constituent particles

Constituent particles are the tiny building blocks — atoms and molecules — that every substance is ultimately made of, however solid or shapeless that substance looks.

• Think of them like LEGO bricks: a wall built from thousands of identical small bricks looks like one solid structure, but it's really made of countless separate pieces packed together.
• This isn't just true for solids like chalk — liquids and gases are built from constituent particles too, just held together more loosely.
• Breaking or grinding something doesn't create new constituent particles or destroy them; it just separates a big chunk into more of the same tiny units, which is why grinding chalk stays chalk.
• Don't confuse this with everyday uses of the word "particle" — dust or smoke particles you can see floating in a sunbeam are themselves made up of enormous numbers of constituent particles, far too small to see even under an ordinary microscope.

Almost everything you learn about matter later — how it dissolves, melts, expands, or reacts — comes down to how these constituent particles behave.