Isotopes

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 neutrons.

Hydrogen Deuterium Tritium

Question and Answer

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.

Question:How many isotopes can one element have? Can an atom have

just any number of neutrons?

Answer: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.

Question:I'm not sure what you mean by "unstable." Do atoms just fall

apart if they don't have the right number of neutrons?

Answer:Well, yes, in a way. Unstable atoms areradioactive: their nuclei change or decay by

spitting out radiation, in the form of particles or electromagnetic waves.

Beta DecayI'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 it isn't,

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.

Question:Does it just kick out one of the neutrons?

Answer: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 tritium).

Question:The neutron turns into a

proton! 3H1 becomes 3He2

Answer: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.

Question: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 that?

Answer: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" of -1:

3H1 => 3He2 + 0e-1Notice 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.

Positrons, Alpha Particles, and Gamma Rays

Question:What happens when an atom

doesn't have enoughneutrons to be stable?

Answer:That's the case with beryllium

7, 7Be4. Click on it in the applet and see what happens.

Question: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 worse.

Answer: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.

Question:So the reaction looks like this:

7Be4 => 7Li3 + 0e1

Answer: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

example.

Question:So all radioactive isotopes decay by

giving off either electrons or positrons?

Answer: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:238U92 => 234Th90 + 4He2

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 high-frequency photon, or electromagnetic

wave, known as a gamma ray.Click on the advanced button for more information about why this happens.

Halflife

Question:The applet lists a "halflife" for each

radioactive isotope. What does that mean?

Answer: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.

Question: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.

Answer: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 8Thanks for Watching

Leader:Magtabog Noel R.Member:

Mariafe TingsonRazel Ann Capulong

Ramel LumapayMelvin VerasMarco David