What is the difference between an element and a compound?

Matter classification: (1) ELEMENT — pure substance of one type of atom, cannot be broken down by chemical means. ~118 known elements (~92 naturally occurring). Examples: gold (Au), oxygen (O₂), carbon (C), iron (Fe), helium (He). Listed on periodic table. (2) COMPOUND — pure substance of two or more elements chemically bonded in fixed proportions. Bonds can be ionic (electron transfer, e.g., NaCl), covalent (electron sharing, e.g., H₂O), or metallic. Can be broken into elements by chemical means. Properties differ from constituent elements (sodium is a reactive metal; chlorine is a toxic gas; together they form table salt). Examples: water (H₂O), salt (NaCl), carbon dioxide (CO₂), glucose (C₆H₁₂O₆). (3) MIXTURE — physical combination of substances retaining their individual properties; variable composition; can be separated physically (filtration, distillation, evaporation). Homogeneous mixtures (solutions): uniform composition (saltwater, air, brass). Heterogeneous mixtures: non-uniform (sand and water, salad, oil and vinegar). Distinctions: physical change (state change, dissolving, mixing) vs chemical change (formation of new substances, color/temperature change, gas production, precipitate). Conservation of mass: in chemical reactions, atoms rearrange but matter is neither created nor destroyed.