Activation Energy Activation energy is the minimum energy needed for reactants to form the transition state.

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1 Absolute Temperature Absolute temperature is measured on the Kelvin scale. Absolute Zero Absolute zero is the coldest possible temperature, at which all molecular motion should theoretically stop. Absolute zero is 0 K or C. Acid-Dissociation Constant The acid-dissociation constant, K a, is the equilibrium constant for an acid undergoing dissociation. Acidic Acidic is used to describe a solution in which the concentration of H + ions is greater than the concentration of OH - ions. Acidic Salt An acidic salt produces an acidic solution when dissolved in water. Activation Energy Activation energy is the minimum energy needed for reactants to form the transition state. Active Site The active site of an enzyme is a pocket in which reactants bind and react. Activity Series of Metals The activity series of metals is a list of metals ordered so that metals higher in the list are more easily oxidized. Actual Yield Actual yield refers to the quantity of product actually obtained in a laboratory situation. Alchemy Alchemy refers to early scientific and philosophical disciplines, which combines elements of chemistry, metallurgy, physics, medicine, astrology, semiotics, mysticism, spiritualism, and art. Alkali Metals The alkali metals are the metals in Group IA of the periodic table. Hydrogen is a nonmetal and is not considered to be an alkali metal. Alkaline Earth Metals The alkaline earth metals are the elements in group IIA of the periodic table. Amorphous Solid An amorphous solid is a non-crystalline solid with little order or structure in the arrangement of its molecules. Ampere The ampere, A, is the SI unit for current. 1 A is 1 coulomb per second. Amphoteric Amphoteric is used to describe compounds that can act as either an acid or a base. Anion Anions are ions that have a negative charge. Anode The anode is the electrode associated with the oxidation half-reaction in an electrochemical cell. Aqueous Solution An aqueous solution is a solution with water as the solvent. Arrhenius Acid An Arrhenius acid is a compound that increases the concentration of H + ions in water. Arrhenius Base An Arrhenius base is a compound that increases the concentration of OH - ions in water. Artificial Radioisotope An artificial radioisotope is a man-made isotope that spontaneously undergoes decay. atm The atm, short for atmosphere, is a unit of pressure. The atmospheric pressure at sea level is approximately 1 atm. Atom An atom is the smallest particle of an element which has all the characteristics of that element. Atomic Absorption Spectroscopy Atomic absorption spectroscopy is a technique used for determining the concentration of an element in a sample. Atomic Emission Spectroscopy Atomic emission spectroscopy is a technique that uses the intensity of light emitted at a particular wavelength to determine the quantity of elements in a sample. Atomic Mass Atomic mass is the mass of an atom's nucleus. Atomic Mass Number or Mass Number The atomic mass number is equal to the sum of the number of protons and neutrons in an atom. Atomic Mass Unit The atomic mass unit is the approximate mass of one proton or one neutron. Atomic Number The atomic number is equal to the number of protons in the nucleus of an atom. Atomic Radius The atomic radius is the radius of an atom. Aufbau Principle The Aufbau principle dictates that when you assign electrons to orbitals, you must assign them one at a time, starting with the lowest-energy orbital, until all of the electrons have been placed in an orbital. Autoionization of Water The autoionization of water is the process by which a small fraction of water molecules dissociate into H + and OH - ions. Avogadro's Law Avogadro's law hypothesizes that at the same temperature and pressure, equal volumes of all gases contain the same number of molecules. Avogadro's Number Avogadro's number is the number of atoms or molecules in one mole, equal to Azimuthal Quantum Number The azimuthal quantum number, l, determines the shape of the subshell in which an electron is located.

2 Barometer A barometer is an instrument for measuring atmospheric pressure. Base-Dissociation Constant The base-dissociation constant, K b, is the equilibrium constant for a base undergoing dissociation. Basic Basic is used to describe a solution in which the concentration of H + ions is less than the concentration of OH - ions. Basic Salt A basic salt produces a basic solution when dissolved in water. Bimolecular Collision A bimolecular collision is a collision occurring between two molecules. Binary Ionic Compound A binary ionic compound consists of only two elements, both of which are ions. Bohr Model A Bohr model depicts an atom with the nucleus at the core of the atom and surrounded by electrons traveling in concentric circles around the nucleus. Boiling Point Boiling point is the temperature at which a liquid evaporates. Bond Dipole The bond dipole is a measure of the polarity of a chemical bond. Bond Length Bond length is the average distance between the nuclei of two bonded atoms in a molecule. Boyle's Law Boyle's law states that at a constant temperature the volume of a confined ideal gas varies inversely with its pressure. Brønsted-Lowry Acid A Brønsted-Lowry acid is a proton donor. Brønsted-Lowry Base A Brønsted-Lowry base is a proton acceptor. Buffer A solution that resists changes in ph when either acid or base is added to it. Buffer Capacity The amount of acid or base that can be added to a buffered solution before the ph begins to change significantly. Buret A buret is a tall, thin tube labeled with volume markings. A buret is typically used for dispensing a standard solution during a titration.

