RESEARCH STARTER

Chemical reactions and collisions

Chemical reactions are processes that involve the transformation of substances through breaking and forming chemical bonds. A fundamental aspect of these reactions is the role of molecular collisions, which provide the energy necessary for reactions to occur. At any temperature, molecules are in constant motion, and as temperature increases, so does the average kinetic energy of these molecules, leading to more frequent and energetic collisions. However, not all collisions result in a chemical reaction; only those that provide sufficient energy to overcome a barrier known as activation energy can lead to a reaction.

The activation energy is critical, as it determines how easily a reaction can take place; lower activation energies typically result in faster reactions. Factors influencing activation energy include temperature and the presence of catalysts, which can increase reaction rates by lowering the required energy for reactants to interact effectively. Additionally, reactions can occur through various energy inputs, such as heat, electricity, or light, each altering the pathway and efficiency of the reaction process.

Understanding chemical reactions and the mechanics of molecular collisions is essential in fields ranging from industrial chemistry to biochemistry, where reactions must be controlled for desired outcomes. This knowledge is not only foundational in scientific research but also has practical applications in everyday life, such as in cooking, manufacturing, and environmental processes.

Full Article

  • Type of physical science: Chemistry
  • Field of study: Chemistry of molecules: nature of chemical bonds

Almost all chemical reactions take place between molecules that, at the moment of reaction, have energies far greater than the average energies of the molecules around them. This energy usually accumulates in a given molecule as a result of its collisions with other molecules. Energy requirements of a given reaction are usually described in terms of a potential energy surface with a high point over which the reacting molecule(s) must pass.

Overview

All atoms are in constant motion. This is true at all temperatures above absolute zero and does not depend on whether the atoms are in a solid, liquid, or gas. It is just as true for atoms in molecules as for atoms moving alone. As the temperature increases, the average value of the energy of motion—kinetic energy—increases. This means that the atoms and molecules move faster as the temperature rises. As an illustration of such motion, the most likely speed of a nitrogen molecule, consisting of two nitrogen atoms, is slightly more than 400 meters per second in air at room temperature. There are three reasons why there is no sense of such rapid motion. First, each molecule is extremely light, so its impact on humans cannot be felt. Second, there are vast numbers of molecules moving randomly in all directions around people. Third, molecules frequently collide with one another and change directions. The typical nitrogen molecule will undergo roughly a billion collisions per second in air.

Molecules exchange energy through collisions, causing their speeds and kinetic energies to constantly shift. At any given moment, only a small fraction of molecules will possess kinetic energies far above average—yet these are the ones most likely to participate in chemical reactions. While the average kinetic energy remains constant at a given temperature, energy is continuously redistributed through molecular collisions, allowing different molecules to temporarily acquire the energy needed for reaction.

Chemical bonds are strong enough that a normal molecule is not damaged by collisions at ordinary temperatures. Negligible reactions occur between the nitrogen and oxygen molecules of the air, despite their ceaseless banging together and bouncing apart. Also, the nitrogen molecules and the oxygen molecules do not significantly react with the water vapor in the air under ordinary atmospheric conditions.

A chemical reaction requires enough energy to weaken bonds, but this does not happen in the typical collision. Instead, it requires a much more energetic collision. Consider the case of two molecules that are about to collide. If they possess energy far in excess of the average as a result of immediately preceding collisions, their collision with each other can be much more energetic than the typical one. If it is, a bond can be weakened, and a reaction may take place.

To have a simple sketch of a reaction, let A-B be a molecule, and C an atom. In this hypothetical reaction, the atom B is to leave A and attach to C: A-B + C → A + B-C

One could imagine the reaction to occur in a simple manner: The molecule A-B gains enough energy through collision to break apart, after which atoms B and C come together to make B-C. If that were the case, the collisions would have to provide all the energy needed to break the bond between A and B. The difference between the reaction in progress and the reactants before the reaction is that A-B has been broken into separate A and B atoms. This requires much energy, and it must come from collisions.

This simple description is inaccurate for most reactions because it overlooks an important factor: C must have some attraction for B. Otherwise, C and B would not join to make the product molecule B-C. Atom C will already have some attraction to B while B is attached to A. That attraction will not be strong enough to pull B away from A unless the molecule A-B is weakened by collision.

