Chapter 2: Chemistry – Study Guide and Key Concepts for General Biology

Chapter 2: Chemistry – Study Guide and Key Concepts

Atomic Structure

This section covers the basic structure of atoms, the fundamental units of matter, and introduces key subatomic particles and their properties.

• Atom: The smallest unit of an element that retains the properties of that element. Atoms consist of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus.

• Subatomic Particles:

  • Proton: Positively charged particle found in the nucleus.

  • Neutron: Neutral particle (no charge) found in the nucleus.

  • Electron: Negatively charged particle found in orbitals around the nucleus.

• Atomic Number (Z): The number of protons in an atom's nucleus. Determines the element's identity.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• Relationship: The number of neutrons can be found by subtracting the atomic number from the mass number: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

• Isotope: Atoms of the same element (same number of protons) with different numbers of neutrons.

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

Example: Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.

Elements, Compounds, and the Periodic Table

Understanding the difference between elements and compounds, and how to use the periodic table to find atomic information.

• Element: A pure substance consisting of only one type of atom.

• Compound: A substance formed when two or more elements are chemically bonded together.

• Periodic Table: A chart organizing elements by increasing atomic number and similar chemical properties. It provides information such as atomic number, symbol, and atomic mass.

Example: Water (H2O) is a compound made of hydrogen and oxygen elements.

Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules and compounds. The main types are covalent, ionic, and hydrogen bonds.

• Covalent Bond: A chemical bond formed when two atoms share one or more pairs of electrons.

• Ionic Bond: A bond formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other.

• Hydrogen Bond: A weak bond between a hydrogen atom (already covalently bonded to a more electronegative atom) and another electronegative atom.

Example: In water, the oxygen and hydrogen atoms are held together by covalent bonds, while hydrogen bonds form between different water molecules.

Electron Shells and Valence Electrons

Atoms have electrons arranged in shells around the nucleus. The outermost shell is called the valence shell, and its electrons determine chemical reactivity.

• Valence Electrons: Electrons in the outermost shell of an atom.

• Determining Valence Electrons: The group number in the periodic table often indicates the number of valence electrons for main-group elements.

Example: Oxygen has 6 valence electrons.

Water Molecule Structure and Hydrogen Bonding

Water molecules have a unique structure and form hydrogen bonds, leading to important biological properties.

• Water Molecule: Consists of two hydrogen atoms covalently bonded to one oxygen atom (H2O).

• Polarity: Water is a polar molecule, with a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• Hydrogen Bonds: Form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.

Example: Three water molecules can be shown with hydrogen bonds connecting the oxygen of one to the hydrogen of another.

Emergent Properties of Water

Water exhibits several unique properties due to its polarity and hydrogen bonding, which are essential for life.

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

• Adhesion: Water molecules stick to other substances.

• High Specific Heat: Water can absorb a lot of heat before its temperature rises.

• High Heat of Vaporization: Water requires significant energy to change from liquid to gas.

• Expansion Upon Freezing: Ice is less dense than liquid water, so it floats.

• Versatile Solvent: Water dissolves many substances due to its polarity.

Example: Cohesion allows water to form droplets; adhesion helps water climb plant vessels (capillary action).

Cohesion vs. Adhesion

Cohesion and adhesion are both properties of water related to hydrogen bonding, but they describe different interactions.

• Cohesion: Attraction between water molecules.

• Adhesion: Attraction between water molecules and other materials.

Example: Water beading on a leaf (cohesion); water climbing up a paper towel (adhesion).

Solutions: Solute, Solvent, and Solution

Solutions are homogeneous mixtures of two or more substances.

• Solute: The substance that is dissolved.

• Solvent: The substance that does the dissolving (in biology, usually water).

• Solution: A homogeneous mixture of solute and solvent.

Example: Saltwater is a solution; salt is the solute, water is the solvent.

Hydrophobic vs. Hydrophilic Substances

Substances can be classified based on their affinity for water.

• Hydrophilic: "Water-loving"; substances that dissolve easily in water (e.g., salts, sugars).

• Hydrophobic: "Water-fearing"; substances that do not dissolve in water (e.g., oils, fats).

Example: Table sugar is hydrophilic; vegetable oil is hydrophobic.

Dissociation of Water and Ions Formed

Water can dissociate into ions, which is important for many biological processes.

• Dissociation: Water molecules can break apart to form hydrogen ions (H+) and hydroxide ions (OH-).

Equation:

$\mathrm{H_2O} \rightleftharpoons \mathrm{H^+} + \mathrm{OH^-}$

pH Scale, Acids, and Bases

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

• pH Scale: Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acid: Substance that increases the H+ concentration in a solution (pH < 7).

• Base: Substance that decreases the H+ concentration (pH > 7).

• Logarithmic Scale: Each pH unit represents a tenfold difference in H+ concentration.

Example: A change from pH 7 to pH 5 means a 100-fold increase in H+ concentration.

Buffers and Their Biological Importance

Buffers are substances that minimize changes in pH, helping to maintain stable conditions in biological systems.

• Buffer: A solution that can resist changes in pH when acids or bases are added.

• Importance: Buffers are crucial in biological systems to maintain homeostasis (e.g., blood pH).

Example: The bicarbonate buffer system in human blood helps maintain a stable pH.

Comparison Table: Types of Chemical Bonds

The following table summarizes the main types of chemical bonds discussed:

Bond Type	Description	Strength	Example
Covalent	Atoms share electron pairs	Strong	H2O (within molecule)
Ionic	Transfer of electrons; attraction between oppositely charged ions	Strong (in dry conditions)	NaCl
Hydrogen	Attraction between a hydrogen atom and an electronegative atom	Weak (individually)	Between water molecules

Additional info:

• Some content about chemical reactions and moles/molarity is intentionally skipped, as per the original note.