Elements, Compounds and Mixtures - Pure and Impure Substances

Pure Substances: These consist of only one type of particles (atoms or molecules). They have a fixed composition and distinct physical and chemical properties. Examples include elements like Gold ($Au$) and compounds like Distilled Water ($H_2O$).

Elements: A pure substance made up of only one kind of atom that cannot be broken down into simpler substances by chemical means. Elements are classified into Metals (e.g., $Fe$, $Cu$), Non-metals (e.g., $H$, $O$), Metalloids (e.g., $Si$, $Ge$), and Noble Gases (e.g., $He$, $Ne$).

Compounds: A pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass. The properties of a compound are entirely different from its constituent elements. For example, $H_2$ (combustible) and $O_2$ (supporter of combustion) combine to form $H_2O$ (extinguishes fire).

Mixtures (Impure Substances): Formed when two or more substances are physically mixed in any proportion without a chemical reaction. Constituents retain their original properties. They can be Homogeneous (uniform composition, e.g., Salt solution) or Heterogeneous (non-uniform, e.g., Sand and Salt).

Atomicity: The number of atoms present in one molecule of an element. It can be Monoatomic ($He$), Diatomic ($O_2$, $N_2$, $H_2$), Triatomic ($O_3$), or Polyatomic ($P_4$, $S_8$).

Chemical Symbols and Formulae: Symbols represent an atom of an element (e.g., $C$ for Carbon), while a formula represents a molecule of a substance (element or compound), showing the number of atoms of each element present (e.g., $CO_2$ contains 1 Carbon atom and 2 Oxygen atoms).