This potential energy surface differs in two respects from the previous one. First, the reaction in progress is represented by a weakened, longer bond between A and B, that is, A–B, while the bond between B and C is forming before A–B is completely broken. Second, this simultaneous bond breaking and bond making lowers the energy that must be supplied to move up to the reaction in progress.

This reduction of the energy required for reaction has great consequences for chemical reactions. Since molecules pass energy back and forth by collision, and since few molecules have energies far in excess of the average at a given moment, the result of taking into account the bond formation between C and B, while the bond between A and B is breaking, is to increase the small number of molecules that have the energy needed for reaction.

The energy needed to move from reactants before reaction up to the reaction in progress is called the activation energy. While temperature and catalysts have traditionally been used to lower this barrier, further research showed that quantum interference and magnetic fields can also influence reaction dynamics by reshaping potential pathways, independent of energy input.

Applications

The activation energy is normally a property of the reaction pathway involving the atoms and molecules. In the usual case, it is far more energy than the average among molecules so that only a tiny fraction of collisions leads to reaction. One of the ways to increase the rate of reaction is to increase the temperature. This increases the average energy of molecules, raising the fraction that can overcome the activation energy barrier. An increase in temperature of, for example, 40 degrees Celsius (104 degrees Fahrenheit) would be invisibly small on the potential energy diagram, but it would have a great influence on the rate of reaction. This is true because raising the temperature increases the tiny fraction of molecules with enough collisional energy to react faster than it raises the average energy of all molecules. This fact is one of the great principles of both physics and chemistry. It is evident in a common observation: Water evaporates much faster after rain in the summer than in the winter. Someone may object that this has nothing to do with chemical reactions because water on the sidewalk and water vapor in the air are the same chemical substance. Nevertheless, the rate of any change that depends strongly on the temperature obeys the same law of nature, whether it is a physical change or a chemical reaction.

One can find the value of the activation energy by studying the rate of reaction at various temperatures. Its numerical value is a property of the particular reaction. A typical activation energy is often significantly greater than the average kinetic energy of the molecules at the temperature of the reaction. The series of experiments needed to find the activation energy will also show that it is essentially constant for many reactions even if the rate is studied at a temperature range as great as 100 degrees Celsius (212 degrees Fahrenheit). If the activation energy is more than ten times as great as the average kinetic energy, then an increase in the latter will have a minor effect on the difference between them.

The potential energy surface can be more complicated. This happens when a reaction is not completed in one step, but first makes a molecule that reacts further to make stable products.

The following reaction between gases illustrates this. The overall reaction is NO2 + CO → NO + CO2, but a careful study of the rate of the reaction shows that it happens in two steps: NO2 + NO2 → NO3 + NO, followed by NO3 + CO → NO2 + CO2.

The reason for this is that excess energy is hard to acquire, and a reaction, even a relatively complicated one, that requires a lower activation energy will generally be favored over one with a higher activation energy. Some of the reactions that take place in living systems are highly complicated.

It has been assumed that reactions take place between molecules in gases. The collisions of interest are among the reactants. In the case of reactions in solution, the reactant is surrounded by molecules of the solvent. If it is to react, it must come into contact with another reactant molecule. Most of its collisions, however, are with the solvent. When two reactant molecules meet, they may collide more than once with each other because the solvent molecules keep them from moving apart immediately after the initial collision.

The rate of a chemical reaction often has serious consequences, and great efforts have been made in research to alter reaction rates. Sometimes, it is desirable to slow down a rate, as when a drug that improves medical care has harmful side effects. More often, however, the scientist wants to increase the speed of a reaction. An increase in temperature will, in some cases, bring about the added reaction rate. At times, the temperature cannot be increased, for example, if the solvent will boil or a reactant will break down at a higher temperature. Also, heating is costly, especially in a manufacturing process that is carried out on large quantities of material. In such cases, the scientist will search for a catalyst—a substance present in the reaction mixture that can lower the activation energy without being used up. In general, a catalyst must bring the reactants together in a way that facilitates the breaking and making of bonds. If the catalyst can do this, the rate of reaction will increase, and the gain is sometimes spectacular, with great savings in production costs. Various important industrial catalysts are metals that have been prepared in a manner that gives them large surface areas.

