other ions or polar molecules. Weak forces such as hydrogen bonds also act to attract atoms to one another.

learning outcomes

You should be able to:

  • Explain why covalent bonds are so strong.

  • Identify examples of polar covalent and nonpolar covalent bonds.

  • Distinguish between hydrophilic and hydrophobic interactions using examples.

  • Identify instances in which van der Waals forces are important.

Question 1

Why is a covalent bond stronger than an ionic attraction when a molecule is in water?

In a covalent bond, the shared electrons between two atoms are actually part of both atoms. They are close together, and it takes a lot of energy to break them apart. In an ionic attraction, the electrons are transferred and the two atoms are relatively far apart, so it takes less energy to break an ionic attraction.

Question 2

Consider the following pairs of bonded atoms:

C—H C=O O—P C—C

For each pair, use Tables 2.2 and 2.3 to determine:

  1. whether the bond is polar or nonpolar

  2. if the bond is polar, which end is δ–

  3. whether the atom pair contributes hydrophilic or hydrophobic properties to a molecule

  1. nonpolar; polar; polar; nonpolar

  2. hydrophobic; hydrophilic; hydrophilic; hydrophobic

  3. C=O: δ– at O; O—P: δ– at O

Question 3

Geckos are lizards that are amazing climbers. A gecko can climb up a glass surface and stick to it with a single toe. Professor Kellar Autumn at Lewis and Clark College and his students and collaborators have shown that each toe of a gecko has millions of micrometer-sized hairs, and that each hair splits into hundreds of 200-nanometer tips that provide intimate contact with a surface. Careful measurements show that a million of these tips could easily support the animal, but it has far more. The toes stick well on hydrophilic and hydrophobic surfaces. Bending the hairs allows the gecko to detach. What kind of noncovalent force is involved in gecko sticking?

This is an example of van der Waals forces, which act over a short distance and do not involve polarity.

The bonding of atoms into molecules is not necessarily a permanent affair. The dynamic aspect of life involves constant change, even at the molecular level. In the next section we will examine how molecules interact with one another—how they break up, how they find new partners, and what the consequences of those changes can be.