Video Transcript
In this video, we will learn how to
describe the radiation produced by decaying atomic nuclei. Let’s start by looking at a single
atom. Atoms are made of three kinds of
particles: positively charged protons, negatively charged electrons, and neutrons
which have no charge. In an atom, protons and neutrons
all bunch together in the core, called the nucleus. As we’ll see, it’s the nucleus of
an atom that causes it to be radioactive or not. Particles in the nucleus are held
together by forces. There are attractive forces between
the particles and also forces that push them apart. Both of these forces are needed for
the nucleus to be balanced and stable. Let’s look now at a quick
example.
Which of the following statements
correctly describes the forces that act on the particles in the nucleus of an
atom? (A) There are no forces on the
particles. (B) There are only attractive
forces on the particles. (C) There are only repulsive forces
on the particles. (D) There are attractive and
repulsive forces on the particles.
The nucleus of an atom is made up
of protons and neutrons. These particles are bunched tightly
together by forces. If there were new forces acting on
these particles, over time, they would drift apart, moving away from one
another. This is not what happens
though. The nucleus does hold together. Considering our next answer option,
if there were only attractive forces on the nucleus, the particles in it would be
squished together. What’s more, they wouldn’t push
back. Over time, the whole nucleus would
collapse to a single point. We know, though, that this isn’t
what happens.
A nucleus is made up of distinct
protons and neutrons. They resist being compressed
altogether. That means there must be some
repulsive force on the particles. But what if there were only
repulsive forces? Then there would be nothing to keep
the nucleus together. The particles in it would fly off
in all directions. Again, this isn’t what happens. So we know option (C) can’t be our
answer. The last choice says there are
attractive and repulsive forces acting on the particles with forces that both push
and pull on the particles. The overall forces on the nucleus
balance out. There are both attractive and
repulsive forces on the particles of a nucleus.
Whenever we have more than one
nucleus, we call them nuclei. Here, we have two nuclei. And notice that they’re
different. The first nucleus has one, two,
three protons and the same number of neutrons. The second nucleus has one, two,
three, four protons as well as four neutrons. The number of protons in a nucleus
gives an atom its identity. Different types of atoms are called
elements. Since this nucleus has a different
number of protons than this nucleus, we can say these nuclei belong to atoms of
different elements.
Now, let’s consider a third
nucleus. Counting the protons, we count one,
two, three, four. This nucleus on the right must be
part of an atom of the same element as the nucleus in the center. But then, look at this. If we count the number of neutrons
in the nucleus on the right, we count one, two, three, four, five. This nucleus contains a different
number of neutrons than this one. We’ve said that the number of
protons in a nucleus determines what type of element the atom is. The number of neutrons tells us
what is called the isotope of that element. Nuclei of atoms of the same element
that have different numbers of neutrons are different isotopes of one another. Knowing this, let’s go back to
thinking about just one atomic nucleus.
We’ve seen that the number of
protons and neutrons in the nucleus tells us which element that atom is as well as
the isotope of that element. When a nucleus is stable, when all
the forces acting on it balance out, that means the number of protons and neutrons
in the nucleus does not change. Examples of elements that are very
stable include tin, iron, and oxygen. If the forces on the particles in a
nucleus do not balance out, however, that nucleus is unstable. An unstable nucleus is one where
the number of protons and neutrons can change. Unstable elements include uranium
and plutonium. An unstable nucleus may at some
point break apart. When this happens, the nucleus can
release what is called radiation.
The process by which a nucleus
gives off radiation is called radioactive decay. Any element that undergoes
radioactive decay is called radioactive. Imagine we have an unstable
nucleus. Through radioactive decay, the
nucleus could release a particle. Here’s something interesting,
though. There is no way to predict exactly
when this will happen. Even if we have an unstable nucleus
and we suspect that at some point it will decay, it’s impossible to predict when
this decay will take place. Radioactive decay is what is called
a spontaneous process. We don’t know when it will happen,
but we do know that it does happen. Let’s look now at a couple of
examples.