3 Calorie A calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1 C. Calorimeter A calorimeter is a device used for measuring the enthalpy change of a reaction. Calorimetry Calorimetry is the laboratory method for determining enthalpy changes. Catalyst A catalyst is a substance which speeds up a reaction without undergoing any net chemical change itself. Cathode The cathode is the electrode associated with the reduction half-reaction in an electrochemical cell. Cathode-Ray Tube A cathode-ray tube is a type of vacuum tube that contains a source of electrons and a fluorescent screen and is used to create images in the form of light emitted from the fluorescent screen. Cation A cation is a positively charged ion. Chain Reaction A chain reaction produces products which lead to further reactions. Chalcogens Chalcogens are the elements located in group VIA of the periodic table. Charles's Law Charles's law is an empirical law which states that the volume of a fixed amount of a gas at constant pressure varies directly with the absolute temperature. Chemical Change or Chemical Reaction A chemical change is a change in a substance that involves the breaking or formation of chemical bonds. Chemical Symbol The chemical symbol for an element is its 1-, 2-, or 3-letter abbreviation, used for chemical reactions. Chemistry Chemistry is a natural science that deals with the study of matter and the changes it undergoes. Chlorofluorocarbon or CFC A chlorofluorocarbon is a stable chemical compound which contains chlorine, fluorine, and carbon and has been used in refrigerants, solvents, and aerosols. Chlorofluorocarbons are harmful to the ozone layer. Closed System A closed system can exchange energy but not matter with its surroundings. CNO Cycle The CNO cycle is a series of fusion reactions which powers stars heavier than the Sun. CNO stands for carbon-nitrogen-oxygen. Coefficient A coefficient is the number of each molecule present in a chemical reaction. Colligative Properties Colligative properties are affected by the amount of solute and not by the identity of the solute, such as the lowering of the vapor pressure, the raising of the boiling point, and the lowering of the freezing point. Combination Reaction A combination reaction is a reaction in which two substances combine to form one compound. Combined Gas Law The combined gas law combines Boyle's law, Charles's law, and Gay-Lussac's law. It is stated mathematically as PV/T = k, where k is a constant. Combustion Reaction A combustion reaction is a reaction in which a compound is burned in the presence of oxygen. Common Ion A common ion is an ion which is present in more than one compound in a solution. Common-Ion Effect The common-ion effect states that a compound's solubility decreases in a solution containing a common ion. Compound A compound is a substance made up of two or more elements that are chemically combined in a specific ratio. Condensation Condensation is the process by which a substance changes from the gas phase to the liquid phase. Conjugate Acid A conjugate acid is a compound which acts as an acid in the reverse reaction. Conjugate Base A conjugate base is a compound which acts as a base in the reverse reaction. Continuous Spectrum A continuous spectrum is a spectrum where all wavelengths are contained in a specified region of the electromagnetic spectrum. Coordinate Covalent Bond In a coordinate covalent bond, both of the bonding electrons come from the same atom. Coulomb The coulomb is the SI unit for electrical charge. Covalent Bond A covalent bond is a chemical bond that is formed by the sharing of one or more electron pairs between two atoms. Critical Mass The critical mass of a radioactive material is the mass needed to sustain a constant rate of fission. Critical Point The critical point is the temperature and pressure at which the liquid and gaseous phases of a substance are no longer distinct. Crystal Lattice A crystal lattice is the three-dimensional arrangement of particles within a crystal. Crystalline Solid A crystalline solid is a solid characterized by an ordered pattern of molecular arrangement.