In the realm of living systems, enzymes are the catalysts, and they often bring about tremendous rate increases for highly specific reactions. Life would be impossible without them. Enzymes are typically large molecules and act by attaching a reactant molecule called a substrate to their surface in a way that makes the substrate enter more easily into reaction.

Beyond traditional methods like heating or catalysis, advances in quantum chemistry have demonstrated that chemical reactions can be controlled using magnetic fields and quantum interference. In a study at the Massachusetts Institute of Technology (MIT), scientists showed that certain reactions can be guided by preparing molecules in specific quantum spin states and applying an external magnetic field. This form of coherent control works not by supplying energy (like heat or light) but by manipulating the quantum wavefunctions of reactant molecules.

Context

Heat is a common energy source for chemical reactions, but other methods like electrical energy, light, and quantum manipulation are also important. In electrolysis, an electric current is applied to a solution or molten substance to drive oxidation or reduction reactions. This process can split water into hydrogen and oxygen gases, offering a practical alternative to thermal decomposition, which would otherwise require temperatures of several thousand degrees. Although temperature has some influence on electrolysis, mainly by affecting ion mobility and reaction rates, high temperatures can also reduce system efficiency and damage electrodes. High-temperature electrolysis systems optimize efficiency by combining electrical and thermal energy, often using waste heat from industrial processes.

In another approach called photochemistry, light drives chemical reactions instead of heat. When a molecule absorbs a photon—especially in the ultraviolet or high-energy visible spectrum—the added energy can be a form of activation energy, allowing an immediate reaction to occur. This light-based mechanism explains why photochemical processes, such as those used in photographic films, remain largely unaffected by changes in ambient temperature. Studies show that photochemical reaction outcomes depend not only on photon absorption but also on the molecule’s surrounding environment, such as solvent and structure. In modern research and industry, lasers can be used to initiate and control reactions with high precision. Chemists began using quantum interference to manipulate reaction pathways in the late twentieth and early twenty-first centuries. Researchers can selectively enhance or suppress certain reaction outcomes by preparing molecules in specific quantum spin states and applying magnetic fields, though external energy input is still required. Advances in quantum chemistry simulations can perform extremely large numbers of calculations per second, reducing reaction modeling time from weeks to minutes and improving the study of collision dynamics and reaction pathways.

Lasers are used as a source of energy for many kinds of reactions. In an industrial reaction, for example, laser light of the right wavelength can cause a reaction to produce more of the desired product and less of the unwanted side products. If light of sufficient energy is absorbed, it can ionize the molecule, which means it loses an electron and becomes electrically charged. This ionization is more than enough energy to activate a molecule, so it may react on the next collision. These reactions between gaseous ions and neutral molecules take place in the upper atmosphere, where molecules receive damaging radiation from the sun, and in lightning discharges. They are also studied extensively in the laboratory. Because the loss of an electron deposits much energy in a molecule, reactions between gaseous ions and molecules frequently happen at rates almost independent of temperature. Because there is no activation energy, and because an electrically charged particle can be attracted to a neutral molecule, the potential energy surface moves down when the molecule and ion first come into contact. In 2025, scientists directly observed the motion of a single electron during a chemical reaction for the first time, providing experimental confirmation of theories about reaction mechanisms and collision dynamics.

If a situation exists in which a substantial fraction of the atoms or molecules are ionized, matter under these conditions is called a plasma. While plasmas are rare on Earth, they are probably the most common form of matter in the universe, because of the great amount of energy available in stars.

