Matter, atoms, and molecules
Use the particle model to explain observable changes without confusing physical and chemical identity.
Particle model#
Matter is commonly described as solid, liquid, gas, or plasma under different conditions. In solids, particles remain in ordered or fixed relative positions while vibrating. Liquids flow because particles move past one another. Gas particles are widely separated and fill available space.
Atoms and elements#
An atom is the smallest unit that retains the chemical identity of an element. An element contains atoms with the same number of protons.
A molecule contains atoms bonded together. A compound contains atoms of at least two different elements in fixed chemical relationships.
Physical changes#
Melting, freezing, evaporation, and condensation change particle arrangement and energy without creating a new substance. Temperature can remain constant during a phase change while energy changes the arrangement.
Common mistakes#
Not every molecule is a compound: oxygen gas contains O₂ molecules made from one element. A mixture is not a compound because its components are not chemically combined in a fixed proportion.
Explain observations#
Use particle spacing, motion, attraction, and energy to connect microscopic models with macroscopic properties such as shape, volume, diffusion, and compressibility.
Key points
- Matter has mass and occupies space.
- Atoms are the basic units of elements.
- Molecules contain bonded atoms; compounds contain more than one element.
- Changes of state are physical changes in arrangement and energy.