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

Atoms: Structure and Properties

An atom is the smallest identifiable unit of matter. Atoms consist of subatomic particles: protons, neutrons, and electrons. The nucleus contains protons and neutrons, while electrons orbit the nucleus.

• Proton: Positively charged particle located in the nucleus.

• Neutron: Neutral particle located in the nucleus.

• Electron: Negatively charged particle located in electron shells around the nucleus.

• Atomic number (Z): Number of protons in the nucleus; defines the element.

• Mass number (M): Sum of protons and neutrons in the nucleus.

• Atomic mass: Actual weight of a specific atom, often close to the mass number.

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

Example: Carbon-12 has 6 protons and 6 neutrons; Carbon-14 has 6 protons and 8 neutrons.

Atomic Symbols and the Periodic Table

Elements are represented by atomic symbols. The periodic table organizes elements by increasing atomic number.

• Atomic number: Number of protons (e.g., C for carbon is 6).

• Mass number: Number of protons plus neutrons.

Formulas:

Electron Arrangement and Valence Electrons

The arrangement of electrons in shells around the nucleus determines chemical behavior.

• First shell holds 2 electrons.

• Subsequent shells hold up to 8 electrons.

• Valence electrons: Electrons in the outermost shell; determine bonding properties.

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

Example: Chlorine has 7 valence electrons; needs 1 more to complete its shell.

Chemical Bonds: Ionic and Covalent

Ionic Bonds

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

• Cation: Positively charged ion (loses electrons).

• Anion: Negatively charged ion (gains electrons).

• Example: Sodium (Na) loses an electron to become Na+; Chlorine (Cl) gains an electron to become Cl-.

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms.

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

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

Example: In water, oxygen is more electronegative than hydrogen, so electrons are pulled closer to oxygen, making water a polar molecule.

Water: Properties and Biological Importance

Water's Polarity and Hydrogen Bonding

Water molecules are polar, with partial negative charge near oxygen and partial positive charge near hydrogen. This allows water molecules to form hydrogen bonds with each other.

• Hydrogen bond: Weak attraction between a hydrogen atom (partially positive) and an electronegative atom (partially negative), such as oxygen or nitrogen.

• Hydrogen bonds are responsible for many of water's unique properties.

Water Stabilizes Temperature

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

• High specific heat: Water requires more energy to change temperature.

• Evaporation: Water absorbs heat when it evaporates, cooling surfaces.

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

Water as a Solvent

Water is an excellent solvent due to its polarity. It dissolves ionic and polar molecules easily.

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

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

• Water forms hydration shells around ions and polar molecules, keeping them dispersed.

Cohesion and Adhesion

Cohesion is the attraction between water molecules due to hydrogen bonding. Adhesion is the attraction between water molecules and other surfaces.

• Cohesion leads to surface tension, allowing water to resist external force.

• Adhesion helps water move up plant roots and stems (capillary action).

Example: Water droplets form beads on a surface due to cohesion; water moves up a straw due to adhesion.

Carbon and Organic Macromolecules

Carbon's Versatility

Carbon atoms can form four covalent bonds, allowing for a variety of complex organic molecules. Carbon skeletons can be linear, branched, or ring-shaped.

• Carbon's ability to bond with H, O, N, S, and P leads to diverse macromolecules.

• Functional groups attached to carbon skeletons determine chemical reactivity.

Major Functional Groups in Organic Molecules

Example: The carboxyl group in amino acids allows them to act as acids in solution.