Principal terms

ACTIVATION ENERGY: the energy, in excess of the average energy of reactant molecules, that is required for molecules to react

ATOM: the smallest and simplest unit of an element that can exist

COVALENT BOND: the sharing of electrons by two atoms, with the result that the atoms are joined together in a molecule

ELECTRON: a fundamental particle with a negative electrical charge

ELEMENTARY REACTION: a reaction that leads to products as a result of a single reactive collision between reactants

ENERGY: the capacity to do work; here, the important kinds of energy are collisional energy and the capacity to stretch and break covalent bonds

KINETIC ENERGY: the energy that an object has when it is in motion; it is always positive, and the kinetic energy that goes into collisions is central to the arguments that follow

POTENTIAL ENERGY: the energy that an object has because it is in a certain position; an object at the top of a hill has potential energy, which is shown by the increase in speed as it moves downhill

REACTION: the process in which one or more molecules, called reactants, gain or lose atoms to become reaction products

SOLVENT: a liquid in which substances are dissolved in a solution


Bibliography

Atkins, Peter W. General Chemistry. W. H. Freeman, 1989.

Bailar, John C., Jr., et al. Chemistry. Harcourt Brace Jovanovich, 1989.

Capella, Larissa G. “Scientists Watch a Single Electron Move during a Chemical Reaction for First Time Ever.” Live Science, 29 Aug. 2025, www.livescience.com/physics-mathematics/particle-physics/scientists-watch-a-single-electron-move-during-a-chemical-reaction-for-first-time-ever. Accessed 14 Apr. 2026.

Ebbing, Darrell D., and Steven D. Gammon. General Chemistry. 11th ed., Houghton Mifflin, 2017.

Gillespie, Ronald, et al. Chemistry. 2nd ed., Allyn & Bacon, 1989.

Kotz, John C., and Keith F. Purcell. Chemistry and Chemical Reactivity. Saunders College Publishing, 1987.

Lauth, Jakob. Physical Chemistry in a Nutshell: Basics for Engineers and Scientists. Springer, 2023.

McQuarrie, Donald A., and Peter A. Rock. General Chemistry. 3rd ed., W. H. Freeman, 1991.

“Physicists Steer Chemical Reactions by Magnetic Fields and Quantum Interference.” MIT News, 8 Mar. 2022, news.mit.edu/2022/physicists-steer-chemical-reactions-magnetic-fields-quantum-interference-0308. Accessed 14 Apr. 2026.

Swayne, Matt. “Scientists Explore Using Quantum Dots to Control Photochemical Reactions.” The Quantum Insider, 17 Jan. 2025, thequantuminsider.com/2025/01/17/scientists-explore-using-quantum-dots-to-control-photochemical-reactions/. Accessed 14 Apr. 2026.

Ware, Skyler. “Ultrafast Quantum Chemistry Engine Could Speed Up the Development of New Medicines and Materials.” Live Science, 22 Feb. 2026, www.livescience.com/technology/computing/ultrafast-quantum-chemistry-engine-could-speed-up-the-development-of-new-medicines-and-materials. Accessed 14 Apr. 2026.

Full Article

  • Type of physical science: Chemistry
  • Field of study: Chemistry of molecules: nature of chemical bonds

Almost all chemical reactions take place between molecules that, at the moment of reaction, have energies far greater than the average energies of the molecules around them. This energy usually accumulates in a given molecule as a result of its collisions with other molecules. Energy requirements of a given reaction are usually described in terms of a potential energy surface with a high point over which the reacting molecule(s) must pass.

Overview

All atoms are in constant motion. This is true at all temperatures above absolute zero and does not depend on whether the atoms are in a solid, liquid, or gas. It is just as true for atoms in molecules as for atoms moving alone. As the temperature increases, the average value of the energy of motion—kinetic energy—increases. This means that the atoms and molecules move faster as the temperature rises. As an illustration of such motion, the most likely speed of a nitrogen molecule, consisting of two nitrogen atoms, is slightly more than 400 meters per second in air at room temperature. There are three reasons why there is no sense of such rapid motion. First, each molecule is extremely light, so its impact on humans cannot be felt. Second, there are vast numbers of molecules moving randomly in all directions around people. Third, molecules frequently collide with one another and change directions. The typical nitrogen molecule will undergo roughly a billion collisions per second in air.

Molecules exchange energy through collisions, causing their speeds and kinetic energies to constantly shift. At any given moment, only a small fraction of molecules will possess kinetic energies far above average—yet these are the ones most likely to participate in chemical reactions. While the average kinetic energy remains constant at a given temperature, energy is continuously redistributed through molecular collisions, allowing different molecules to temporarily acquire the energy needed for reaction.

