Atoms, Ions, and Molecules: The Building Blocks of Chemical Evolution

Atoms and Subatomic Particles

An atom is the smallest identifiable unit of matter. Four types of atoms—hydrogen (H), carbon (C), nitrogen (N), and oxygen (O)—make up most of the matter found in living organisms.

• Subatomic particles include protons (positively charged), neutrons (neutral), and electrons (negatively charged).

• Protons and neutrons are found in the nucleus; electrons orbit the nucleus.

• Mass number (M) = number of protons + number of neutrons

• Atomic number = number of protons

• Atomic mass = actual weight of a specific atom (often similar to mass number)

Isotopes are atoms of the same element with different numbers of neutrons. The atomic weight is the average of all atomic masses of naturally occurring isotopes.

Atomic Symbols and the Periodic Table

The periodic table organizes elements by atomic number. Each element has a unique symbol and atomic number.

• Example: Carbon (C) has atomic number 6, meaning it has 6 protons.

• Most biological molecules are composed of H, C, N, O, P, and S.

Electron Arrangement and Valence Electrons

The arrangement of electrons around the nucleus determines chemical behavior.

• First shell fits 2 electrons; subsequent shells fit 8 electrons.

• Valence electrons are electrons in the outermost shell.

• Atoms are stable when their outer shell is full (usually 8 electrons).

• Atoms with unpaired valence electrons tend to form chemical bonds.

Octet Rule

• Atoms tend to gain, lose, or share electrons to achieve a full outer shell.

Chemical Bonds: Ionic and Covalent

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in charged ions.

• Example: Sodium (Na) transfers an electron to Chlorine (Cl), forming Na+ and Cl-.

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms.

• Example: Two hydrogen atoms share electrons with one oxygen atom to form H2O.

Nonpolar vs. Polar Covalent Bonds

• Nonpolar covalent bonds: Electrons are shared equally (e.g., H2).

• Polar covalent bonds: Electrons are shared unequally, creating partial charges (e.g., H2O).

Water: Properties and Biological Importance

Water Is Polar

Water molecules have polar covalent bonds, resulting in a partial negative charge near oxygen and partial positive charges near hydrogen.

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

Hydrogen Bonding in Water

Hydrogen bonds are weak attractions between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

• Hydrogen bonding gives water its unique properties.

Water Stabilizes Temperature

Water absorbs and releases heat slowly, helping to stabilize temperature in organisms and environments.

• High specific heat: Water requires a lot of energy to change temperature.

• High heat of vaporization: Water absorbs much energy before evaporating.

Water as a Solvent

Water is an excellent solvent due to its polarity.

• It dissolves ionic and polar molecules (hydrophilic substances).

• Nonpolar molecules (hydrophobic substances) do not dissolve in water.

Cohesion and Adhesion

Cohesion is the attraction between water molecules due to hydrogen bonding.

• Leads to surface tension, allowing water to resist rupture at the surface.

Adhesion is the attraction between water molecules and other surfaces.

• Important for processes like water transport in plants.

Carbon and Organic Macromolecules

Carbon's Versatility

Carbon atoms have four valence electrons, allowing them to form strong covalent bonds and complex molecules.

• Carbon forms the backbone of organic molecules.

• Organic molecules can be large and diverse due to carbon's bonding ability.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that determine chemical reactivity.

Summary Table: Key Properties of Water

Key Equations

Example

• Water molecule (H2O): Two hydrogen atoms covalently bonded to one oxygen atom, with polar covalent bonds resulting in partial charges and hydrogen bonding.

• Carbon atom: Four valence electrons allow bonding with other atoms to form organic molecules such as carbohydrates, proteins, lipids, and nucleic acids.

Additional info: These notes expand on the original content by providing definitions, examples, and tables for clarity and completeness.