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

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Chapter 2: Chemistry – Study Guide and Key Concepts

Atomic Structure and Elements

This section covers the basic structure of atoms, the nature of elements and compounds, and the subatomic particles that make up all matter.

Atom: The smallest unit of matter that retains the properties of an element. Atoms consist of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus.
Element: A pure substance consisting of only one type of atom, defined by its atomic number (number of protons).
Compound: A substance formed when two or more different elements are chemically bonded together in fixed proportions.
Subatomic Particles:
- Proton: Positively charged particle found in the nucleus. Determines the atomic number.
- Neutron: Neutral particle found in the nucleus. Contributes to atomic mass.
- Electron: Negatively charged particle orbiting the nucleus. Involved in chemical bonding.
Atomic Number (Z): The number of protons in an atom's nucleus. Identifies the element.
Mass Number (A): The total number of protons and neutrons in the nucleus.
Relationship:
Isotope: Atoms of the same element with different numbers of neutrons (and thus different mass numbers).
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.

Chemical Bonds

Chemical bonds are the forces that hold atoms together in 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 an electronegative atom) and another electronegative atom (often oxygen or nitrogen).

Example: In water (H2O), the oxygen and hydrogen atoms are held together by covalent bonds, while hydrogen bonds form between different water molecules.

Periodic Table and Electron Shells

The periodic table organizes elements by atomic number and properties. Electron configuration determines chemical reactivity.

Finding Information: The periodic table provides atomic number, symbol, atomic mass, and sometimes electron configuration.
Valence Electrons: Electrons in the outermost shell. The number of valence electrons determines an atom's chemical properties and bonding behavior.

Example: Oxygen has 6 valence electrons; it tends to form two covalent bonds to fill its outer shell.

Water Molecule Structure and Hydrogen Bonding

Water's unique properties arise from its molecular structure and hydrogen bonding.

Water Molecule: Consists of two hydrogen atoms covalently bonded to one oxygen atom (H2O). The molecule is polar, 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 hydrogen of one molecule to the oxygen of another.

Emergent Properties of Water

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

Cohesion: Water molecules stick to each other due to hydrogen bonding. Responsible for surface tension.
Adhesion: Water molecules stick to other substances, aiding processes like capillary action.
High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, stabilizing environments.
High Heat of Vaporization: Water requires significant energy to evaporate, helping organisms cool via sweating or transpiration.
Expansion Upon Freezing: Ice is less dense than liquid water, so it floats, insulating aquatic life in winter.
Versatile Solvent: Water dissolves many substances, facilitating chemical reactions in cells.

Example: Water's cohesion allows insects to walk on its surface; its solvent properties enable transport of nutrients in blood.

Solutions and Solubility

Solutions are homogeneous mixtures of two or more substances. Water is often the solvent in biological systems.

Solute: The substance dissolved in a solution.
Solvent: The substance that dissolves the solute (in biology, usually water).
Solution: A homogeneous mixture of solute and solvent.
Hydrophilic: Substances that dissolve easily in water (e.g., salts, sugars).
Hydrophobic: Substances that do not dissolve in water (e.g., oils, fats).

Example: Table salt (NaCl) is hydrophilic and dissolves in water; oil is hydrophobic and does not mix with water.

Water Dissociation, pH, Acids, Bases, and Buffers

Water can dissociate into ions, affecting pH. The pH scale measures the concentration of hydrogen ions in a solution.

Water Dissociation:
pH Scale: Measures hydrogen ion concentration;
Acid: Substance that increases the hydrogen ion concentration of a solution (pH < 7).
Base: Substance that reduces the hydrogen ion concentration (pH > 7).
pH Units: Each unit represents a tenfold difference in [H+]. A change of 2 pH units = 100x difference; 3 units = 1000x difference.
Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions. Buffers are crucial for maintaining stable pH in biological systems.

Example: Blood contains bicarbonate buffer to maintain pH near 7.4.

Comparison Table: Types of Chemical Bonds

Bond Type	Definition	Strength	Example
Covalent	Atoms share electron pairs	Strong	H2O molecule
Ionic	Transfer of electrons creates charged ions	Strong (in dry conditions)	NaCl (table salt)
Hydrogen	Attraction between H (attached to O or N) and another electronegative atom	Weak (individually)	Between water molecules

Comparison Table: Hydrophilic vs. Hydrophobic Substances

Property	Hydrophilic	Hydrophobic
Affinity for Water	High	Low
Chemical Nature	Polar or charged	Nonpolar
Examples	Salts, sugars	Oils, fats

Additional info:

Some content about chemical reactions and moles/molarity is intentionally skipped, as per the original note.
For drawing models of atoms or water molecules, refer to textbook diagrams for visual representation.