Chemical bonds are strong enough that a normal molecule is not damaged by collisions at ordinary temperatures. Negligible reactions occur between the nitrogen and oxygen molecules of the air, despite their ceaseless banging together and bouncing apart. Also, the nitrogen molecules and the oxygen molecules do not significantly react with the water vapor in the air under ordinary atmospheric conditions.

A chemical reaction requires enough energy to weaken bonds, but this does not happen in the typical collision. Instead, it requires a much more energetic collision. Consider the case of two molecules that are about to collide. If they possess energy far in excess of the average as a result of immediately preceding collisions, their collision with each other can be much more energetic than the typical one. If it is, a bond can be weakened, and a reaction may take place.

To have a simple sketch of a reaction, let A-B be a molecule, and C an atom. In this hypothetical reaction, the atom B is to leave A and attach to C: A-B + C → A + B-C

One could imagine the reaction to occur in a simple manner: The molecule A-B gains enough energy through collision to break apart, after which atoms B and C come together to make B-C. If that were the case, the collisions would have to provide all the energy needed to break the bond between A and B. The difference between the reaction in progress and the reactants before the reaction is that A-B has been broken into separate A and B atoms. This requires much energy, and it must come from collisions.

This simple description is inaccurate for most reactions because it overlooks an important factor: C must have some attraction for B. Otherwise, C and B would not join to make the product molecule B-C. Atom C will already have some attraction to B while B is attached to A. That attraction will not be strong enough to pull B away from A unless the molecule A-B is weakened by collision.

This potential energy surface differs in two respects from the previous one. First, the reaction in progress is represented by a weakened, longer bond between A and B, that is, A–B, while the bond between B and C is forming before A–B is completely broken. Second, this simultaneous bond breaking and bond making lowers the energy that must be supplied to move up to the reaction in progress.

This reduction of the energy required for reaction has great consequences for chemical reactions. Since molecules pass energy back and forth by collision, and since few molecules have energies far in excess of the average at a given moment, the result of taking into account the bond formation between C and B, while the bond between A and B is breaking, is to increase the small number of molecules that have the energy needed for reaction.

The energy needed to move from reactants before reaction up to the reaction in progress is called the activation energy. While temperature and catalysts have traditionally been used to lower this barrier, further research showed that quantum interference and magnetic fields can also influence reaction dynamics by reshaping potential pathways, independent of energy input.

Applications

The activation energy is normally a property of the reaction pathway involving the atoms and molecules. In the usual case, it is far more energy than the average among molecules so that only a tiny fraction of collisions leads to reaction. One of the ways to increase the rate of reaction is to increase the temperature. This increases the average energy of molecules, raising the fraction that can overcome the activation energy barrier. An increase in temperature of, for example, 40 degrees Celsius (104 degrees Fahrenheit) would be invisibly small on the potential energy diagram, but it would have a great influence on the rate of reaction. This is true because raising the temperature increases the tiny fraction of molecules with enough collisional energy to react faster than it raises the average energy of all molecules. This fact is one of the great principles of both physics and chemistry. It is evident in a common observation: Water evaporates much faster after rain in the summer than in the winter. Someone may object that this has nothing to do with chemical reactions because water on the sidewalk and water vapor in the air are the same chemical substance. Nevertheless, the rate of any change that depends strongly on the temperature obeys the same law of nature, whether it is a physical change or a chemical reaction.

One can find the value of the activation energy by studying the rate of reaction at various temperatures. Its numerical value is a property of the particular reaction. A typical activation energy is often significantly greater than the average kinetic energy of the molecules at the temperature of the reaction. The series of experiments needed to find the activation energy will also show that it is essentially constant for many reactions even if the rate is studied at a temperature range as great as 100 degrees Celsius (212 degrees Fahrenheit). If the activation energy is more than ten times as great as the average kinetic energy, then an increase in the latter will have a minor effect on the difference between them.

The potential energy surface can be more complicated. This happens when a reaction is not completed in one step, but first makes a molecule that reacts further to make stable products.

