Chemistry studies matter and how it changes. All matter is made of atoms — tiny particles composed of protons (positive), neutrons (neutral), and electrons (negative).

Elements are pure substances made of one type of atom. The periodic table organizes all known elements by atomic number. Elements in the same column (group) share similar properties. Metals are on the left, nonmetals on the right.

Atoms bond together to form compounds and molecules. Ionic bonds form when atoms transfer electrons (metals + nonmetals). Covalent bonds form when atoms share electrons (nonmetals + nonmetals).

In a chemical reaction, reactants transform into products. The law of conservation of mass states that matter is neither created nor destroyed — the total mass of reactants equals the total mass of products. Chemical equations must be balanced to reflect this.