Helium, Alpha Particle, Radioactivity
In class we are studying nuclear radiation. When
discussing alpha particles being composed of 2 protons and 2 neutrons
(actually the nucleus of a helium atom), a student asked me "if an alpha
particle is truly the nucleus of a helium atom, why isn't helium
considered a radioactive element or substance?"
A radioactive element sends energy away, carried in the motion of a
radioactive particle or ray. A Helium atom does not send its nucleus away
Math, Science, Engineering
Illinois Central College
A radioactive substance is one that EMITS some sort of "elementary"
particle (electron, proton, neutron, or an atomic nuclear fragment such as
an alpha particle), or some high energy electromagnetic radiation (X-rays,
gamma rays, etc.). With rare exception (electron capture) the particle or
radiation emanates from the atomic nucleus. This distinguishes
radioactivity from atomic processes such as absorption, emission,
fluorescence, or ionization where the radiation/electron originates from
the surrounding electrons rather than from the atomic nucleus itself.
Since elemental helium does not emit any particles or radiation, it is
not classified as "radioactive".
One additional comment: The student's question is a good one. It shows
perception about the topic and is by no measure a trivial question.
"Radiation" and "Radioactive" basically refer to what things do, not what
they "are made of".
"Radiation" in this context is a particle with high enough energy (speed)
to do some damage to molecules it hits.
It can be made of a photon, electron, proton, helium nucleus, or any other
nucleus or particle.
It is often called "ionizing radiation" to specify that it at least has
enough energy to bump some electrons out of place,
and in living tissue some chemical reactions may follow which are
We should specify a word for a higher degree of damage, such as
which refers to things which can bump whole atoms out of their molecules
That is obviously damaging.
A helium atom is usually just sitting there, with no net energy, no speed,
no damaging abilities.
If you take one and accelerate it to 10 MeV or more, it would definitely
be called radiation.
Conversely, after an alpha particle has spent all its energy plowing
through air or any other medium,
it slows to a stop and picks up a couple of electrons from its new
and then we call it a helium atom.
A "Radioactive substance" is a substance which emits ionizing radiation
from its nuclei, which "makes radiation".
Normal helium atoms emit nothing, and make no radiation.
Radioactive radium metal, on the other hand, emits these helium nuclei
with high energy;
it makes radiation, so it is called radioactive.
Radiation is kind of like light, fleeting.
Radioactive refers a substance with stored energy, like a battery that
can power the light bulb making light.
Radioactive atoms are tiny bombs.
Radiation is their explosion, the fragments which fly out.
I hope that clarifies it for your students-
Radioactive means unstable -- eventually to decay and emit radiation or
particles. Alpha particles are one possible product of radioactive decay,
and they might be emitted in an excited state, from which they will very
quickly decay by emitting gamma radiation (extremely high-frequency light --
higher than x rays). But once they have reached their lowest energy state,
they are very stable.
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Update: June 2012