Chapter 2: The Chemical Context of Life – General Biology Study Notes

Chapter 2: The Chemical Context of Life

Learning Objectives

This chapter introduces the chemical principles essential for understanding biological processes. Students should be able to:

• Define elements and their properties.

• Recall the three main subatomic particles and their location within the atom.

• Diagram the electron distribution for an element given its number of electrons.

• Differentiates between the three main types of chemical bonds: ionic, covalent, and hydrogen bonds.

• List the six properties of water that make it an important molecule for life.

• Describe the significance of pH and how to convert concentration of H+ to pH.

Elements and Atoms

Definition and Importance of Elements

Elements are substances that cannot be broken down by chemical reactions. They are the basic building blocks of matter.

• Examples: Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O)

• These elements are most important in living organisms.

Atomic Structure

An atom consists of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus.

• Protons: Positively charged particles in the nucleus

• Neutrons: Neutral particles in the nucleus

• Electrons: Negatively charged particles orbiting the nucleus

• Atomic number: Number of protons in the nucleus

• Mass number: Number of protons plus neutrons

• Isotopes: Atoms of the same element with different numbers of neutrons

Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Electron Shells and Valence Electrons

Electrons are arranged in shells around the nucleus. The outermost shell is called the valence shell, and electrons in this shell are called valence electrons.

• Valence electrons determine the chemical reactivity of an atom.

• Atoms are most stable when their valence shell is full.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules and compounds.

• Covalent bonds: Atoms share electrons to fill their valence shells.

• Ionic bonds: Atoms transfer electrons, resulting in charged ions that attract each other.

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to O, N, or F and another electronegative atom.

Example: Water molecules are held together by hydrogen bonds.

Electronegativity and Polarity

Electronegativity is the tendency of an atom to attract shared electrons in a bond.

• When atoms with different electronegativities form covalent bonds, the bond is polar.

• Polar covalent bonds have unequal sharing of electrons.

• Nonpolar covalent bonds have equal sharing of electrons.

Example: In water (H2O), oxygen is more electronegative than hydrogen, making the molecule polar.

Properties of Water

Unique Properties of Water

Water is essential for life due to its unique chemical and physical properties.

• Cohesion: Water molecules stick to each other due to hydrogen bonding.

• Adhesion: Water molecules stick to other surfaces.

• Surface tension: Water has a high surface tension due to hydrogen bonds.

• Specific heat: Water can absorb a lot of heat before changing temperature.

• Heat of vaporization: Water requires a lot of energy to evaporate.

• Density: Ice is less dense than liquid water, allowing it to float.

• Solvent properties: Water is an excellent solvent for polar and ionic substances.

Biological Significance

These properties of water are crucial for maintaining life and regulating temperature in organisms and environments.

Acids, Bases, and pH

Definition and Measurement of pH

pH is a measure of the concentration of hydrogen ions (H+) in a solution.

• Acid: Substance that increases H+ concentration

• Base: Substance that decreases H+ concentration

• Neutral: pH = 7

• Acidic: pH < 7

• Alkaline (basic): pH > 7

Formula:

$pH = -\log_{10}[H^+]$

Example: A solution with [H+] = 0.01 M has a pH of 2.

Buffers

Buffers are chemical systems that help maintain the homeostasis of the acid-base balance in biological systems.

• They resist changes in pH when acids or bases are added.

Key Terms and Definitions

• Element: Pure substance consisting of one type of atom

• Atom: Smallest unit of an element

• Electron, Proton, Neutron: Subatomic particles

• Mass number: Sum of protons and neutrons

• Atomic number: Number of protons

• Isotope: Atom with same number of protons but different number of neutrons

• Valence electron: Electron in the outermost shell

• Covalent bond: Bond formed by sharing electrons

• Single/double covalent bond: Sharing one/two pairs of electrons

• Electronegativity: Atom's ability to attract electrons

• Nonpolar/polar covalent bond: Equal/unequal sharing of electrons

• Ionic bond: Bond formed by transfer of electrons

• Ion: Charged atom or molecule

• Solvent, solute: Dissolving medium and dissolved substance

• Hydrophilic/hydrophobic: Water-loving/water-fearing

• Cohesion/adhesion: Attraction between same/different molecules

• Specific heat: Energy required to raise temperature

• Heat of vaporization: Energy required for evaporation

• Density: Mass per unit volume

• Neutral, basic, acidic: pH categories

• Buffer: Substance that stabilizes pH

Table: Comparison of Chemical Bonds

Bond Type	Mechanism	Strength	Example
Covalent	Sharing of electrons	Strong	H2O, O2
Ionic	Transfer of electrons	Strong (in dry conditions)	NaCl
Hydrogen	Attraction between H and electronegative atom	Weak	Between water molecules

Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Water molecules stick together	Transport in plants
Adhesion	Water molecules stick to other surfaces	Capillary action
Surface tension	High resistance at water surface	Supports small organisms
Specific heat	High energy required to change temperature	Temperature regulation
Heat of vaporization	High energy required for evaporation	Cooling mechanism
Density	Ice floats on water	Aquatic life survival
Solvent properties	Dissolves polar and ionic substances	Biochemical reactions

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Tables have been inferred and organized for comparison and classification purposes.