4 Dalton's Law Dalton's law states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture. Decay Constant The decay constant is the rate constant used to describe the kinetics of radioactive disintegration. Decomposition Reaction A decomposition reaction involves the breakdown of a reactant into smaller, simpler products. Degenerate Orbitals Degenerate orbitals are orbitals at identical energy levels. Deposition Deposition is the process that occurs when a substance changes from a gas directly into a solid by cooling. Diatomic Molecule A diatomic molecule is composed of two atoms only. Diffusion Diffusion describes the process by which molecules spread out to uniformly occupy a space. Dipole A dipole is the separation of positive and negative charges between two atoms. Dipole Moment The dipole moment of a bond or molecule is the measure of its polarity. Dipole-Dipole Interactions Dipole-dipole interactions are attractive interactions between molecules with permanent dipoles. Dissolution Dissolution is the process of dissolving a substance in a liquid, and is largely dependent on intermolecular forces. Distillation Distillation is the process in which a liquid mixture is separated into its components based on differences in boiling points. Double Displacement Reaction A double displacement reaction occurs when ions of two different molecules switch places forming two entirely different compounds. Dynamic Equilibrium In a dynamic equilibrium, two (or more) processes occur at the same rate with no net change.

5 Effusion Effusion is the movement of a gas through a small hole in a container. Electrolysis Electrolysis is the process of using electrical energy to decompose a compound into its elements. Electrolytic Cell An electrolytic cell uses an external energy supply to pump electrons in the reverse direction that the electrons tend to go spontaneously. Electrolytic cells are used to drive non-spontaneous oxidation-reduction reactions. Electromotive Force Electromotive force, emf, is a measure of the tendency for electrons to flow in an electrochemical cell. Electron An electron is a subatomic particle with a negative charge, which is found in orbitals surrounding an atomic nucleus. Electrons are much smaller than protons and neutrons. Electron Affinity Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gaseous state. Electron Cloud An electron cloud is a region of space around an atomic nucleus in which electrons are likely to exist. Electron Configuration An electron configuration shows the specific distribution of electrons in atomic orbitals of atoms. Electron Shell An electron shell refers to a grouping of electrons surrounding the nucleus of an atom. Electronegativity Electronegativity is a measure of the tendency of an atom to attract the electrons it shares with another atom in a bond. Electronic Balance An electronic balance is an instrument used to measure mass. Electroplating Electroplating is the process by which a thin film of one metal is deposited, or plated, onto another metal. Element An element is a pure chemical substance that cannot be decomposed into simpler substances by chemical means. Elementary Particles Elementary particles cannot be subdivided into smaller particles. Elementary Reaction An elementary reaction occurs in one step, without the formation of any intermediates. Elementary Step An elementary step is a reaction in which reactants form products without forming any intermediates. An elementary step involves only a single collision. Empirical Formula An empirical formula is the formula of a compound in which the subscripts are reduced to their smallest whole-number ratio. Empirical Knowledge Empirical knowledge is knowledge that is gained through actual observation and experimentation. Endothermic Endothermic is used to describe reactions which absorb heat and have a positive enthalpy change. Endpoint The endpoint is the point at which a chemist stops a titration. Energy Energy is the capacity to do work. Enthalpy Enthalpy is the heat content of a system at constant pressure. Entropy Entropy is a measure of the randomness or disorder of a system. Enzyme An enzyme is a large molecule which catalyzes a specific reaction in an organism. Equilibrium Equilibrium occurs when the forward rate of reaction equals the reverse rate of reaction. Equilibrium Constant The equilibrium constant, K, expresses the ratio of product concentration to reactant concentration in an equilibrium system. Equivalence Point The equivalence point is the point at which moles of base equals moles of acid in a titration. Erlenmeyer Flask An Erlenmeyer flask is a triangular-shaped flask with a wide bottom and narrow opening. Escape Velocity Escape velocity is the velocity necessary to overcome the pull of gravity. Evaporation Evaporation occurs when a substance changes from the liquid phase to the gas phase. Excess Reactant An excess reactant is a reactant that has some amount remaining after a chemical reaction. Excited Electron An excited electron is an electron that has absorbed energy and moved to a higher energy level. Exothermic Exothermic is used to describe reactions which release heat and have a negative enthalpy change.

