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 serve as the fundamental building blocks for all substances. In biological systems, just a few elements—hydrogen (H), carbon (C), nitrogen (N), and oxygen (O)—make up the majority of living matter.

• Atom: The smallest unit of an element, consisting of a nucleus (protons and neutrons) surrounded by electrons.

• Subatomic particles:

  • Proton: Positively charged particle in the nucleus.

  • Neutron: Neutral particle in the nucleus.

  • Electron: Negatively charged particle orbiting the nucleus.

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. $M = \text{protons} + \text{neutrons}$

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

• 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.

Example: Carbon-12 and Carbon-14 are isotopes of carbon, differing in their number of neutrons.

Atomic Symbols and the Periodic Table

The periodic table organizes elements by atomic number and groups elements with similar properties. Atomic symbols provide information about the number of protons, neutrons, and electrons.

• Mass number = protons + neutrons

• Atomic number = protons

• Most of the time, atomic mass is close to the mass number.

Example: The atomic symbol for carbon is 12C, where 12 is the mass number and C is the element symbol.

Electron Arrangement and Valence Electrons

The arrangement of electrons around the nucleus determines how elements interact chemically. The octet rule states that atoms are most stable when their outer shell is full, typically with 8 electrons (except for the first shell, which holds 2).

• First shell: 2 electrons

• Other shells: 8 electrons

• Valence electrons: Electrons in the outermost shell, important for chemical bonding.

• Valence: Number of unpaired valence electrons.

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

Chemical Bonds and Molecules

Ionic and Covalent Bonds

Atoms form chemical bonds to achieve stable electron configurations. The two main types of bonds are ionic and covalent.

• Ionic bond: Formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other.

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

Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl- ions, which bond ionically to make NaCl (table salt).

Polar and Nonpolar Covalent Bonds

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

• Polar covalent bond: Electrons are shared unequally, resulting in 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.

Properties of Water

Water Is Polar

Water molecules have a bent shape and polar covalent bonds, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other.

• Hydrogen bond: A weak attraction between a hydrogen atom (partially positive) and an electronegative atom (like oxygen) in another molecule.

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 requires a lot of energy to change temperature.

• High heat of vaporization: Water requires a lot of energy to evaporate.

Example: Oceans and lakes moderate climate by absorbing heat in summer and releasing it in winter.

Water Is an Excellent Solvent

Water's polarity allows it to dissolve many ionic and polar substances, making it an excellent solvent for biological reactions.

• Hydrophilic: Substances that dissolve in water (e.g., salts, sugars).

• Hydrophobic: Substances that do not dissolve in water (e.g., oils, fats).

Example: Table salt (NaCl) dissolves in water because the positive and negative ions are surrounded by water molecules.

Water Is Cohesive and Adhesive

Water molecules stick to each other (cohesion) and to other surfaces (adhesion) due to hydrogen bonding.

• Cohesion: Attraction between water molecules, leading to surface tension.

• Adhesion: Attraction between water molecules and other substances.

Example: Water moves up plant stems through capillary action, a result of cohesion and adhesion.

Carbon and Organic Macromolecules

Importance of Carbon

Carbon is the backbone of organic molecules due to its ability to form four covalent bonds, allowing for a diversity of stable, complex structures.

• Forms chains, rings, and branches.

• Can bond with H, O, N, P, S, and other carbons.

Example: Glucose (C6H12O6) is a simple sugar with a carbon backbone.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactions of those 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	-C=O	Polar, found in sugars	Acetone
Hydroxyl	-OH	Makes compounds more soluble in water	Ethanol
Phosphate	-PO4	Stores energy, found in ATP	ATP
Sulfhydryl	-SH	Forms disulfide bonds in proteins	Cysteine

Example: The carboxyl group in amino acids gives them acidic properties, while the amino group acts as a base.