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As an example of this in the field of chemistry, the combination of 22.99 grams of [[Sodium]] with 35.45 grams of [[Chlorine]] (those quantities are one [[mole]] each, or [[Avogadro's number]] of atoms) yields 58.44 grams of Sodium Chloride.

As an example of this in the field of chemistry, the combination of 22.99 grams of [[Sodium]] with 35.45 grams of [[Chlorine]] (those quantities are one [[mole]] each, or [[Avogadro's number]] of atoms) yields 58.44 grams of Sodium Chloride.

Over time, the understanding of this principle has developed.  But in the early twentieth century it became clear that radioactivity seemed to violate both the Law of Mass Conservation and the classic "Law of Conservation of Elements".  It was found that small changes in mass occur when elements undergo radioactive disintegration.  The effect was only observed for nuclear transformations.  For a while this led to the belief that the law does not apply to nuclear reactions, and holds only for other phenomena.  This may still be taught in some elementary-schools.

Over time, the understanding of this principle has developed.  But in the early twentieth century it became clear that radioactivity seemed to violate both the Law of Mass Conservation and the classic "Law of Conservation of Elements".  It was found that small changes in mass occur when elements undergo radioactive disintegration.  The effect was only observed for nuclear transformations.  For a while this led to the belief that the law does not apply to nuclear reactions, and holds only for other phenomena.  This false notion may still be taught to some elementary-school children.

That notion is not true.  It would imply that nuclear phenomena involve a different sort of fundamental physics (laws of motion, etc.) from other phenomena.  The fundamental laws of physics are the same for all phenomena.

That notion is not true.  It would imply that nuclear phenomena involve a different sort of fundamental physics (laws of motion, etc.) from other phenomena.  But that isn't so—the fundamental laws of physics are the same for all phenomena.

The explanation relates to [[potential energy]].  Recall that, from the Law of Energy Conservation, energy is conserved.  Kinetic energy (the energy of motion that you can "see") may seem to disappear, but it is really only turning into potential energy.  That "hidden" potential energy can be turned back into kinetic energy.  The sum of potential energy and kinetic energy is constant.

The explanation relates to [[potential energy]].  Recall that, from the Law of Energy Conservation, energy is conserved.  Kinetic energy (the energy of motion that you can "see") may seem to disappear, but it is really only turning into potential energy.  That "hidden" potential energy can be turned back into kinetic energy.  The sum of potential energy and kinetic energy is constant.

But when analyzing nuclear reactions, the potential-energy-equivalent mass becomes (barely) measurable, and, when methods of measuring atomic weights became sufficiently accurate in the 1910s and 1920s, it was noticed.

But when analyzing nuclear reactions, the potential-energy-equivalent mass becomes (barely) measurable, and, when methods of measuring atomic weights became sufficiently accurate in the 1910s and 1920s, it was noticed.

For example, a Radium atom has a mass (atomic weight) of 226.0254 amu (atomic mass units).  It decays by [[alpha decay]] into a Radon atom (222.0176 amu) and a Helium atom (alpha particle, 4.0026 amu.)  The discrepancy is .0052 amu.  This is the potential-energy-equivalent mass of the Uranium atom.  That is, .0052 amu of the Uranium atom's mass was this potential-energy-equivalent mass.  It disappeared when the potential energy was converted into kinetic energy.  The various conversion factors among amu, kilograms, Joules, and electron volts, along with the conversion factor of 1.11<math>\times</math>10^-17 given above, yields a potential energy loss of 4.864 MeV.  The observed alpha particle kinetic energy is 4.784 MeV, and the recoil energy of the Radon atom is about 0.08 Mev, for a total kinetic energy of 4.864 MeV.

For example, a Radium atom has a mass (atomic weight) of 226.0254098 amu (atomic mass units).  It decays by [[alpha decay]] into a Radon atom (222.0175777 amu) and a Helium atom (alpha particle, 4.0026033 amu.)  The discrepancy is .0052288 amu.  This is the potential-energy-equivalent mass of the Radium atom.  That is, .0052288 amu of the Radium atom's mass was this potential-energy-equivalent mass.  It disappeared when the potential energy was converted into kinetic energy.  The various conversion factors among amu, kilograms, Joules, and electron volts, along with the conversion factor of 1.11<math>\times</math>10^-17 given above, yields a potential energy loss of 4.870596 MeV.  This got converted into the kinetic energy of both the alpha particle and the recoiling Radon atom.  Using the laws of conservation of energy and of momentum, one can calculate the ratio of alpha energy to recoil energy; it is the same as the ratio of Radon atom mass to Helium atom mass&mdash;55.5 to 1. Hence the recoil energy of the Radon atom is expected to be about 0.0868 Mev, and the expected alpha particle kinetic energy is 4.784 MeV.  This is the observed alpha particle energy.

The Law of Mass Conservation is believed by some to be in conflict with religious sensibilities, apparently relating to the initial creation of the universe, or its ultimate fate.  While the exact circumstances and causes of the "big bang" are discussed by cosmologists and philosophers, the Law of Mass Conservation is accepted for everything in between.

The Law of Mass Conservation is believed by some to be in conflict with religious sensibilities, apparently relating to the initial creation of the universe, or its ultimate fate.  While the exact circumstances and causes of the "big bang" are discussed by cosmologists and philosophers, the Law of Mass Conservation is accepted for everything in between.