6 First Ionization Energy First ionization energy is the amount of energy required to remove the outermost electron from a neutral atom. First Law of Thermodynamics The first law of thermodynamics states that the total energy in the universe is constant, and that energy cannot be created or destroyed. Formula Unit A formula unit is the lowest whole number ratio of ions in an ionic compound. Free Energy Free energy, G, is a thermodynamic quantity that can be used to determine the spontaneity of a reaction. Free Radical A free radical is a highly reactive atom or molecule containing an unpaired electron. Freezing Freezing occurs when a substance moves from the liquid phase to the solid phase. Frequency Frequency is determined by establishing a specific point in space and measuring the number of wave peaks that pass through that point in a given time. Fundamental Particles Fundamental particles cannot be subdivided into smaller particles.

7 Galvanic Cell A galvanic cell uses spontaneous oxidation-reduction reactions to perform work. Gas A gas is a low density substance that has no definite shape or volume. Gay-Lussac's Law Gay-Lussac's law is an empirical law which states that the pressure of a gas with fixed volume and mass is directly proportional to the absolute temperature of the gas. Graduated Cylinder A graduated cylinder is a measuring device most often used for measuring liquid volumes. Graham's Law of Effusion Graham's law of effusion states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. Greenhouse Effect The greenhouse effect is the retention of part of the sun's energy in the earth's atmosphere as heat. Greenhouse Gases Greenhouse gases are any gases whose absorption of the sun's radiation is responsible for the greenhouse effect, including carbon dioxide, methane, ozone, and fluorocarbons. Ground State The ground state of an atom, ion, or molecule is its lowest energy state. Group A group is a column of the periodic table. Elements that share a group have similar properties.

8 Half-Life Half-life is the amount of time it takes for a radioisotope to decay to half of its original amount. Half-Reaction A half-reaction represents either the oxidation part or the reduction part of an oxidation-reduction reaction. Halogens Halogens are the elements found in Group VIIA of the periodic table. Heat Heat is the energy transferred from a warm system to colder surroundings. Heat of Combustion Heat of combustion, or enthalpy of combustion, is the enthalpy change involved in a reaction in which a compound is burned in the presence of oxygen. Heat of Formation Heat of formation, or enthalpy of formation, is the enthalpy change involved in a reaction in which a compound is formed from its elements. Heat of Fusion Heat of fusion is the amount of thermal energy required to convert a solid into a liquid at its melting point. Heat of Vaporization Heat of vaporization is the amount of thermal energy required to convert liquid to vapor at its boiling point. Heating Curve A heating curve is created by graphing how a substance's temperature changes while being heated. Heisenberg Uncertainty Principle The Heisenberg uncertainty principle states that it is impossible to know exactly both the position and the momentum of a particle the size of an electron. Henderson-Hasselbalch Equation The Henderson-Hasselbalch equation is used to calculate ph from pk a and the ratio of base to acid. Hess's Law Hess's law states that the sum of enthalpy changes for a series of reaction steps equals the net enthalpy change for the net reaction. Heterogeneous Catalyst A heterogeneous catalyst is a catalyst that exists in a different phase than the reactants. Heterogeneous Equilibrium A heterogeneous equilibrium is an equilibrium that involves more than one phase of matter. Heterogeneous Mixture A heterogeneous mixture is a mixture which is not uniform in composition. Homogeneous Catalyst A homogeneous catalyst is a catalyst that exists in the same phase as the reactants. Homogeneous Equilibrium A homogeneous equilibrium is an equilibrium that involves only one phase of matter. Homogeneous Mixture A homogeneous mixture is a mixture which is uniform in composition. Hund's Rule Hund's rule states that electrons begin to pair only after each orbital in a sublevel contains one electron. Hybrid Orbital A hybrid orbital is formed by combining two or more atomic orbitals. Hydrogen Bonding Hydrogen bonding is an intermolecular force between a hydrogen atom and an electronegative atom. Hydronium Ion A hydronium ion, H 3 O +, is formed when water gains a hydrogen ion. Hydroxide Ion A hydroxide ion, OH -, is formed when water loses a hydrogen ion. Hygroscopic Hygroscopic describes a substance which tends to absorb water from the air.

