## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

KS4 Atomic Structure:
At the turn from the 19th into the 20th century scientists such as Marie Curie began discovering and investigating strange invisible "radiations" that came from a range of elements and their isotopes.
In this second section we learn that not all atoms are stable and that some are quite dangerously unstable and "radioactive".

The nuclei of some elements are naturally unstable!

Unstable nuclei - what does this mean?

It means that without any prompting from outside they will disintegrate (sometimes quickly, sometimes slowly) by giving out one or more types of radioactivity (more on these later).
As they do this, they become more and more stable.
They stop this process when they become fully stable.

This is a random process (meaning we can't predict when the disintegration of individual atoms within an unstable material will take place) called radioactive decay and the elements that do this are said to be radioactive.

You might like to think of it like this:

An unstable / radioactive nucleus is like a mound of sand that is piled up in a tall tower shape; it won't stay in that shape will it? It will gradually collapse as bits of it fall off until it reaches a flatter, more stable state.
A stable / non-radioactive nucleus is, on the other hand, already like the stable pile of sand; it does not need to release any grains of sand in order for it to become any more stable. Before we move on to discuss the 3 types of radioactivity given out by unstable nuclei it is very important to notice that all of this is about the NUCLEUS; it is the nucleus that might be stable or unstable and it is the nucleus that gives out one or more of these 3 types of radioactivity.

The three main types of radioactivity

When these 3 things were discovered at the turn of the 19th into the 20th century they were simply named after the first 3 letters of the Greek alphabet, making them very easy to remember.
So, they are Alpha, Beta and Gamma (α, β, γ).

Alpha, α
When an unstable nucleus gives out Alpha it gives out one particle consisting of 2 protons and 2 neutrons, making it quite a large, heavy particle (compared to other particles, that is). It is known as an Alpha Particle.
Quick Question: what other atom has a nucleus with 2 protons and 2 neutrons (or an atomic number of 2 and a mass number of 4) ? Look at the tables above. Its Helium! So, an Alpha particle is identical to a Helium nucleus. Interesting! • Alpha particles have a very short range in air (only about 5 cm) and
• are easily stopped by paper.
• Both of the above are a consequence of the very strong ionising power of alpha particles causing them to run out of energy very quickly as they ionise all the surrounding particles that they encounter.

NB. Since the nucleus loses protons when it emits an Alpha Particle, its atomic number changes, so it becomes a nucleus of a different element- amazing!

Beta, β
When an unstable nucleus gives out Beta it gives out an electron ! (you might want to read this again.)
This is strange because electrons are not normally found inside the nucleus, are they?
What happens is - a neutron in the nucleus changes into both a proton and an electron (thus preserving the overall charge) and the electron is ejected at high speed from the nucleus as a Beta Particle. • Beta particles have a range of up to a metre in air and
• are stopped by a few mm (about 5 mm)of Aluminium .
• Both of the above are a consequence of the weaker ionising power of beta particles enabling them to travel further before losing their energy.

NB. Since the nucleus gains a proton when it emits a Beta Particle, its atomic number changes, so it becomes a nucleus of a different element- again, amazing!

Gamma, γ
The third of our three main types of radioactivity is unlike the first 2. When it was discovered it was thought to be similar, so it was named accordingly, but eventually it was found to be quite different.
Whilst alpha and beta are "particles" of matter, gamma radioactivity is a burst of energy in the form of electromagnetic radiation (it's also known as gamma waves or gamma rays and is part of the electromagnetic spectrum).
Once again, like with alpha and beta, gamma is emitted from an unstable nucleus; it will emit a burst of gamma rays in order to release excess energy, making itself more stable. • Gamma rays have a range of at least a kilometre in air and
• are stopped by about 10 cm of Lead (lead is one of the most dense metals).
• Both of the above are a consequence of the very weak ionising power of gamma rays, enabling them to travel great distances and penetrate materials deeply before losing their energy.

NB. Since the nucleus neither loses or gains any protons when it emits a Gamma Ray, the atomic number does not change, so the nucleus does not become the nucleus of a different element. There is a fourth type of radioactivity (less common than the 3 main types).
Certain nuclei that are very rich in neutrons are, as a consequence, "too heavy" and therefore often unstable. These nuclei can become a bit more stable by emitting a neutron. We call this type of radioactivity Neutron Emission. Pretty obvious, don't you think?

NB. Just like in gamma radioactivity, since the nucleus neither loses or gains any protons when it undergoes Neutron Emission, the atomic number does not change, so the nucleus does not become the nucleus of a different element.

