Particle

In IGCSE sciences, particle is the universal term describing the tiny individual constituents of matter ¹. Depending on the substance, a particle may be:

1. An Atom: The smallest individual unit of an element (e.g., helium, iron).
2. A Molecule: Two or more non-metal atoms chemically bonded together (e.g., $\text{O}_2$, $\text{H}_2\text{O}$).
3. An Ion: An atom or group of atoms that has gained or lost electrons, acquiring an electrical charge (e.g., $\text{Na}^+$, $\text{Cl}^-$).

Classification of Particles

           +---------------------------+
           |         PARTICLE          |
           |   (Microscopic Entity)    |
           +-------------+-------------+
                         |
      +------------------+------------------+
      |                  |                  |
      v                  v                  v
  [ Atom ]         [ Molecule ]          [ Ion ]
Single element     Bonded atoms      Charged species
  e.g., He, Fe      e.g., O2, H2O     e.g., Na+, Cl-

Role in Kinetic Particle Theory

According to Kinetic Particle Theory, the physical state and bulk behavior of any substance depend on three key particle features ^{1 2}:

• Arrangement: Regular ordered pattern or random disorder.
• Separation: Tightly packed in contact or separated by large empty spaces.
• Motion & Kinetic Energy: Vibrating about fixed positions or moving freely at speeds proportional to temperature.

Related

• Atom - Single chemical entity retaining elemental properties.

• Molecule - Group of covalently bonded atoms.

• Kinetic Particle Theory - Model explaining macroscopic states from particle dynamics.

• State of Matter - Macroscopic phases determined by particle forces and spacing.

• Intermolecular Space - Separation and empty volume between particles across states of matter.

• Diffusion - Net transport of particles driven by random motion.

• Chemistry - Foundational science area.