Changes

Neutrons, by themselves, decay.  This follows from the mass difference between the neutron and the resultant proton and electron.  But, when the entire nucleus is taken into account, it's often a different story.

Neutrons, by themselves, decay.  This follows from the mass difference between the neutron and the resultant proton and electron.  But, when the entire nucleus is taken into account, it's often a different story.

Consider the decay of Argon-39 to Potassium-39.  This is a standard beta-minus decay.  The Argon atom has 18 protons, 21 neutrons, and 18 electrons, and an atomic mass of 38.964313.  The Potassium atom has 19 protons, 20 neutrons, and 19 electrons, and an atomic mass of 38.963706487.  The beta-plus particle (electron) ejected by the nucleus joins the electron cloud, since the atomic number has gone up by one.  The mass decrease is .0006065 amu, so the decay is energetically possible, releasing .0006065 amu of equivalent energy.

Consider the decay of Argon-39 to Potassium-39.  This is a standard beta-minus decay.  This Argon atom has 18 protons and 21 neutrons, and an nuclear mass of 38.95443856 amu.  The Potassium atom has 19 protons and 20 neutrons, and a nuclear mass of 38.95328347 amu.  The beta-minus particle (electron) ejected by the nucleus weighs .00054858 amu.  So the total mass decrease is .000606513 amu, so the decay is energetically possible, releasing .000606513 amu of equivalent energy.

Now consider the hypothetical beta-plus decay of Argon-37 to Chlorine-37.  The Argon atom has 18 protons, 19 neutrons, and 18 electrons, and an atomic mass of 36.96677631.  The Chlorine atom has 17 protons, 20 neutrons, and 17 electrons, and an atomic mass of 36.9659025.  This would give an excess of .0008737 amu, but the decay released a positron.  Furthermore, an electron was released into the atom's electron cloud.  These two particles add up to .00109716 amu.  So there would be an overall mass ''increase'' of .00022343 amu.  This decay cannot happen.

Now consider the hypothetical beta-plus decay of Argon-37 to Chlorine-37.  This Argon atom has 18 protons and 19 neutrons, and a nuclear mass of 36.9569019 amu.  The Chlorine atom has 17 protons and 20 neutrons, and a nuclear mass of 36.9565767 amu.  The beta-plus particle (positron) ejected by the nucleus weighs .00054858 amu.  There would be an overall mass ''increase'' of .00022343 amu.  This decay cannot happen.

But suppose that, instead of releasing an electron and positron, the excess electron from the atom's electron cloud simply went down into the nucleus.  The nuclear transformation would be:

But suppose that, instead of releasing an electron and positron, the excess electron from the atom's electron cloud simply went down into the nucleus.  The nuclear transformation would be: