Atoms, Ions, Molecules, and Water: Foundations of Chemical Evolution in Biology

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

Atoms and Subatomic Particles

Atoms are the smallest identifiable units of matter and form the basis of all chemical substances. Four elements—hydrogen (H), carbon (C), nitrogen (N), and oxygen (O)—make up the majority of matter in living organisms.

• Subatomic particles: Atoms are composed of protons (positively charged), neutrons (neutral), and electrons (negatively charged).

• Nucleus: The center of the atom, containing protons and neutrons.

• Electrons: Orbit the nucleus in defined energy levels or shells.

Particle	Charge	Atomic Mass Unit (amu)	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	~0	Electron cloud

• Mass number (M): The sum of protons and neutrons in the nucleus. $\text{Mass number} = \text{protons} + \text{neutrons}$

• Atomic number (Z): The number of protons in the nucleus. $\text{Atomic number} = \text{protons}$

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

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

Atomic Structure and Electron Arrangement

The arrangement of electrons around the nucleus determines how elements interact chemically.

• Electron shells: Electrons occupy energy levels (shells) around the nucleus.

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

• Valence electrons: Electrons in the outermost shell; determine chemical reactivity.

• Valence: Number of unpaired valence electrons; indicates bonding capacity.

Chemical Bonds and Molecules

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Cation: Atom that loses electrons and becomes positively charged.

• Anion: Atom that gains electrons and becomes negatively charged.

• Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl- ions, which combine to form NaCl (table salt).

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. These are strong bonds that hold molecules together.

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

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

Water: Properties and Biological Importance

Polarity of Water

Water is a polar molecule due to the unequal sharing of electrons between hydrogen and oxygen atoms. This results in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen bonds: Weak attractions between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.

Water as a Solvent

Water's polarity makes it an excellent solvent, capable of dissolving many ionic and polar substances.

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

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

• Hydration shell: Water molecules surround and isolate ions or polar molecules, aiding dissolution.

Water Stabilizes Temperature

Water absorbs and releases heat more slowly than most other substances, helping to stabilize temperatures in organisms and environments.

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

• Evaporative cooling: As water evaporates, it removes heat, cooling the surface.

• Ice is less dense than liquid water: Ice floats, insulating aquatic environments.

Cohesion, Adhesion, and Surface Tension

Water molecules exhibit cohesion (attraction to each other) and adhesion (attraction to other substances), both due to hydrogen bonding.

• Cohesion: Responsible for surface tension, allowing water to resist rupture at the surface.

• Adhesion: Enables water to climb up plant roots and stems (capillary action).

• Example: Water moving from roots to leaves in plants.

Carbon and Organic Macromolecules

Importance of Carbon

Carbon atoms form the backbone of organic molecules. With four valence electrons, carbon can form up to four covalent bonds, allowing for a diversity of stable, complex molecules.

• Versatility: Carbon can form single, double, or triple bonds and can bond with many elements (H, O, N, S, P).

• Functional groups: Specific groups of atoms attached to carbon skeletons that determine the chemical properties and reactivity of organic molecules.

Group	Structure	Properties	Example
Amino	-NH2	Acts as a base; attracts protons	Glycine
Carboxyl	-COOH	Acts as an acid; donates protons	Acetic acid
Carbonyl	-CO (aldehyde or ketone)	Polar; found in sugars	Acetone
Hydroxyl	-OH	Makes compounds more soluble in water	Ethanol
Phosphate	-PO4	Stores energy; found in ATP	Phospholipids
Sulfhydryl	-SH	Forms disulfide bonds in proteins	Cysteine

Additional info: Functional groups are critical in determining the types of chemical reactions organic molecules can undergo, influencing biological processes such as enzyme activity and cell signaling.