A particle can mean various things. You can have a particle of dust, which is just a tiny bit of dust. It just means a bit of something. Or a "sub-atomic particle" - basically anything smaller than an atom. Or you can talk about an "elementary particle" such as an electron. A particle which has no known smaller components. A molecule is two or more atoms bound together by electrical forces called chemical bonds. Oxygen gas is like this, each molecule has two oxygen atoms. A compound is a substance containing atoms of more than one element bound together. It might be a whole load of molecules, or as in Sodium Chloride, the whole lot may be bound together.

The word particle is used in different senses in different areas of science eg In chemistry: Molecule Atom neutron Photon Colloidal particle in colloid chemistry, a one-phase system of two or more components In physics: Subatomic particle, which may be either: Elementary particle, a particle of which larger particles are composed, also called a fundamental particle Composite particle, a bound state between several elementary particles Point particle, an idealized particle that does not have any volume See also list of particles, or for a non-technical introduction to the topic, Introduction to Particles In other contexts: Particle (ecology), in marine and freshwater ecology, a small object Grammatical particle, in linguistics, a useful catch-all lacking a strict definition Particle system, in computer graphics, a technique to simulate certain fuzzy phenomena Particle (band), a 2000 jam band from Los Angeles, California Particle Man, the hero of a song by They Might Be Giants If you mean an atom, then it smallest particle of a chemical element that retains its chemical properties. (of course, atoms can be subdivided into subatomic particles called protons, electrons and neutrons, but by dividing the atom, you lost chemical properties). In science, a molecule is a combination of two or more atoms in a definite arrangement held together by chemical bonds.[1] Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure substance that still retains its composition and chemical properties.[2] Certain pure substances (e.g., metals, molten salts, crystals, etc.) are best understood as being composed of networks or aggregates of atoms or ions instead of molecular units. In the molecular sciences, a "molecule" is a sufficiently stable, electrically neutral entity composed of two or more atoms.[3] The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition. [4] For gases and certain molecular liquids and solids (such as water and sugar), molecules are the smallest division of matter which retains chemical properties; however, there are also many solids and liquids which are made of atoms, but do not contain discrete molecules (such as salts, rocks, and liquid and solid metals). Thus, while molecules are common on Earth (making up all of the atmosphere and most of the oceans), most of the mass of the Earth (much of the crust, and all of the mantle and core) is not made of identifiable molecules, but rather represents atomic matter in other arrangements, all of which lack the particular type of small-scale order that is associated with molecules. Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom. The term "molecule" in gases has been used as a synonym for the fundamental particles of the gas, whatever their structure. This definition results in a few types of gases (for example inert elements that do not form compounds, such as helium), which has "molecules" consisting of only a single atom. A chemical compound is a chemical substance consisting of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. The ratio of each element is usually expressed by chemical formula. For example, water (H2O) is a compound consisting of two hydrogen atoms bonded to an oxygen atom. The atoms within a compound can be held together by a variety of interactions, ranging from covalent bonds to electrostatic forces in ionic bonds. A continuum of bond polarities exist between the purely covalent bond (as in H2) and ionic bonds. For example H2O is held together by polar covalent bonds. Sodium chloride is an example of an ionic compound. (this is a combination of information from Wikipedia) hope this helps.