9 Ideal Gas An ideal gas is a hypothetical gas whose behavior doesn't deviate from the kinetic-molecular theory postulates. Ideal Gas Law The ideal gas law states that the product of pressure and the volume of an ideal gas is directly proportional to the absolute temperature and the number of moles of the gas, usually written as PV = nrt. Indicator An indicator is a substance which signals the equivalence point of a titration, usually by changing color once the equivalence point is reached. Inert Inert describes a chemically inactive element, compound, or material. Initial Rate The initial rate is the rate of the reaction at time 0. Inner-Transition Metals The inner-transition metals are the elements in the lanthanide and actinide series. Inorganic Compound An inorganic compound is a substance which does not contain a carbon-hydrogen bond. Intermediate An intermediate is a compound which is formed in one step of a reaction mechanism and consumed in a later step. Internal Energy Internal energy is the energy contained within a system. Ion-Dipole Force An ion-dipole force is the result of electrical interactions between an ion and the slight positive or negative charge present on a polar molecule. Ionic Bond An ionic bond is a chemical bond formed by the attraction between two oppositely charged ions. Ionic Compound An ionic compound is a compound made from ionic bonds. Ionic Radius The ionic radius is a measure of the size of an ion. Ionization Ionization is the adding or subtracting of electrons from a neutral atom or molecule. Isolated System An isolated system is a system that can exchange neither energy nor matter with its surroundings. Isotope Each isotope of an element contains a different number of neutrons in its nucleus.

10 Joule The joule, J, is the standard unit for energy.

11 Kelvin The kelvin, K, is the SI unit for temperature. Kinetic Energy Kinetic energy is the energy of motion. Kinetic-Molecular Theory (KMT) The kinetic-molecular theory is a theory that describes the nature of ideal gases.

12 Laser (or LASER) Laser stands for Light Amplification by Stimulated Emission of Radiation. Lattice Energy Lattice energy is the energy required to completely separate the ions in an ionic solid. Law of Conservation of Mass The law of conservation of mass states that matter cannot be created or destroyed. Law of Triads The law of triads states that when elements are placed in ascending order of their atomic masses, groups of three elements having similar properties are obtained, and the atomic mass of the middle element of the triad is the average of the atomic masses of the other two elements of the triad. Le Châtelier's Principle Le Châtelier's principle states that whenever a system is disturbed, the system will counteract that change by establishing a new equilibrium. Lewis Acid A Lewis acid is an electron pair acceptor. Lewis Base A Lewis base is an electron pair donor. Lewis Dot Structure or Lewis Structure A Lewis dot structure is the representation of a molecule by showing its atomic symbol and only valence electrons. Limiting Reactant The limiting reactant is the substance in a chemical reaction that limits the amount of product that can be formed. Line Spectrum A line spectrum is an image of colored lines which represents the emission of energy from an atom or molecule. Liquid A liquid is a form of matter between a gas and a solid that has a definite volume, but no definite shape. London Dispersion Forces London dispersion forces are weak attractive forces between temporary dipoles. Lone Pair A lone pair is a pair of electrons that is not shared by other atoms.

13 Magnetic Quantum Number The magnetic quantum number, m l, designates the orbital in which an electron is located. Main Group Elements The main group elements are the elements located in the s-block and p-block of the periodic table. Manometer A manometer is an instrument used to measure pressure. Mass Mass is a measure of the amount of matter in an object. Mass Number Mass number, A, is the sum of the neutrons and protons in an atom. Melting Melting occurs when a substance moves from the solid phase to the liquid phase. Meniscus The meniscus is the curve at the surface of a liquid. Metallic Bonding Metallic bonding occurs when electrons in a solid are delocalized, moving freely throughout the solid. Metallic Solid A metallic solid is a crystalline solid containing metal atoms. Metalloid A metalloid is an element that has some properties of metals and some properties of nonmetals. Millimeter of Mercury (mm Hg) A millimeter of mercury is a unit of pressure equal to the pressure exerted by a column of mercury 1 millimeter high. Molality Molality, m, is a measure of concentration equal to the moles of solute per kilogram of solvent. Molarity Molarity, M, is a measure of the number of moles of solute dissolved in a liter of solution. Mole The mole is the SI base unit for measuring the amount of a substance. 1 mole of molecules is Avogadro's number of that molecule. Mole Fraction Mole fraction is the number of moles of a component divided by the total number of moles of all the components present. Molecularity Molecularity is the number of molecules that collide in an elementary step. Molecule A molecule is the combination of two or more atoms. Monatomic Ion A monatomic ion is an ion made up of only one atom. Monomers Monomers are molecules that can combine with others of the same kind to form a polymer. Monoprotic Acids Monoprotic acids contain only one dissociable proton.

