Particle models use symbols or spheres for atoms and links for joining. Read the key: colours are conventions, not actual atom colours. A joined A–A pair represents an elemental molecule; a joined A–B pair represents a compound molecule when the model describes discrete molecular particles. Count atoms separately from joined groups.
A 2D diagram can make types and connections easy to count. A 3D model shows spatial arrangement more clearly, but sticks are not literal solid rods. A formula summarises composition and normally omits exact shape. Use the key and joining pattern, not a familiar colour alone, to choose the supported classification or formula.
Example 1
A diagram contains three separate joined A–B pairs. How many compound molecules and atoms are shown?
Three molecules and six atoms: each pair is one molecule containing two atoms.
Example 2
A key says blue = nitrogen and white = hydrogen. One blue sphere joins three white spheres. What is the formula?
NH3: one nitrogen and three hydrogen atoms.
Watch out:Colours and rods are modelling conventions; they identify atom types and connections.
Reading particle representations practice | Year 8 Science | Ascent Arcade