The Rate of Disintegration

The rate at which an unstable nucleus decays is known as the Activity of the nucleus.

Activity is measured in becquerel (Bq), named after the French Physicist Henri Becquerel, who together with his assistant, Marie Curie, carried out the first investigations into the nature of radioactivity in the last few years of the 19th century.

In practical situations radioactivity is usually detected using radioactivity counters connected to a Geiger-Muller tube. Such instruments are often said to measure the "count rate" of a radioactive source.
A count-rate of 1 count per second is the same as 1 Bq (1 disintegration per second).

Rather than repeat this material twice, we will cover this final part of 4.4.2.1 in section 4.4.3

Time to have a go at a few basic questions.

#### Nuclear Equations

If you look back to the previous section you will see that there is a comment after each description of Alpha, Beta and Gamma radioactivity about what happens to the emitting nucleus after it has sent out its alpha particle, beta particle or gamma ray.
This section is all about what happens to the emitting nucleus when radioactive decay occurs, and we will learn that we can represent the decay using some very neat and simple equations.

First of all a reminder; we said, in the previous section, that an Alpha particle is identical to a Helium nucleus, so it is convenient for us to use its symbol to represent the Alpha Particle, ie

4 2 He

The Alpha Particle is equivalent to a Helium Nucleus because both consist of 2 protons and 2 neutrons; so both have a Mass number of 4 and an Atomic number of 2.

OK, now we can discuss Alpha decay.
An example of a radioactive element that is known to emit Alpha particles is Radon-219.
Radon-219 has an Atomic number of 86 and a Mass number of 219. We represent it like this:

219 86 Rn

To show it emitting an Alpha particle, we use an arrow followed by the Alpha particle symbol:

219 86 Rn → 4 2 He + ?

So, the above equation shows Radon-219 emitting an Alpha particle and leaving behind.....?

For now, let's call this unknown element X, with a Mass number m and an Atomic number a.

219 86 Rn → 4 2 He + m a X

What is this unknown element X?
Well, to answer the question, all we have to do is find the value of its Atomic number, a, because its the Atomic number (the number of protons) that defines the element. However, whilst we are finding a, we may as well find the Mass number m.

To find either a or m, all we do is "balance" the numbers on the left and right of the arrow.
For a: The two numbers on the right have to add up to equal the number on the left, 86.
So, a must be 86 - 2 = 84.

For m: The two numbers on the right have to add up to equal the number on the left, 219.
So, m must be 219 - 4 = 215.

Rewriting the complete Alpha decay equation:

219 86 Rn → 4 2 He + 215 84 X

So, the final question is, what element has an Atomic number of 84?
And the answer is Polonium, Po.

219 86 Rn → 4 2 He + 215 84 Po

Now we know that an atom of Radon-219 decays by emitting an Alpha particle and in the process causes the mass and atomic number (and hence the charge) of the nucleus to decrease by 4 and 2 respectively.
Furthermore, becuase of the change to the atomic number, the nucleus changes into the nucleus of a different element, Polonium-215.

Its worth reading the above sentence again, especially the words in bold.
Only via radioactive processes like Alpha decay can one element change into another! It is quite a remarkable think. Its a nuclear thing, not a chemical thing. No chemical reaction of any kind can cause this change.

Beta decay.
First of all a reminder; we said, in the previous section, that a Beta particle is identical to an electron (that has been emitted from the nucleus), so it is convenient for us to use the following symbol to represent the Beta Particle, ie

0 -1 e

Why is the Beta particle's Mass number zero?
Because, being an electron, there are zero protons and zero neutrons!
Slightly harder: Why is the Beta particle's Atomic number -1?
Because an electron is equivalent to a proton charge of -1.

OK, now we can discuss Betaa decay.
An example of a radioactive element that is known to emit Beta particles is Carbon-14.
Carbon-14 has an Atomic number of 6 and a Mass number of 14. We represent it like this:

14 6 C

To show it emitting an Beta particle, we use an arrow followed by the Beta particle symbol:

14 6 C → 0 -1 e + m a X

As we did with the Alpha decay example, to find m and a we simply balance the mass and atomic numbers on either side of the arrow.
So, m = 14 - 0 = 14.
and, a = 6 - -1 = 7

Rewriting the complete Beta decay equation:

14 6 C → 0 -1 e + 14 7 X

So, which element has an Atomic number of 7?

Our final decay equation is:

14 6 C → 0 -1 e + 14 7 N

Like Alpha decay, when Beta decay occurs, an atom of a radioactive element changes into an atom of a different element.
Unlike Alpha decay, however, there is no change to the mass number, but the atomic number (and hence the charge) increases by 1.