The following reaction between gases illustrates this. The overall reaction is NO2 + CO → NO + CO2, but a careful study of the rate of the reaction shows that it happens in two steps: NO2 + NO2 → NO3 + NO, followed by NO3 + CO → NO2 + CO2.

The reason for this is that excess energy is hard to acquire, and a reaction, even a relatively complicated one, that requires a lower activation energy will generally be favored over one with a higher activation energy. Some of the reactions that take place in living systems are highly complicated.

It has been assumed that reactions take place between molecules in gases. The collisions of interest are among the reactants. In the case of reactions in solution, the reactant is surrounded by molecules of the solvent. If it is to react, it must come into contact with another reactant molecule. Most of its collisions, however, are with the solvent. When two reactant molecules meet, they may collide more than once with each other because the solvent molecules keep them from moving apart immediately after the initial collision.

The rate of a chemical reaction often has serious consequences, and great efforts have been made in research to alter reaction rates. Sometimes, it is desirable to slow down a rate, as when a drug that improves medical care has harmful side effects. More often, however, the scientist wants to increase the speed of a reaction. An increase in temperature will, in some cases, bring about the added reaction rate. At times, the temperature cannot be increased, for example, if the solvent will boil or a reactant will break down at a higher temperature. Also, heating is costly, especially in a manufacturing process that is carried out on large quantities of material. In such cases, the scientist will search for a catalyst—a substance present in the reaction mixture that can lower the activation energy without being used up. In general, a catalyst must bring the reactants together in a way that facilitates the breaking and making of bonds. If the catalyst can do this, the rate of reaction will increase, and the gain is sometimes spectacular, with great savings in production costs. Various important industrial catalysts are metals that have been prepared in a manner that gives them large surface areas.

In the realm of living systems, enzymes are the catalysts, and they often bring about tremendous rate increases for highly specific reactions. Life would be impossible without them. Enzymes are typically large molecules and act by attaching a reactant molecule called a substrate to their surface in a way that makes the substrate enter more easily into reaction.

Beyond traditional methods like heating or catalysis, advances in quantum chemistry have demonstrated that chemical reactions can be controlled using magnetic fields and quantum interference. In a study at the Massachusetts Institute of Technology (MIT), scientists showed that certain reactions can be guided by preparing molecules in specific quantum spin states and applying an external magnetic field. This form of coherent control works not by supplying energy (like heat or light) but by manipulating the quantum wavefunctions of reactant molecules.

Context

Heat is a common energy source for chemical reactions, but other methods like electrical energy, light, and quantum manipulation are also important. In electrolysis, an electric current is applied to a solution or molten substance to drive oxidation or reduction reactions. This process can split water into hydrogen and oxygen gases, offering a practical alternative to thermal decomposition, which would otherwise require temperatures of several thousand degrees. Although temperature has some influence on electrolysis, mainly by affecting ion mobility and reaction rates, high temperatures can also reduce system efficiency and damage electrodes. High-temperature electrolysis systems optimize efficiency by combining electrical and thermal energy, often using waste heat from industrial processes.

In another approach called photochemistry, light drives chemical reactions instead of heat. When a molecule absorbs a photon—especially in the ultraviolet or high-energy visible spectrum—the added energy can be a form of activation energy, allowing an immediate reaction to occur. This light-based mechanism explains why photochemical processes, such as those used in photographic films, remain largely unaffected by changes in ambient temperature. Studies show that photochemical reaction outcomes depend not only on photon absorption but also on the molecule’s surrounding environment, such as solvent and structure. In modern research and industry, lasers can be used to initiate and control reactions with high precision. Chemists began using quantum interference to manipulate reaction pathways in the late twentieth and early twenty-first centuries. Researchers can selectively enhance or suppress certain reaction outcomes by preparing molecules in specific quantum spin states and applying magnetic fields, though external energy input is still required. Advances in quantum chemistry simulations can perform extremely large numbers of calculations per second, reducing reaction modeling time from weeks to minutes and improving the study of collision dynamics and reaction pathways.

Lasers are used as a source of energy for many kinds of reactions. In an industrial reaction, for example, laser light of the right wavelength can cause a reaction to produce more of the desired product and less of the unwanted side products. If light of sufficient energy is absorbed, it can ionize the molecule, which means it loses an electron and becomes electrically charged. This ionization is more than enough energy to activate a molecule, so it may react on the next collision. These reactions between gaseous ions and neutral molecules take place in the upper atmosphere, where molecules receive damaging radiation from the sun, and in lightning discharges. They are also studied extensively in the laboratory. Because the loss of an electron deposits much energy in a molecule, reactions between gaseous ions and molecules frequently happen at rates almost independent of temperature. Because there is no activation energy, and because an electrically charged particle can be attracted to a neutral molecule, the potential energy surface moves down when the molecule and ion first come into contact. In 2025, scientists directly observed the motion of a single electron during a chemical reaction for the first time, providing experimental confirmation of theories about reaction mechanisms and collision dynamics.

If a situation exists in which a substantial fraction of the atoms or molecules are ionized, matter under these conditions is called a plasma. While plasmas are rare on Earth, they are probably the most common form of matter in the universe, because of the great amount of energy available in stars.

Principal terms

ACTIVATION ENERGY: the energy, in excess of the average energy of reactant molecules, that is required for molecules to react

ATOM: the smallest and simplest unit of an element that can exist

COVALENT BOND: the sharing of electrons by two atoms, with the result that the atoms are joined together in a molecule

ELECTRON: a fundamental particle with a negative electrical charge

ELEMENTARY REACTION: a reaction that leads to products as a result of a single reactive collision between reactants

ENERGY: the capacity to do work; here, the important kinds of energy are collisional energy and the capacity to stretch and break covalent bonds

KINETIC ENERGY: the energy that an object has when it is in motion; it is always positive, and the kinetic energy that goes into collisions is central to the arguments that follow

POTENTIAL ENERGY: the energy that an object has because it is in a certain position; an object at the top of a hill has potential energy, which is shown by the increase in speed as it moves downhill

REACTION: the process in which one or more molecules, called reactants, gain or lose atoms to become reaction products

SOLVENT: a liquid in which substances are dissolved in a solution


Bibliography

Atkins, Peter W. General Chemistry. W. H. Freeman, 1989.

Bailar, John C., Jr., et al. Chemistry. Harcourt Brace Jovanovich, 1989.

Capella, Larissa G. “Scientists Watch a Single Electron Move during a Chemical Reaction for First Time Ever.” Live Science, 29 Aug. 2025, www.livescience.com/physics-mathematics/particle-physics/scientists-watch-a-single-electron-move-during-a-chemical-reaction-for-first-time-ever. Accessed 14 Apr. 2026.

Ebbing, Darrell D., and Steven D. Gammon. General Chemistry. 11th ed., Houghton Mifflin, 2017.

Gillespie, Ronald, et al. Chemistry. 2nd ed., Allyn & Bacon, 1989.

Kotz, John C., and Keith F. Purcell. Chemistry and Chemical Reactivity. Saunders College Publishing, 1987.

Lauth, Jakob. Physical Chemistry in a Nutshell: Basics for Engineers and Scientists. Springer, 2023.

McQuarrie, Donald A., and Peter A. Rock. General Chemistry. 3rd ed., W. H. Freeman, 1991.

“Physicists Steer Chemical Reactions by Magnetic Fields and Quantum Interference.” MIT News, 8 Mar. 2022, news.mit.edu/2022/physicists-steer-chemical-reactions-magnetic-fields-quantum-interference-0308. Accessed 14 Apr. 2026.

Swayne, Matt. “Scientists Explore Using Quantum Dots to Control Photochemical Reactions.” The Quantum Insider, 17 Jan. 2025, thequantuminsider.com/2025/01/17/scientists-explore-using-quantum-dots-to-control-photochemical-reactions/. Accessed 14 Apr. 2026.

Ware, Skyler. “Ultrafast Quantum Chemistry Engine Could Speed Up the Development of New Medicines and Materials.” Live Science, 22 Feb. 2026, www.livescience.com/technology/computing/ultrafast-quantum-chemistry-engine-could-speed-up-the-development-of-new-medicines-and-materials. Accessed 14 Apr. 2026.

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