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 Structure and Electron Arrangement

The arrangement of electrons around the nucleus determines how atoms interact and form chemical bonds.

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

• The first shell holds up to 2 electrons; subsequent shells hold up to 8 electrons.

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

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

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

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: An atom that loses electrons and becomes positively charged.

• Anion: An atom that gains electrons and becomes negatively charged.

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

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms, creating molecules.

• 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).

Example: In water (H2O), oxygen is more electronegative than hydrogen, so electrons are pulled closer to oxygen, making it partially negative and hydrogens partially positive.

Properties of Water

Water is Polar

Water molecules have 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 many 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 absorbs a lot of energy before it evaporates.

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 a universal solvent in biological systems.

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

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

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

Water is Cohesive and Adhesive

Cohesion and adhesion are properties of water that result from hydrogen bonding.

• Cohesion: Water molecules stick to each other, contributing to surface tension.

• Adhesion: Water molecules stick to other substances, aiding processes like capillary action.

Example: Water moves up plant stems from roots to leaves due to 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 complex and diverse molecular structures.

• Forms stable bonds with H, O, N, P, S, and other carbon atoms.

• Can create chains, rings, and branched structures.

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	Formula	Property	Example
Amino	-NH2	Acts as a base, attracts protons	Glycine (an amino acid)
Carboxyl	-COOH	Acts as an acid, donates protons	Acetic acid
Carbonyl	-CO-	Polar, found in sugars	Acetone
Hydroxyl	-OH	Polar, forms hydrogen bonds	Ethanol
Phosphate	-PO4	Transfers energy, found in ATP	ATP (adenosine triphosphate)
Sulfhydryl	-SH	Forms disulfide bonds in proteins	Cysteine (an amino acid)

Example: The carboxyl group in amino acids gives them acidic properties, while the amino group gives them basic properties.