Review atomic structure and use chemical formulas to count atoms and distinguish elements, molecules, and compounds, as groundwork for analyzing chemical reactions.
Every substance is built from atoms: tiny particles with protons and neutrons packed in a central nucleus and electrons moving around it. An element is a substance made of only one type of atom (defined by its number of protons); oxygen and carbon are elements. A molecule forms when two or more atoms bond together; if those atoms are all the same element, like two oxygen atoms bonded as O2, it is a molecule but not a compound. A compound is a molecule made of two or more DIFFERENT elements bonded together, like water, H2O, made of hydrogen and oxygen.
Chemical formulas are shorthand for exactly which atoms, and how many, make up a molecule. A subscript number applies only to the element symbol directly before it: in CO2, the "2" belongs to oxygen only, meaning one carbon atom and two oxygen atoms. This atom-counting skill matters this month, since the equations for photosynthesis and cellular respiration, and every chemical reaction after this, are really just chemical formulas rearranging -- you need to count atoms accurately to check that a reaction is balanced.
Mastery looks like: They can count the atoms of each element in a given formula and correctly classify a substance as an element, a molecule, or a compound.
Common stumbles: Applying a subscript to the entire formula instead of just the element directly before it; assuming any molecule with more than one atom must be a compound.