Chapter 2 Elements Reviw Sheet

Chapter 2: The Chemical Context of Life

I. Elements and Compounds

The study of biology requires an understanding of the chemical elements and compounds that make up living organisms. This section introduces the basic building blocks of matter and their relevance to life.

• Element: A substance that cannot be broken down into other substances by chemical means. Examples include carbon, oxygen, and hydrogen.

• Compound: A substance consisting of two or more elements combined in a fixed ratio. Example: water (H2O).

• Essential Elements: Elements required for an organism to survive, grow, and reproduce. In humans, these include oxygen, carbon, hydrogen, nitrogen, and others.

• Trace Elements: Elements required by an organism in minute quantities, such as iron and iodine.

• Atoms: The smallest unit of an element that retains its properties.

• Subatomic Particles: Atoms are composed of protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Atomic Number: The number of protons in an atom, which determines the element.

• Mass Number: The sum of protons and neutrons in an atom.

• Atomic Mass: The total mass of an atom, typically measured in Daltons.

Example: Carbon has an atomic number of 6 (6 protons) and a typical mass number of 12 (6 protons + 6 neutrons).

II. Isotopes

Isotopes are variants of a particular chemical element that differ in neutron number, and thus in mass number.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Radioactive Isotope: An isotope that decays spontaneously, giving off particles and energy.

• Stable Isotope: An isotope that does not undergo radioactive decay.

• Applications: Radioactive isotopes are used in medical imaging and dating fossils.

Example: Carbon-12 and Carbon-14 are isotopes of carbon; Carbon-14 is radioactive and used in radiocarbon dating.

III. Electron Configuration and Chemical Properties

The arrangement of electrons in an atom determines its chemical behavior. Electrons are organized in shells around the nucleus.

• Electron Shells: Energy levels where electrons are found. The first shell holds up to 2 electrons, the second up to 8, etc.

• Valence Electrons: Electrons in the outermost shell, which are involved in chemical bonding.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a full outer shell (usually 8 electrons).

Example: Oxygen has 6 valence electrons and tends to form two bonds to complete its octet.

IV. Chemical Bonds

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

• Ionic Bond: Formed when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other.

• Covalent Bond: Formed when two atoms share one or more pairs of electrons.

• Polar Covalent Bond: Electrons are shared unequally, creating partial charges.

• Nonpolar Covalent Bond: Electrons are shared equally.

• Hydrogen Bond: A weak bond between a hydrogen atom and an electronegative atom (such as oxygen or nitrogen).

• Van der Waals Forces: Weak attractions between molecules due to temporary partial charges.

Example: Water molecules are held together by hydrogen bonds, which give water its unique properties.

V. Chemical Formulas and Reactions

Chemical formulas represent the composition of compounds, while chemical reactions describe the transformation of substances.

• Molecular Formula: Shows the number and type of atoms in a molecule (e.g., H2O).

• Structural Formula: Shows the arrangement of atoms in a molecule.

• Chemical Reaction: The process by which substances are transformed into different substances.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of a chemical reaction.

• Balanced Equation: An equation where the number of atoms of each element is the same on both sides.

• Chemical Equilibrium: The point at which the forward and reverse reactions occur at the same rate.

Example: The reaction of hydrogen and oxygen to form water: $2H_2 + O_2 \rightarrow 2H_2O$

VI. Ions and Ionic Compounds

Ions are charged particles formed when atoms gain or lose electrons. Ionic compounds are formed from the electrostatic attraction between cations and anions.

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

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

• Electronegativity: The ability of an atom to attract electrons in a covalent bond.

Example: Table salt (NaCl) is formed from Na+ and Cl- ions.

VII. Table: Types of Chemical Bonds

The following table summarizes the main types of chemical bonds and their properties:

Bond Type	Definition	Strength	Example
Ionic	Transfer of electrons between atoms	Strong (in solid state)	NaCl
Covalent	Sharing of electron pairs between atoms	Strong	H2O
Hydrogen	Attraction between H and electronegative atom	Weak	Between water molecules
Van der Waals	Temporary attractions due to partial charges	Very weak	Between nonpolar molecules

VIII. Additional info:

• Understanding the chemical context of life is foundational for topics such as metabolism, genetics, and cell biology.

• Knowledge of chemical bonds and reactions is essential for interpreting biological processes at the molecular level.