Answer to Question #2549 Submitted to "Ask the Experts"

Q

In the decay of radium to radon we lose an alpha particle, He ++. The radium atom has 88 protons, 88 electrons, and 138 neutrons. When it loses an alpha particle, radon has 86 protons, 136 neutrons, and 88 electrons. Overall the atom is negatively charged. Why is this not shown in the equation? I found this very confusing and difficult to explain to a chemistry class. Can you help please? The same applies to beta particles with ¹⁴C going to ¹⁴N—does the N atom carry an overall positive charge?

A

The ejection of the alpha particle from the ²²⁶Ra nucleus immediately creates unstable conditions at the atomic level, which are followed by rapid adjustments. The ejected alpha particle can collide with electrons as it exits the atom, and it produces large numbers of ionizations as it slows down in neighboring material. The residual ²²²Rn nucleus also recoils from the emission of the alpha particle, leading to further disruptions of atomic electrons. An abundance of electrons are thus set free and positive ions are created by the event of alpha-particle emission. The free electrons are quickly captured by the positive ions, bringing things back to an altered, but again electrically neutral, state. (These processes occur in about 10^-12 s in local regions of a track in water.) The ²²²Rn nucleus ends up with its compliment of 86 electrons as a neutral atom, and the alpha particle will ultimately capture two electrons to form a neutral helium atom. Neglecting all the transient conditions in between, one can describe the alpha-particle decay by symbolizing the initial and final atomic species involved:

²²⁶Ra ----> ²²²Rn + ⁴He.

This represents the net result of what occurs: "before" and "after."

The same kinds of events happen with the beta decay of ¹⁴C, which you mention. In this case, however, an electron (the beta particle) and an anti-neutrino are created in the nuclear disintegration, resulting in an increase of the nuclear charge by one unit. Total charge is always conserved.

James E. Turner, CHP, PhD