Atomic vs. Molecular Reactions

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Difference between Atomic Reaction and Molecular Reaction

Atomic reactions and molecular reactions are similar in that they result in a perceivable change in the appearance and state of the matter in question. Both may also result in the release and/or transfer of energy, sometimes in dramatic ways.


Nuclear physics defines an atomic or nuclear reaction as a process during which nuclei or nuclear particles collide with each other. This collision results in products that are sometimes radically different from the particles of the initial matter. While atomic reactions can and sometimes do result from the colliding of three particles, this is far less likely to occur than atomic reactions involving two nuclei, given the reduced probability of three or more nuclei to collide with each other at the same time.

Some types of atomic reactions may be spontaneous, as in the case of radioactive decay, or they may result from initiation by a particle. This is what happens in most nuclear reactions. In some cases, the particles may collide and go on their separate ways without change. This process is not referred to as an atomic reaction, but an elastic collision.

Molecular reactions can occur with virtually any type or combination of substances, and they may occur with or without an external force. As with an atomic reaction, a molecular reaction may occur spontaneously, or it may be triggered by an external force.

In Action

Atomic reactions come in many different forms, with some being almost commonplace. Among the most familiar types of atomic reactions are fusion reactions, in which two light nuclei combine to form a heavier nucleus. This reaction often results in the discarding of additional particles such as protons or neutrons in order to conserve momentum. Fission reactions are also commonplace in science and industry, and they are characterized by a very heavy nucleus splitting into two or three pieces spontaneously, or due to absorption of additional light particles such as neutrons. Finally, spallation occurs when a nucleus comes in contact with a particle with sufficient energy and momentum to fragment it, or cause several smaller fragments to detach.

Molecular reactions may occur from everyday household substances, such as commercial cleaning products. Many of these contain strong chemicals that may interact with other substances–not necessarily chemicals–in unpredictable and sometimes dangerous ways.


During an atomic reaction, kinetic energy is often applied in order for the reaction to take place. In most nuclear reactions, the total energy is conserved, with the "missing" rest mass therefore having to reappear as kinetic energy released as a result of the reaction. The source is most often the nuclear binding energy, and its amount can be determined by Einstein's mass-energy equivalence formula, the notorious E = mc².

Molecular reactions can occur with or without the presence of extraneous substances. That being said, they commonly occur in controlled laboratory settings.

Similarities and Differences

Atomic reaction

  • A process during which nuclei or nuclear particles collide with each other
  • Far less likely to occur with three or more nuclei

Molecular reaction

  • Can occur with virtually any type or combination of substances
  • May occur with or without an external force


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