F.4 Chemistry

Atom, ion and molecule are PARTICLES:
• An atom is a elementary particles which is neutral in charge and contains one nucleus with some electrons surrounding the nucleus. For example, an Na atom contains a nucleus (which has 11 protons) with 11 electrons surrounding the nucleus.
• An ion is a charge particle. A cation (positive ion) is positively charged and an anion (negative ion) is negatively charged. An ion may be formed from one atom (e.g. Na+ ion and Cl- ion) or from more than one atom (e.g. NH4+ ion and CO32- ion).
• A molecule is a neutral particle which can be exist under certain conditions. A molecule may contain one atom (e.g. noble gases) or more than one atom (e.g. CO2 molecule and NH3­ molecule).

Element and compound are PURE SUBSTANCES:
• An element is a pure substance which contains one kind of atoms only (e.g. a jar of O2, some I2 solid).
• An element is a pure substance which contains more than one kind of atoms (e.g. a jar CO2, a bottle of H2O).
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