MYP Chemistry: The Particle Model of Matter

The particle model says all matter is made of tiny particles (atoms, ions or molecules) in constant motion, with spaces between them. The arrangement, movement and energy of these particles differ between solids, liquids and gases, and explain why each state behaves the way it does.

Solids have particles tightly packed in a fixed, ordered arrangement, vibrating on the spot. Liquids have particles close together but disordered, able to slide past each other. Gases have particles far apart, moving freely and randomly at high speed with almost no attraction between them.

Diffusion is the net movement of particles from an area of higher concentration to lower concentration, caused by their random constant motion. It happens fastest in gases, slower in liquids, and is essentially undetectable in solids because particles there can't move freely between fixed positions.

Changes of state — melting, freezing, evaporating, condensing, subliming — happen when energy is transferred in or out, changing how fast particles move and how strongly they're held together. Adding energy weakens the forces between particles and increases their movement; removing energy does the reverse.

During a change of state, energy supplied goes entirely into breaking or forming the forces between particles, not into increasing their average kinetic energy. That's why temperature plateaus at the melting or boiling point until the whole sample has changed state, then starts rising again.

Draw circles of equal size and spacing to represent particles: tightly packed touching circles in rows for solids, touching but irregular circles for liquids, and widely spaced circles for gases. Label the arrangement, spacing and (if asked) add arrows showing vibration or free movement.

The most common command terms are 'describe' (arrangement, movement, spacing), 'explain' (why a property occurs, using particle behaviour as the reason), and 'compare' (states or processes side by side). MYP Criterion A assessments also use 'outline' and 'state' for shorter recall answers.

The biggest one I see marking mock papers: students explain properties using the substance itself rather than its particles — saying 'the water expands' instead of 'the particles move further apart and gain kinetic energy.' Always frame your explanation around particle behaviour, not the bulk material.

It's usually assessed under MYP Sciences Criterion A (Knowing and Understanding), which asks you to describe scientific knowledge and apply it to explain phenomena. Expect short-answer questions on diagrams, changes of state, and diffusion, often building toward Criterion C or D tasks involving real experiments.

Not conceptually — most students grasp the three states quickly. Where marks get lost is in the writing: vague descriptions that don't mention arrangement, movement and energy specifically enough. Students who practise past-paper wording, not just the diagrams, tend to do noticeably better.

The particle model is the foundation for DP topics like the kinetic theory of gases, states of matter and intermolecular forces, and phase changes in thermodynamics. Students who understood particle arrangement and energy properly at MYP 4-5 pick up DP gas laws and enthalpy calculations much faster.

Look for resources that combine clear diagrams with short-answer practice questions modelled on Criterion A wording, since that's where most marks are actually lost. On RevisionPrep, the MYP Chemistry Revision Notes and Topical Worksheets cover the particle model with exactly this diagram-plus-practice approach.