JUNE 2026 Ultimate Chemistry Regents Review | EVERYTHING YOU NEED TO KNOW (every topic reviewed)

Atoms are composed of electrons (negative charge, negligible mass, orbit the nucleus), protons (positive charge, 1 amu, in nucleus), and neutrons (neutral charge, 1 amu, in nucleus). The nucleus is dense and positively charged, while electrons contribute to the atom's volume. Changes in electrons form ions (cations and anions), changes in neutrons form isotopes, and changes in protons change the element itself.

Isotopes have the same number of protons but different numbers of neutrons, leading to different masses. The mass number is the sum of protons and neutrons. Average atomic mass is calculated by multiplying the mass of each isotope by its abundance and summing these values. Examples like Carbon-13 and Carbon-14 are used to illustrate this.

Electrons orbit the nucleus in shells, with higher energy levels further from the nucleus. An atom in the ground state has electrons in their lowest energy shells. An excited state occurs when electrons absorb energy (e.g., from light/photons) and jump to higher energy orbitals. When they return to their ground state, they release energy, often as light, connecting to the electromagnetic spectrum.

The electromagnetic spectrum classifies waves based on wavelength and frequency. Wavelength and frequency are inversely related. Energy increases as wavelength decreases and as frequency increases. This concept is crucial for understanding how electrons absorb and release energy in the form of light.

Valence electrons are the electrons in the outermost orbital and dictate an atom's reactivity. The periodic table is organized by increasing atomic number (number of protons). Elements are categorized as metals, non-metals, and metalloids, each with distinct properties. Groups (columns) and periods (rows) denote specific characteristics and trends.

Across a period (left to right), electronegativity and ionization energy increase, while atomic radius decreases due to increasing nuclear charge. Down a group, atomic radius increases, while electronegativity and ionization energy decrease due to added electron shells and increased distance from the nucleus.

Ionic bonding involves the complete transfer of electrons between a metal and a non-metal, forming charged ions (cations and anions) that attract each other. Covalent bonding involves the sharing of electrons between two non-metals. Lewis dot structures and the S=N-A formula help determine the number of bonds (single, double, triple) and lone pairs.

Bond polarity arises from unequal sharing of electrons due to differences in electronegativity. Polar bonds have partially positive and negative ends. Molecular polarity depends on both bond polarity and molecular symmetry. Symmetrical molecules with polar bonds can still be non-polar overall (e.g., CO2).

IMFs are attractive forces between molecules. Hydrogen bonding is the strongest IMF, occurring between hydrogen and highly electronegative atoms (N, O, F). Dipole-dipole forces occur between polar molecules. London dispersion forces are the weakest, present in all molecules but dominant in non-polar ones. Stronger IMFs lead to higher boiling points.

Ionic compounds are named by the metal then the non-metal with an '-ide' ending. Multivalent metals require Roman numerals to indicate charge. Covalent compounds use prefixes (mono-, di-, tri-) to denote the number of each atom. Polyatomic ions have specific names.

Five main types of reactions: synthesis (A + B -> C), decomposition (C -> A + B), single replacement (AB + C -> AC + B), double replacement (AB + CD -> AC + BD), and combustion (hydrocarbon + O2 -> CO2 + H2O).

Chemical equations must be balanced to conserve mass, ensuring the same number of atoms of each element on both reactant and product sides. Stoichiometry uses mole ratios (coefficients) to predict the amount of product formed or reactant consumed. Grams can be converted to moles using molar mass.

A mole (Avogadro's number) relates particles to a measurable mass. Molar mass is used to convert between grams and moles. Percent composition is the mass of a specific element in a compound divided by the total molar mass of the compound. Empirical formulas represent the simplest whole-number ratio of elements in a compound, while molecular formulas show the actual number of atoms.

Matter exists as solids (low entropy, fixed structure), liquids (medium entropy, fluid), and gases (high entropy, random motion). Phase changes include melting, vaporization, condensation, freezing, sublimation (solid to gas), and deposition (gas to solid). Kinetic energy is proportional to temperature, with gases having the highest kinetic energy.

Heating curves illustrate temperature changes and phase transitions as heat is added. Q = MCΔT calculates heat absorbed/released when temperature changes. Q = MHf (heat of fusion) and Q = MHv (heat of vaporization) calculate heat for melting/freezing and boiling/condensation, respectively.

The combined gas law (P1V1/T1 = P2V2/T2) describes how pressure, volume, and temperature of a gas relate. Pressure and volume are inversely proportional, while temperature is directly proportional to both pressure and volume.

Solutions consist of a solute (what dissolves) and a solvent (what does the dissolving). Solubility refers to how much solute can dissolve in a solvent. Ionic and polar compounds are generally soluble in water. Saturated solutions hold the maximum solute, unsaturated hold less, and supersaturated hold more than the maximum at a given temperature.

Solubility is affected by temperature and pressure. High temperature and high pressure generally increase solubility. Concentration is quantified by molarity (M = moles/volume). The dilution equation (M1V1 = M2V2) is used to calculate changes in concentration upon dilution.

Acids (pH 0-7) are H+ donors, and bases (pH 7-14) are H+ acceptors. The pH scale measures acidity, with lower pH indicating higher H+ concentration. Indicators are used to test pH.

Endothermic reactions absorb heat (ΔH > 0), typically occurring when bonds are broken. Exothermic reactions release heat (ΔH < 0), occurring during bond formation. Energy diagrams illustrate these changes, showing activation energy and the relative energy of reactants and products.

Collision theory states that molecules must collide with proper energy and orientation to react. Factors increasing reaction rate include higher temperature, higher concentration, increased surface area, and the presence of a catalyst (which lowers activation energy by providing an alternative reaction pathway).

Equilibrium occurs in reversible reactions where the forward and reverse reaction rates are equal, and reactant/product concentrations remain constant. Le Chatelier's principle states that if an equilibrium system is disturbed, it will shift to counteract the disturbance and re-establish equilibrium.

Redox reactions involve the transfer of electrons. Oxidation is loss of electrons (OIL), and reduction is gain of electrons (RIG). Oxidation numbers track electron movement. The substance getting oxidized is the reducing agent, and the substance getting reduced is the oxidizing agent.

Galvanic or voltaic cells (batteries) generate electricity spontaneously from redox reactions. The anode is where oxidation occurs (negative electrode, loses mass), and the cathode is where reduction occurs (positive electrode, gains mass). Electrons flow from anode to cathode.

Electrolytic cells require an external power source (battery) to drive a non-spontaneous redox reaction, effectively forcing the movement of electrons in the opposite direction of a galvanic cell.

Radioactive decay is the spontaneous breakdown of unstable atomic nuclei, emitting particles or energy. Common types include alpha decay (emits a helium nucleus), beta decay (emits an electron), positron emission (emits a positron), and gamma radiation (emits pure energy).

Nuclear equations must balance both mass numbers and atomic numbers. Alpha particles are least penetrating, while gamma rays are most penetrating. Half-life is the time required for half of a radioactive sample to decay, calculated using the formula: Amount Remaining = Initial Amount * (1/2)^n, where n is the number of half-lives.

Nuclear fission is when a large nucleus splits into smaller nuclei (e.g., nuclear reactors). Nuclear fusion is when two smaller nuclei combine to form a larger nucleus (e.g., in the sun), releasing immense energy.