2.2 Molecules & Chemical Compounds

2.2 Molecules & Chemical Compounds

Atomic structure (Section 2.1) explains single atoms. Aircraft materials are rarely free isolated atoms. They are molecules, compounds, alloys, and mixtures. Module 2 expects you to define these terms cleanly, recognise three bonding types at technician depth, and connect them to fuels, metals, and corrosion without drifting into full organic chemistry.

Molecules vs Compounds

Molecules

A molecule is a particle formed when two or more atoms are held together by chemical bonds. The atoms may be of the same element or of different elements.

• Elemental molecules: O₂ (oxygen gas), N₂ (nitrogen gas), H₂ — two atoms of one element bonded together.
• Compound molecules: H₂O (water), CO₂ (carbon dioxide), C₈H₁₈ (a representative hydrocarbon formula used when discussing petrol/gasoline-type fuels).

Not every compound is molecular in the simple “discrete molecule” sense: ionic solids such as sodium chloride form extended lattices of ions rather than isolated NaCl molecules. At Module 2 depth, the exam still expects you to know that compounds can form via different bonding arrangements.

Compounds

A chemical compound is a pure substance containing two or more different elements chemically combined in a fixed proportion by mass.

Classic teaching point: sodium (reactive metal) and chlorine (toxic gas) form sodium chloride (table salt / ionic solid) — properties of the compound are not a simple average of the free elements.

Molecule vs compound trap:

• All compound molecules are molecules, but not all molecules are compounds (O₂ is a molecule of an element).
• All compounds contain more than one element; elemental molecules do not.

Chemical Bonding at Module 2 Depth

Bonding explains how atoms stick together and why materials behave as conductors, brittle salts, flexible polymers, or strong metals. Three models are enough for AML Module 2.

1. Ionic bonding

• Occurs typically between metals and non-metals.
• Electrons are transferred from metal atoms to non-metal atoms, forming cations and anions.
• Electrostatic attraction holds the oppositely charged ions in a lattice.
• Ionic solids are often brittle, high melting, and may conduct when molten or dissolved (mobile ions) but not as dry solids.

Aviation relevance: many corrosion products and salts involve ionic species in water films; electrolyte solutions accelerate electrochemical attack on airframe metals. Battery electrolytes and plating baths also rely on ionic conduction.

2. Covalent bonding

• Occurs typically between non-metal atoms.
• Electrons are shared in pairs so each atom can achieve a stable outer shell configuration (Module 3 will expand valence ideas).
• Molecules may be gases, liquids, or soft solids at room temperature depending on size and intermolecular forces; giant covalent networks (e.g. diamond-like structures) are hard and high-melting.

Aviation relevance:

• Hydrocarbon fuels (Jet A-1 / kerosene-type, avgas components) are largely covalent molecular compounds of carbon and hydrogen.
• Combustion products CO₂ and H₂O are covalent molecules.
• Many sealants, adhesives, and polymer composites use covalent carbon-based chemistry.

3. Metallic bonding

• Metal atoms form a lattice of positive ions surrounded by a sea of delocalised electrons shared across the structure.
• Delocalised electrons explain electrical and thermal conductivity, malleability, and ductility of metals used in airframes and wiring.

Aviation relevance: aluminium alloys, steels, copper wiring, and titanium structures all rely on metallic bonding for strength and conductivity. When metallic bonding is disrupted at a surface by oxide films or corrosion products, electrical bonding paths and structural integrity can both suffer.

These models are simplified; real materials (especially alloys and composites) combine features. Exam questions usually ask for the dominant bonding idea matching the material class.

Pure Substances vs Mixtures

Pure substances

A pure substance has a uniform and definite composition throughout:

• Elements (pure Al, pure O₂),
• Compounds (pure H₂O, pure CO₂).

At a given pressure, a pure substance has characteristic melting and boiling points (subject to impurities shifting those points slightly).

Mixtures

A mixture contains two or more substances not chemically combined in fixed proportions. Components retain many of their own properties and can often be separated by physical methods (filtration, distillation, magnetic separation, settling).

Alloys — critical for airframe thinking

Aircraft structural metals are almost never pure elements. Alloys are mixtures (or intermediate phases) designed for strength, corrosion resistance, machinability, and temperature performance. Key Module 2 point: an alloy is not a single pure element; composition can vary within specification ranges, unlike a stoichiometric compound’s fixed formula.

Example conceptual contrast:

• Al₂O₃ (aluminium oxide) is a compound with fixed Al:O ratio — a common corrosion / anodising product context.
• 2024-type aluminium alloy is an engineered mixture based on aluminium with copper and other additions — composition bands, not a single fixed molecule.

Fuels, Alloys, and Corrosion Chemistry — Forward Links

Module 2 plants seeds used heavily in later modules and hangar practice.

Fuels

• Jet fuels are complex mixtures of many covalent hydrocarbon compounds, plus controlled additives.
• Water contamination is a mixture problem (free water / entrained water), not a new fuel compound.
• Combustion chemically rearranges fuel and oxygen into CO₂, H₂O, and other products — compound formation with large energy release (thermodynamics later).

Alloys and materials

• Selecting a fastener alloy or skin alloy is choosing a mixture with certified mechanical and corrosion properties.
• Heat treatment and work hardening change microstructure and properties without changing the fact that the material remains a multi-component metallic system.

Corrosion chemistry (preview)

Corrosion is not “rust appearing from nowhere.” Typical sequence at technician level:

1. Metal atoms at a surface lose electrons (oxidation) → metal cations.
2. Electrons flow to a cathode region where reduction occurs (often involving oxygen and water).
3. Ions combine with oxygen, water, or other species to form compounds such as oxides, hydroxides, or salts.
4. Presence of an electrolyte (ionic solution — moisture + salts) completes the cell and accelerates attack.

Dissimilar metals in contact (different electrochemical tendencies) create galvanic cells — bonding paths and isolation procedures exist precisely because ionic/electron transfer is real physics, not jargon.

Exam Strategy for Section 2.2

• If asked whether something is a molecule, compound, element, or mixture, apply definitions strictly (fixed composition? one element? chemically bonded?).
• Match bonding type to material class: metals → metallic; salts/electrolytes → ionic character; fuels/organics → covalent.
• Remember alloys are mixtures designed for engineering properties; oxides and salts formed in corrosion are compounds.
• Keep answers at Module 2 depth: name the bond behaviour (transfer / share / delocalise) without inventing advanced orbital theory.