Chemical Bonds in Biology

Electronegativity and Bond Types

Atoms form chemical bonds by sharing or transferring electrons. The type of bond depends on the difference in electronegativity between the atoms involved.

• Electronegativity: The tendency of an atom to attract electrons in a chemical bond.

• If the electronegativity difference is between 0.5 and 1.7, the bond is a polar covalent bond (unequal sharing of electrons).

• If the difference is greater than 1.7, the bond is typically ionic (one atom steals the electron from the other).

• Example: In water (H2O), oxygen (3.5) minus hydrogen (2.1) gives a difference of 1.4, resulting in a polar covalent bond.

Example: An older child (more electronegative atom) gets the toy (electrons) more of the time than a younger child (less electronegative atom).

Ions and Ionic Bonds

When atoms gain or lose electrons, they become ions. Oppositely charged ions attract each other to form ionic bonds.

• Ion: An atom or molecule with an electric charge due to loss or gain of electrons.

• Cation: Positively charged ion (e.g., Na+).

• Anion: Negatively charged ion (e.g., Cl-).

• Ionic bond: The electrostatic attraction between oppositely charged ions (e.g., NaCl, table salt).

In living organisms: Covalent bonds are generally stronger than ionic bonds.

Types of Chemical Bonds

• Non-polar covalent bond: Equal sharing of electrons (e.g., O2 molecule).

• Polar covalent bond: Unequal sharing of electrons (e.g., H2O).

• Ionic bond: Transfer of electrons (e.g., NaCl).

• Hydrogen bond: Weak attraction between a hydrogen atom (attached to an electronegative atom) and another electronegative atom. Hydrogen bonds do not combine atoms into molecules but occur between molecules.

Strength comparison: Hydrogen bonds are much weaker than covalent bonds (about 1/20 as strong).

Water Molecule and Its Properties

Structure and Polarity of Water

Water (H2O) is a polar molecule due to the difference in electronegativity between oxygen and hydrogen. The oxygen atom is partially negative, while the hydrogen atoms are partially positive.

• Polarity allows water molecules to form hydrogen bonds with each other.

Facts About Water

• A large percentage of living things are made up of water (e.g., humans: 65%, plants: 78%, jellyfish: 95%).

• About 2/3 of the Earth's surface is covered by water.

• Life originated in water, and most chemical reactions in life take place in water.

States of Water

• Water can exist as a solid, liquid, or gas under natural conditions.

Properties of Water

Water has several unique properties that are essential for life.

1. Ability to Moderate Temperature

• Specific heat: The amount of energy required to raise the temperature of 1 gram of a substance by 1°C.

• Water has a high specific heat (1 calorie/gram/°C), which helps moderate Earth's climate and organisms' body temperatures.

• Heat of vaporization: The amount of energy required to convert 1 gram of a liquid to a gas. For water, this is 540 calories, much higher than alcohol (0.6 cal), iron (0.1 cal), or air (0.004 cal).

Example: Sweating cools the body as water evaporates, removing heat. Bees drink dew to cool down.

2. Ice Floats

• Unlike most substances, solid water (ice) is less dense than liquid water, so ice floats.

• This is due to the hydrogen bonds holding water molecules further apart in ice than in liquid water.

Example: In a pond, ice forms on the surface, insulating the water below and allowing aquatic life to survive in winter.

3. Water as a Solvent

• Water is known as the "universal solvent" because it can dissolve many substances, especially polar and ionic compounds.

• Example: Salt (NaCl) dissolves easily in water, while sugar (a polar molecule) also dissolves due to attraction to water molecules.

4. Cohesion

• Cohesion: The attraction between like molecules (e.g., water molecules stick to each other via hydrogen bonds).

• Cohesion causes surface tension, allowing small insects or objects to rest on the water's surface.

Example: The basilisk lizard can run across water due to surface tension.

5. Adhesion

• Adhesion: The attraction between different types of molecules (e.g., water molecules sticking to glass).

• Adhesion helps water climb up plant vessels and stick to other surfaces.

Example: Water between two glass slides makes them difficult to pull apart due to adhesion.

6. Capillary Action

• Capillary action is the movement of water within narrow spaces due to the combination of cohesion and adhesion.

• It allows water to travel from the roots to the leaves in plants.

• The meniscus is the curve seen at the surface of a liquid in a tube, caused by capillary action.

Example: In a thin tube, water rises higher than the surrounding liquid due to hydrogen bonding and capillary action.

Summary Table: Properties of Water

Additional info: The notes reference demonstrations and analogies (e.g., children with toys, lizards running on water) to illustrate concepts. These are common in introductory biology courses to help students visualize molecular interactions.