Which of the following does an
atomic nucleus become if it loses only neutrons when it undergoes radioactive
decay? (A) A different element from the
one it was before the decay. (B) A different isotope of the
element it was before the decay.
Here, we’re thinking about a
nucleus that undergoes radioactive decay and loses only neutrons in the process. Atomic nuclei are made up of two
types of particles, protons and neutrons. The number of protons in a nucleus
tells us what element that nucleus is a part of. The number of neutrons tells us the
isotope of the element that the nucleus is a part of. Therefore, radioactive decay where
only neutrons are lost does not change the element that that nucleus is a part
of. What it does change is the isotope
of the element that that nucleus was a part of before the decay. We choose answer option (B).
Let’s look at another example.
Which of the following statements
about the prediction of radioactive decay of unstable atomic nucleus is correct? (A) The time at which an unstable
atomic nucleus will decay can be predicted. (B) The time at which an unstable
atomic nucleus will decay cannot be predicted.
We see that the only difference
between these answers is whether the timing of nuclear decay can or cannot be
predicted. When the nucleus of an atom goes
through radioactive decay, it emits or gives off particles or energy. This process is described as
spontaneous. This is because we can’t say
exactly when it will happen. In general, radioactive decay is a
spontaneous process. The time at which an unstable
nucleus decays can’t be predicted.
Let’s now look at some details of
what takes place when a nucleus decays. When a nucleus is unstable, it may
experience radioactive decay. When this happens, the nucleus
gives off radiation. One type of radiation is called
alpha radiation. This symbol here is the Greek
letter 𝛼. A second type is called beta
radiation. Here is the Greek letter 𝛽. Finally, there’s a third sort of
radiation, gamma radiation. Alpha and beta radiation, both
consist of particles. Gamma radiation is different. It consists of electromagnetic
waves. Let’s look now at one last
example.
Which of the following statements
correctly describes the possible compositions of the radiation emitted by unstable
atomic nuclei decaying? (A) The radiation emitted by
unstable atomic nuclei decaying includes particles and electromagnetic waves. (B) The radiation emitted by
unstable atomic nuclei decaying consists only of particles. (C) The radiation emitted by
unstable atomic nuclei decaying consists only of electromagnetic waves.
We’re thinking here of radiation
emitted by decaying nuclei. Let’s consider the three different
types of radiation a decaying nucleus can give off. First, there is what is called
alpha radiation. Alpha radiation consists of a
particle. Then, there’s beta radiation. Beta radiation is also a particle
given off by a decaying nucleus. Then, there’s gamma radiation. Unlike alpha and beta radiation,
gamma radiation consists of electromagnetic waves. Considering all three types of
radiation, we see they include particles as well as electromagnetic waves. We choose answer option (A), the
radiation emitted by unstable atomic nuclei decaying includes particles and
electromagnetic waves.
We may wonder just what is it that
makes a given nucleus stable or unstable? It comes down to the number of neutrons in
the nucleus compared to the number of protons. For smaller nuclei, when the number
of protons and neutrons is the same, the nucleus tends to be stable rather than
unstable. However, the more the number of
neutrons exceeds the number of protons in the nucleus of an isotope, the more likely
it is that all nuclei of that isotope are unstable, likely to decay.
Let’s finish this lesson by
reviewing a few key points. In this video, we learn that atomic
nuclei are made of protons and neutrons held together by a force. The number of protons tells what
element a nucleus is a part of. The number of neutrons tells the
isotope of that element. Nuclei can be stable or
unstable. Unstable nuclei break down
spontaneously through a process called radioactive decay. Radioactive decay causes the
emission of radiation. There are three types of radiation:
alpha, beta, and gamma radiation. Finally, a nucleus is stable or
unstable based on the number of protons it has compared to the number of neutrons it
has. In general, the more the number of
neutrons exceeds the number of protons, the more likely it is that that isotope is
unstable. This is a summary of
radioactivity.