14 Natural Radioisotope A natural radioisotope is a naturally occurring isotope that spontaneously undergoes decay. Net Ionic Equation A net ionic equation is a chemical reaction that is completely balanced and is reduced to the ions that are actually involved in the reaction. Network Solid A network solid is a substance in which the atoms are bonded by covalent bonds. Neutral Neutral is used to describe a solution in which the concentration of H + ions is equal to the concentration of OH - ions. Neutralization Reaction A neutralization reaction occurs when an acid and base are mixed. Water is the product of a neutralization reaction. Neutron A neutron is a subatomic particle that has no charge. Noble Gases or Inert Gases The noble gases are the elements of group VIIIA, which have very low chemical reactivity. Nonmetals The nonmetals are the elements in the upper right corner of the periodic table, as well as hydrogen in the upper left corner, which lack the properties of metals. Non-Spontaneous Reaction A non-spontaneous reaction requires a continual input of energy to power the reaction. Normal Boiling Point The normal boiling point is the temperature at which a liquid boils at one atm of pressure. Normal Melting Point The normal melting point is the temperature at which a solid melts at one atm of pressure. Nuclear Fission Nuclear fission is the splitting of large nuclei. Nuclear Fusion Nuclear fusion is the combining of small nuclei into larger nuclei. Nuclear Transmutation Nuclear transmutation is the conversion of one element into another one. Nucleus The nucleus is the center of an atom containing protons and neutrons. Nuclide Symbol The nuclide symbol of an element represents its chemical symbol, atomic number, and atomic mass.

15 Octahedral Molecular Geometry Octahedral molecular geometry is a molecular geometry that describes six atoms or ligands symmetrically based around a central atom, creating the vertices of an octahedron. Octet Rule The octet rule refers to the fact that atoms from the main-group elements form molecules with their valence shells sharing eight electrons. Open System An open system can exchange both matter and energy with its surroundings. Orbital An orbital represents an area of the electron cloud which can contain a pair of electrons. Order The order of a reactant is the exponent of that reactant in a rate law. The order of a reaction is the sum of all the orders in the rate law. Organic Compound An organic compound is a substance which contains a carbon-hydrogen bond. Organic compounds often contain carbon-carbon and carbon-oxygen bonds as well. Oxidant or Oxidizing Agent An oxidant is a reactant that removes electrons from another reactant during an oxidation-reduction reaction. Oxidation State The oxidation state of an atom refers to the degree of its oxidation, based on how the bonding electrons tend to distribute themselves. Oxidation-Reduction Reaction or Redox Reaction An oxidation-reduction reaction, or redox reaction, involves an exchange of electrons between reactants. Ozone Layer The ozone layer is the layer of the atmosphere where ozone is most concentrated.

16 Partial Pressure Partial pressure is the pressure that an individual gas exerts in a mixture of other gases. Pascal The pascal is the SI unit for pressure. 1 pascal is equal to 1 newton per square meter. Pauli Exclusion Principle The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers. Percent Ionization Percent ionization is the percentage of how many moles of acid or base are ionized out of the total moles of acid or base. Percent Yield Percent yield is a measure of the efficiency of product formation and collection. Periodic Periodic refers to something that appears at regular intervals. Periodic Table The periodic table is a list of elements in order of the number of protons in their nuclei. ph ph is a measure of the acidity of a solution. ph Meter A ph meter is an instrument which directly measures the ph of a solution. ph Paper ph paper changes color when it is dipped into a solution. The color is used to determine the ph of the solution. Phase Boundary On a phase diagram, a phase boundary is a solid line which represents the division of one phase from another. Phase Diagram A phase diagram is used to show temperature-pressure relationships of different phases of a substance. Phosphorescent Phosphorescent describes a substance which absorbs electromagnetic radiation, such as light, and then slowly re-emits that radiation. Photodissociation A photodissociation is a chemical reaction occurring when photons strike a chemical compound and cause one or more bonds to be broken. Photoelectric Effect The photoelectric effect is the observation that shining light on some metals causes that metal to emit electrons. Photoionization Photoionization occurs when a photon collides with an atom or molecule and causes that atom or molecule to emit an electron. Photon A photon is a bundle of electromagnetic energy. Physical Change A physical change is a change in a substance that does not change the chemical identity of that substance. Pi Bond A pi bond is a covalent bond formed when the two lobes of a p orbital overlap the two lobes of another p orbital. Pipette A pipette is a measuring device most often used for measuring liquid volumes. Planck's Constant Planck's constant, h, is used to show the sizes of quanta in quantum mechanics. The accepted value of Planck's constant is J s. Plasma Plasma is a fourth state of matter, in which electrons become stripped from their nuclei. Plasma occurs at very high temperatures. poh poh is a measure of the basicity of a solution. Polar Bond A polar bond is a type of covalent bond with unequally shared electrons. The bond will have a slightly negative charge on one end and a slightly positive charge on the other end. Polar Molecule A polar molecule is a molecule in which one part of the molecule has a slightly positive charge and one part has a slightly negative charge. Polyatomic Ion A polyatomic ion is a group of atoms covalently bonded together in such a way that the entire group has a charge associated with it. Polymer A polymer is a large molecule formed by the bonding of a series of smaller, simpler molecules called monomers. Polymerization Polymerization is the process of bonding two or more monomers to form a polymer. Polyprotic Acid A polyprotic acid contains more than one dissociable proton. Positron A positron is an electron with a positive charge. Postulate A postulate is a basis for an argument. Potential Energy Potential energy is stored energy. Precipitate A precipitate is a solid formed in a reaction. Pressure Pressure is the force per unit area, represented by the equation P = F/A. Principal Quantum Number The principal quantum number, n, represents the distance of an electron from the nucleus. Products Products are substances formed as a result of a chemical reaction. Proton A proton is a subatomic particle that has a positive charge. Proton-Proton Chain The proton-proton chain is a series of fusion reactions which powers the Sun.

