Particles and models: atoms and waves
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Atoms and composition of matter
Atoms are the fundamental, very small units of matter. Atoms combine to form molecules or exist as single atoms in elements; ions result from atoms losing or gaining electrons. Atomic size limits direct observation, so models describe structure, bonding and behaviour at a useful scale. Atoms consist of a nucleus and electrons, but the particle model treats atoms as indivisible particles for many macroscopic explanations. This simplification yields correct predictions for bulk properties while excluding subatomic behaviour.
Particle model of matter
The particle model represents solids, liquids and gases as collections of particles with different arrangements and motions. In solids, particles are closely packed and vibrate about fixed positions, causing a fixed shape and volume. In liquids, particles move past each other while remaining close, causing a fixed volume but variable shape. In gases, particles move rapidly and widely separated, causing variable shape and volume and high compressibility. The particle model links microscopic motion to macroscopic effects: increased temperature causes faster particle motion, which causes expansion or increased pressure where motion is constrained.
Changes of state and energy
Heating supplies energy to particles, which increases kinetic energy and overcomes attractive forces between particles. Increased kinetic energy causes melting or boiling when particle motion becomes great enough to change arrangement and freedom of movement. Cooling removes energy, which reduces kinetic energy and allows attractive forces to pull particles into closer, more ordered arrangements. Latent heat describes energy absorbed or released during a change of state without temperature change, because energy changes the arrangement or bonding rather than particle speed.
Diffusion and Brownian motion
Diffusion occurs when particles move from regions of higher concentration to regions of lower concentration because random particle motion produces net spreading. Higher temperature increases particle speed and therefore increases the rate of diffusion. Smaller particles diffuse faster than larger ones because fewer collisions and lower mass reduce resistance to motion. Brownian motion provides visible evidence of particle motion when tiny particles suspended in a fluid move randomly due to collisions with fluid molecules. Observation of Brownian motion supports the existence and motion of particles.
Gas behaviour and pressure
Gas pressure results from particles colliding with container walls. Increased particle speed or increased number of particles causes more frequent or more forceful collisions, which increases pressure. Compression reduces the space between particles, increasing collision frequency and pressure. The relation v = f × λ describes wave speed, not particle motion, but particle kinetic theory provides quantitative links between temperature and pressure for ideal gases when particle interactions are negligible.
Wave models of light and sound
The wave model represents light and sound as energy transfer by oscillations. Sound requires a material medium and travels as longitudinal mechanical waves through particle vibrations. Light travels as transverse electromagnetic waves and does not require a medium. Wave properties such as wavelength, frequency and amplitude explain reflection, refraction, diffraction and interference. Changes in frequency change pitch for sound and colour for light, and changes in wavelength at constant speed produce frequency changes when entering new media.
Limitations and appropriate use of models
Models simplify reality and therefore have limits. The particle model does not explain electronic structure, chemical bonding in full detail, or quantum effects at atomic scales. The wave model explains many behaviours of light but cannot fully describe interactions where particle-like behaviour appears, such as photons in the photoelectric effect. Appropriate use of a model requires awareness of scale and purpose: the particle model suits explanations of states, pressure and diffusion; the wave model suits explanations of interference and sound transmission. More advanced models replace simple models where predictions conflict with observations.
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