Video Transcript
The following nuclear equation
shows gamma radiation by radon. What is the name of element X? What element symbol should replace
X?
All right, taking a look at this
nuclear equation, we see that we’re starting out with this element, which we’re told
is radon. The atomic number of radon is
86. It has 86 protons in its nuclear
core. And its mass number, the sum of the
number of protons and the number of neutrons in its nucleus, is 222. So then, we can refer to this
isotope of radon as radon-222. This radon nucleus experiences
nuclear decay, and it emits a gamma ray. This is the Greek symbol for that
letter 𝛾 representing this radiation. Once the gamma ray is emitted from
the radon nucleus, there’s this leftover element, element X.
Starting off, we want to solve for
the name of this element. And then, we want to know what
element symbol should replace X. These two questions are closely
connected, and there are a couple of different ways to answer them. One way is to consider that what’s
being emitted here is gamma radiation. And we can recall that a gamma ray,
a photon, a packet of electromagnetic energy, has no mass to it. And it also has no electric
charge. This means that when we consider
gamma radiation as a part of an overall nuclear equation, whatever is left over
after the gamma ray is emitted — in our case, it’s this element X — will have the
same mass and the same electric charge as what emitted the gamma ray in the first
place. In other words, this element here
and this element here are the same.
Now, there’s a second way to see
this. And that is by looking at the
atomic number and the mass number of this unknown element. When we compare these values to the
equivalent values on the left-hand side of our nuclear equation, we see that they’re
the same. Both our radon nucleus and our
nucleus of element X have an atomic number of 86 and a mass number of 222. This is a second way of seeing that
these two elements must be the same. And therefore, the name of element
X is the name of this element. Element X is radon. The next question is related. It says, what element symbol should
replace X? Well, if X is radon and the
element’s symbol for radon is capital Rn, then that tells us that that same symbol
should go in place of capital X.