17 Quadratic Formula The quadratic formula is used to determine the value of x in a quadratic equation. Quantum Mechanics Quantum mechanics is used to show the energies and spatial distributions of electrons in atoms and molecules. Quantum Numbers The four quantum numbers give an electron's position in an atom. Quark A quark is an elementary particle which is found in protons and neutrons.

18 Radioactivity Radioactivity is the disintegration of an unstable atomic nucleus to a more stable form. Rate Constant The rate constant is a temperature-dependent constant present in a reaction's rate law. Rate Law A rate law is an equation that relates reactant concentration and reaction rate. Rate-Limiting Step A rate-limiting step is the slowest step of a reaction mechanism, which is used to determine the rate law. Reactants Reactants are substances present before a chemical reaction takes place. Reaction Kinetics Reaction kinetics is the study of reaction rates and the conditions that can be used to change them. Reaction Mechanism A reaction mechanism is the sequence of elementary steps that occur in a reaction. Reaction Quotient The reaction quotient expresses the ratio of product concentration to reactant concentration at any point in a reaction's path to equilibrium. Real Gas A real gas is a gas which does not behave like an ideal gas. Redlich-Kwong Equation The Redlich-Kwong equation can be used to calculate the pressure of a real gas. It is similar to the van der Waals equation, but includes a temperature factor in the pressure term. Root-Mean-Square Speed The root-mean-square speed is a measure of the speed of particles in a gas.

19 Salt A salt is a compound which is formed when one or more hydrogen ions in an acid are replaced with another ion. Saturated Solution A saturated solution is a solution in which there is a dynamic equilibrium between undissolved solid and dissolved ions. Scientific Notation Scientific notation is a method of writing numbers that takes the place of using standard decimal notation when values are very large or very small. Second Law of Thermodynamics The second law of thermodynamics states that the entropy of the universe tends to increase. Sigma Bond A sigma bond is a single bond formed when the lobe of a p orbital overlaps the lobe of another p orbital. Single-Displacement Reaction A single-displacement reaction is a type of reaction that occurs when an element or ion moves out of one compound and into another. Solid A solid is a substance which has a fixed shape and volume. Solubility Solubility is the quantity of a compound that dissolves to form a saturated solution. Solubility-Product Constant The solubility-product constant, K sp, is the equilibrium constant used when a slightly soluble solid is dissolved in water. Solute A solute is a substance that is dissolved in a solution. Solvent A solvent is the substance in which a solute is dissolved. Specific Heat Specific heat is the amount of heat needed to raise the temperature of 1 gram of a substance by 1 C. Spectroscopy Spectroscopy is the study of light emitted or absorbed by a substance. Spin Quantum Number The spin quantum number, m s, describes the quantum state of an electron. Spontaneous Reaction A spontaneous reaction does not require a continuous input of energy to occur. Standard A standard is a solution of base whose concentration is known accurately. Standard Enthalpy of Formation The standard enthalpy of formation is the enthalpy change that occurs when a compound is formed under standard conditions. Standard Entropy Standard entropy is the entropy value for substances in their standard states. Standard Oxidation Potential The standard oxidation potential is the emf associated with an oxidation reaction at standard conditions. Standard Reduction Potential The standard reduction potential is the emf associated with a reduction reaction at standard conditions. Standardization Standardization is the process of accurately determining a standard's concentration. Stoichiometry Stoichiometry refers to the quantitative relationships between elements and compounds as they undergo chemical changes. Strong Acid A strong acid completely dissociates in water. Strong Nuclear Force The strong nuclear force holds subatomic particles together in a nucleus. Subatomic Particles Subatomic particles are part of the structure of an atom. Subcritical Mass A subcritical mass of a radioactive material is too small to sustain a fission reaction. Sublimation Sublimation is the process in which a substance changes from a solid directly into a gas. Subshell A subshell is a set of electron orbitals which have the same principal and azimuthal quantum numbers. Supercritical Mass A supercritical mass of a radioactive material is more than large enough to sustain a fission reaction. Surroundings The surroundings are everything except for the system. Synthesis Reaction A synthesis reaction is a reaction in which two or more reactants are combined to form a single product. System The system is whatever is being focused on.

