Particle model limits and implications
Bonding, structure, and the properties of matter • Bonding and substance properties
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Key concepts
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Basic particle model assumptions
Particles are discrete, tiny, and in constant motion. Solids have closely packed particles with fixed positions; liquids have close particles that move past one another; gases have widely spaced, rapidly moving particles. The model uses particle arrangement and motion to explain density, shape, and diffusion.
Representation as solid inelastic spheres
Particles often appear as solid, inelastic spheres in diagrams to simplify collisions and packing. That representation treats collisions as perfectly inelastic or perfectly elastic without internal deformation. The simplification makes calculations and conceptual diagrams easier but omits surface interactions, non-spherical shapes, and rotational motion.
No forces between particles: limiting factor
A particle model that omits forces between particles cannot explain cohesion, condensation, or melting points. Cause: absence of attraction or repulsion terms. Effect: inability to predict why a gas condenses to a liquid or why solids require energy input to melt. Interparticle forces determine whether particles stay together and how energy changes affect state.
Energy, vibrations and changes of state
Changes of state depend on changes in energy and on overcoming interparticle forces. Cause: added thermal energy increases particle kinetic energy and vibrational amplitude. Effect: particles move sufficiently to overcome attractions and change state. A model with rigid, inelastic spheres cannot represent changes in internal vibrational energy or gradual weakening of attractions during heating.
Why atoms lack bulk properties
Single atoms do not display macroscopic properties such as hardness, conductivity, melting point or colour. Cause: bulk properties arise from many-body interactions and collective behaviour. Effect: properties such as electrical conductivity or tensile strength emerge only when atoms form extended structures with specific bonding and arrangement.
When the particle model requires extension
The basic model requires extension when explaining melting, boiling, surface tension, electrical conduction and precise heat capacity. Cause: these phenomena depend on bonding type, electron behaviour, and molecular polarity. Effect: models that include intermolecular forces, ionic/covalent/metallic bonding, and energy quantisation give accurate predictions.
Key notes
Important points to keep in mind