The basic structure of an atom was established through experiments by scientists like Ernest Rutherford (gold foil experiment, 1911) and Niels Bohr (planetary model, 1913). Protons carry a positive electrical charge, electrons carry a negative charge, and neutrons are neutral. In a neutral atom, the number of electrons equals the number of protons. The atomic number (Z) uniquely identifies an element – no two elements share the same atomic number. The mass number is the total number of protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons (e.g., carbon-12, carbon-13, carbon-14). The electron cloud accounts for over 99% of the atom's volume but only a tiny fraction of its mass, as electrons are about 1/1836 the mass of a proton.

Dmitri Mendeleev created the first widely recognized periodic table in 1869, arranging elements by atomic mass and predicting properties of undiscovered elements. The modern periodic table arranges elements by atomic number (proton count). There are 7 periods (horizontal rows) and 18 groups (vertical columns). Group 1 elements (alkali metals) are highly reactive, while Group 18 elements (noble gases) are inert due to complete valence shells. The periodic table also distinguishes metals (left and center), nonmetals (right), and metalloids (stair-step line). Elements in the same period have the same number of electron shells but increasing valence electrons. This organization allows chemists to predict how elements will behave in chemical reactions, making the periodic table one of chemistry's most powerful tools.

Ionic bonds form when one atom donates one or more electrons to another atom, creating oppositely charged ions that attract. Common example: sodium (Na) donates an electron to chlorine (Cl), forming Na⁺ and Cl⁻, which combine as NaCl (table salt). Covalent bonds form when atoms share electron pairs. Water (H₂O) has two covalent bonds: each hydrogen shares an electron with oxygen. Methane (CH₄) has four covalent bonds. Metallic bonds explain why metals conduct electricity and heat well – electrons move freely throughout the metal lattice. Bond strength varies: covalent bonds are generally strongest within molecules, while ionic compounds have high melting points but can dissolve in water. Understanding chemical bonds explains everything from DNA structure (hydrogen bonds) to industrial materials (alloys).

Antoine Lavoisier established the Law of Conservation of Mass in the late 18th century, which became fundamental to chemistry. For example, when methane (CH₄) burns in oxygen (O₂), the reaction produces carbon dioxide (CO₂) and water (H₂O). The number of atoms of each element must balance: CH₄ + 2O₂ → CO₂ + 2H₂O. Chemical reactions are classified into types: synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), double replacement (AB + CD → AD + CB), and combustion (hydrocarbon + O₂ → CO₂ + H₂O). Factors affecting reaction rates include temperature, concentration, surface area, and catalysts (which speed reactions without being consumed). Enzymes are biological catalysts essential for life processes. Balancing chemical equations is a key skill in stoichiometry, which calculates reactant and product quantities.

The pH scale was introduced by Danish chemist Søren Sørensen in 1909 while working at the Carlsberg Laboratory. "pH" stands for "potential of hydrogen" or "power of hydrogen." The mathematical relationship is pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per liter. Examples: stomach acid (pH ≈ 1.5-3.5), lemon juice (pH ≈ 2), vinegar (pH ≈ 3), coffee (pH ≈ 5), milk (pH ≈ 6.5), pure water (pH = 7), blood (pH ≈ 7.4), baking soda solution (pH ≈ 8.5), ammonia (pH ≈ 11), and drain cleaner (pH ≈ 14). Acids taste sour (citric acid in lemons) and can corrode metals; bases taste bitter and feel slippery (soap). Buffer solutions resist pH changes and are crucial in biological systems – human blood maintains pH between 7.35 and 7.45 for survival. Indicators like litmus paper (red in acid, blue in base) or universal indicator (rainbow color range) measure pH.

Balancing chemical equations is essential for accurate stoichiometric calculations. Consider the combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O (unbalanced). To balance: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Count atoms: Left side has 3 C, 8 H, 10 O; right side has 3 C, 8 H, 10 O. Never change subscripts (e.g., turning H₂O into H₂O₂ would represent hydrogen peroxide, a different substance). Special techniques include balancing atoms that appear in only one reactant and one product first, leaving oxygen and hydrogen for last. Polyatomic ions (like SO₄²⁻, NO₃⁻) can often be balanced as units. Redox reactions (oxidation-reduction) require balancing both atoms and charge. Practice examples: H₂ + O₂ → H₂O (balanced: 2H₂ + O₂ → 2H₂O), and Fe + O₂ → Fe₂O₃ (balanced: 4Fe + 3O₂ → 2Fe₂O₃). Stoichiometry uses balanced equations to convert between masses, moles, and numbers of molecules.