20 Temperature Temperature is a measure of the average kinetic energy of the molecules (or atoms) in a system. Termolecular Collision A termolecular collision is a collision occurring between three molecules. Theoretical Yield Theoretical yield is the amount of product predicted to form based on stoichiometric calculations alone. Thermometer A thermometer is a device that measures temperature. Third Law of Thermodynamics The third law of thermodynamics states that as the temperature approaches absolute zero, entropy approaches a minimum. Titration Titration is a technique used to determine the quantity of a known compound. Tokamak A tokamak is a container that uses a strong magnetic field to contain high-temperature plasmas. Torr The torr is a non-si unit of pressure that is equal to the pressure necessary to support a column of mercury one millimeter high. Transition Metals The transition metals are the elements in the d-block of the periodic table. Transition State A transition state is formed as colliding reactant molecules make and break bonds. Triple Point The triple point is the point of the phase diagram at which all three phases of matter are in equilibrium. Triple-Beam Balance A triple-beam balance is a device used to measure mass.

21 Unimolecular Process A unimolecular process involves only one reactant molecule. A decomposition reaction would be an example of a unimolecular process. Unit Cell A unit cell is the smallest repeating unit within a crystal lattice. Unsaturated Solution An unsaturated solution is a solution in which more solute can be dissolved.

22 Valence Bond Theory Valence bond theory states that covalent bonds are formed when atomic orbitals overlap and electrons are shared. Valence Electrons Valence electrons are the electrons in the s and p orbitals of the outer shell of the atom. Valence Number Valence number is the total number of electrons in the s and p orbitals of the outer shell of the atom. Van der Waals Equation The van der Waals equation is a form of the ideal gas law, which accounts for the attraction between particles and the volume of the gas. Van der Waals Forces Van der Waals forces are intermolecular interactions including ion-dipole, dipole-dipole, and London dispersion forces. Vapor Pressure Vapor pressure is the pressure exerted by the vapor phase when it is in equilibrium with the liquid phase. Volt The volt, V, is the SI unit for electromotive force. Voltaic Cell A voltaic cell uses spontaneous oxidation-reduction reactions to perform work. Volume Volume is the amount of space a substance takes up. Volumetric Pipet A volumetric pipet is used to very accurately measure the volume of a solution and dispense it. VSEPR VSEPR stands for Valence Shell Electron Pair Repulsion. It is used to determine molecular geometry based upon the extent of electron pair electrostatic repulsion.

23 Wavelength Wavelength is the distance from peak to peak in a wave. Wave-Particle Duality Wave-particle duality is the idea that all energy has the properties of both waves and particles. Weak Acid A weak acid is an acid which only partially dissociates in water. Weak Base A weak base is a base which only partially dissociates in water. Weak Nuclear Force The weak nuclear force gives off small particles when a radioactive nucleus disintegrates, in order to stabilize the nucleus. Work Work is the energy needed to move an object from one place to another.

24 Zone of Stability In a graph of neutron-to-proton ratios for isotopes, the zone of stability, or belt of stability, is the region of the graph corresponding to non-radioactive isotopes.