Its also worth noting that the atom it changes into might still be radioactive (for example Polonium is radioactive), so it will continue to decay and change into another element. The process will continue until it arrives at a stable, none radioactive element.

Gamma decay.
Since Gamma radioactivity is the emission of energy in the form of gamma electromagnetic radiation from an unstable nucleus and is not the emission of a particle of matter (as in Alpha or Beta decay) there are no nuclear changes associated with Gamma decay.

When Gamma decay occurs, a nucleus simply releases energy and by doing so it becomes more stable (a bit like letting air out from an overblown balloon). So, Gamma decay causes no change to the mass or atomic number of the nucleus.

Time for a few more questions

#### Decay is Random but there is a Pattern to it!

We stated at the start of this page that "radioactive decay is a random process", meaning that nobody can predict when a particular radioactive nucleus will emit one or more of the various types of decay that we know about.

However, when we observe any radioactive material (meaning- many atoms) over a reasonable time scale, we notice that there is a pattern to the decay.

We find that the number of the radioactive nuclei keeps halving after the same amount of time goes by.
eg. We might have a radioactive material which starts off with 100 radioactive nuclei and we find that after say 10 minutes, there are only 50 of the original radioactive nuclei left (because 50 have decayed). Then after another 10 minutes we find that there are only 25 left. After another 10 minutes there will be only 12 or 13 left. And so on.

Time
(mins)
Number of Nuclei
Remaining
0 100
10 50
20 25
30 12.5
40 6.25
50 3.13

For our example material, we say that it has a Half Life of 10 minutes because it quite litterally loses half of itself every 10 minutes, doesn't it?

If we plotted a graph of the number of radioactive nuclei against time, it would look like this: The Half Life is a time period and you should be able to see that after each Half Life the number of nuclei present in the material falls to one half of what was present.
Just in case you can't see this on the above graph, have a look at the following graph: The graph shows that after every Half Life, T1/2, the number of nuclei falls to half of its current amount.

Since the Activity of a sample will be proportional to the size of the sample or to the number of nuclei present, we have another way of defining half-life and of drawing half-life graphs.

A graph of activity (in counts per second or in Bq) against time would look like this for a sample that had a half life of 15 seconds, starting at an activity of 1000 counts per sec. You should be able to see that the activity falls from 1000 to 500 in 15 secs (the half-life), then it falls from 500 to 250 in another 15 secs and so on.

So we have an alternative definition for half-life; both are equally correct and you can use either of them:

The best thing about "half-life" is that the graph ALWAYS has exactly the same shape no matter how you define it or what sample is used; if you look at the 3 graphs drawn so far, the shape of the curve is always the same; a smooth "downhill" slope.

Also, questions involving half-life are never difficult so long as you understand what we have said so far, so make sure you do; then have a look at the following examples.

#### The range of half-lives

You should notice from the above examples that the half-life values of radioactive materials can vary widely.
We have mentioned materials with a half-life of a few seconds and we have mentioned others with a half-life of a number of days.
In fact half-lives can range from tiny fractions of a microsecond to billions of years, eg. Uranium-238 has a half-life of 4.5 billion years, which is about the age of the Earth!

Have a go at the following questions.

An extremely common misconception (or wrong idea) about radioactivity is that if you are exposed to (eg put near, but not touching) a radioactive material, you become contaminated, such that even when you move away or leave the vicinity of the material, you remain contaminated.

This is a misconception, a wrong idea.

You do, however, become irradiated, meaning that you are exposed to the alpha, beta or gamma radioactivity that might come from the radioactive source. And depending on the dose (strength of the source and the time of exposure) of the irradiation you might be adversely affected. But you are not contaminated and nothing needs to be cleaned from you or your clothes.

So you can't be contaminated if you don't come into contact with a radioactive source, but you can be irradiated.

To be contaminated you have to come into direct contact with the radioactive material but this could be with the tiniest particles of the material, eg it is possible to breath in radioactive particles or to ingest them with food.
Contamination with material or particles of material outside the body is dangerous because the contaminating products will produce harmful radioactivity, but it can usually be safely cleaned off, but contamination through inhalation or ingesting is much more serious since it can't be easily "cleaned out" (eg. Russian agents killed a journalist in London a number of years ago; they did it by slipping a tiny amount of radioactive polonium into his food; once inside his body the journalist died within a week or so due to damage caused by radioactivity inside his body!)