Atoms of the same element can have different
numbers of neutrons; the different possible
versions of each element are called isotopes.
For example, the most common isotope of
hydrogen has no neutrons at all; there's also a
hydrogen isotope called deuterium, with one
neutron, and another, tritium, with two
If you want to refer to a certain
isotope, you write it like this: AXZ. Here
X is the chemical symbol for the
element, Z is the atomic number, and
A is the number of neutrons and
protons combined, called the mass
number. For instance, ordinary
hydrogen is written 1H1, deuterium
is2H1, and tritium is 3H1.
How many isotopes can one element
have? Can an atom have
just any number of neutrons?
No; there are "preferred" combinations of
neutrons and protons, at which the forces
holding nuclei together seem to balance
best. Light elements tend to have about as
many neutrons as protons; heavy elements
apparently need more neutrons than
protons in order to stick together. Atoms
with a few too many neutrons, or not quite
enough, can sometimes exist for a
while, but they're unstable.
I'm not sure what you mean by
"unstable." Do atoms just fall apart if
they don't have the right number of
Well, yes, in a way. Unstable atoms
areradioactive: their nuclei change
or decay by spitting out radiation, in
the form of particles or
I'm going to illustrate how radioactive
decay works with the help of an
isotope table applet, which should
now be open in a separate window. If
There are several ways in which
radioactive atoms can decay. Here's
one example: suppose an atom has
too many neutrons to be stable.That's
the case with tritium, 3H1.
Does it just kick out one of the
No, it can't do that; the neutrons are
stuck too firmly where they are. What
it can do...well, I'll let you see for
yourself. In the applet, click on the
button labeled H3 (for hydrogen 3, or
The neutron turns into a
proton! 3H1 becomes 3He2
Right. An unstable isotope of hydrogen
has converted itself into a stable
isotope of helium. You'll notice
that 3H1 and 3He2 have the same mass
number, which is good, because mass
has to be conserved.
There is a problem, though. Electric
charge also has to be conserved.
Hydrogen has only one proton, and
helium has two, so you'd end up with
twice as much positive charge as you
started with. How do you get around
When 3H metamorphoses into helium
3, it also gives off an electron--which
has hardly any mass, and is endowed
with a negative charge that exactly
cancels one proton. This process is
known as beta decay, and the electron
is called a beta particle in this context.
You can write out the nuclear reaction
involved in the beta decay of tritium
by giving the electron a "mass
number" of 0 and an "atomic number"
3H => 3He + 0e Notice that the mass
numbers on each side add up to the
same total (3 = 3 + 0), and so do the
charges (1 = 2 + -1). This must always
be true in any nuclear reaction.
What happens when an atom
doesn't have enoughneutrons to
That's the case with beryllium
7Be . Click on it in the applet and
see what happens.
It decays to lithium 7--so a proton
turns into a neutron. That makes
sense...but how do you deal with
the electric charge problem now?
Going from Be to
Li, you lose charge; emitting an
electron would just make things
Right...so instead you emit
a positron--a particle that's just
like an electron except that it has
opposite electric charge. In nuclear
reactions, positrons are written
this way: 0e1.
So the reaction looks like this:
7Be => 7Li + 0e
Good. The applet will show you
many other decays that produce
either electrons or positrons; it's
easy to tell which, by the
"direction" in which the decay
moves. Sometimes it even takes
more than one decay to arrive at a
stable isotope; try 18Ne or 21O, for
So all radioactive isotopes decay by
giving off either electrons or
No, there are other possibilities.
Some heavy isotopes decay by
spitting out alpha particles. These
are actually helium 4 nuclei-clumps of two neutrons and two
protons each. A typical alpha decay
looks like this:
238U => 234Th + 4He
There's also a third type of radioactive
emission. After alpha or beta decay, a
nucleus is often left in an excited
state--that is, with some extra energy.
It then "calms down" by releasing this
energy in the form of a very highfrequency photon, or electromagnetic
wave, known as a gamma ray.
Click on the advanced button for more
information about why this happens.
The applet lists a "halflife" for each
radioactive isotope. What does
The halflife is the amount of time it
takes for half of the atoms in a
sample to decay. The halflife for a
given isotope is always the same ;
it doesn't depend on how many
atoms you have or on how long
they've been sitting around.
For example, the applet will tell you
that the halflife of beryllium 11 is
13.81 seconds. Let's say you start
with, oh, 16 grams of 11Be. Wait 13.81
seconds, and you'll have 8 grams left;
the rest will have decayed to boron 11.
Another 13.81 seconds go by, and
you're left with 4 grams of 11Be; 13.81
seconds more, and you have 2
grams...you get the idea.
Hmmm...so a lot of decays happen
really fast when there are lots of
atoms, and then things slow down
when there aren't so many. The
halflife is always the same, but
the half gets smaller and smaller.
That's exactly right
Notice how the decays are fast and
furious at the beginning and slow
down over time; you can see this
both from the color changes in the
top window and from the graph.
You'll also notice that the pattern
of atoms in the top picture is
random-looking, and different each
time you run the applet, but the
graph below always has the same
shape. It's impossible to predict
when a specific atom is going to
decay, but you can predict
the number of atoms that will
decay in a certain time period
That’s the report for Group 8
Thanks for Watching
Leader:Magtabog Noel R.
Razel Ann Capulong
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