Understanding Matter: Atoms, Molecules, and States

The Nature of Matter

Matter is composed of atoms and empty space.

Molecules and crystals are associations of atoms, whether identical or different, always present in a fixed number or proportion.

Chemical elements are substances made of identical atoms.

Compounds are substances formed by atoms, molecules, or crystals of different types.

How Elements Appear in Nature

• Atomic: Atoms exist as isolated and widely separated particles.
• Molecular: Independent particles formed by identical atoms, usually two or three.
• Crystalline: Elements consisting of an endless number of identical atoms, closely packed and perfectly ordered.

How Compounds Appear in Nature

• Molecular: Comprised of independent particles formed by a fixed number of different atoms.
• Crystalline: Clusters of an indeterminate number of different atoms in a constant, orderly proportion.

Kinetic Theory of Matter

• Matter consists of particles.
• Particles exert short-range forces: attractive when separated and repulsive when very close.
• Particles are in a state of permanent agitation.
• There is a vacuum between particles.

Temperature is a property that indicates the average energy content of particles.

Changes of State

• Fusion: When heating ice, water molecules increase their vibration and volume.
• Evaporation: In liquids, particles vibrate and move. Particles that reach the surface with sufficient energy to overcome cohesive forces escape to form a gas.
• Boiling: When the temperature rises enough for the entire liquid mass to have particles with sufficient energy to overcome cohesive forces, gas bubbles form and rise due to lower density.

Temperature and Phase Transitions

For example, water melts at 0°C and boils at 100°C.

State Change Terminology

• Solid to Liquid: Fusion
• Liquid to Gas: Vaporization
• Solid to Gas: Sublimation
• Gas to Solid: Reverse Sublimation
• Liquid to Solid: Solidification
• Gas to Liquid: Condensation