How did differences in atomic shape and size explain the properties of matter for Democritus?

For Democritus, observable qualitative differences in matter—such as hardness, taste, or color—are not inherent properties of the atoms themselves. Instead, these qualities are merely conventional impressions caused by the varying configurations, shapes, sizes, and arrangements of atoms moving through the void.12

Democritus explained physical properties through the mechanical interaction of these quantitative differences:

• Solidity and Texture: Atoms of substances like water are smooth and round, allowing them to roll over one another, whereas atoms of iron are rough and jagged, enabling them to hook together and form solid bodies.2
• Taste: Sensations like sweetness are produced by specific shapes, such as round atoms, which interact with the senses in particular ways.2
• Color: Visual perceptions are determined by the position and shape of atoms. For example, smooth and flat atoms, which cast no shadow, produce the sensation of white, while rough and uneven atoms produce the sensation of black.2

Ultimately, because all phenomena are composed of the same eternal, indivisible atoms, qualitative changes are simply the result of atoms rearranging their combinations.12