Physics- Unit 1b

Atomic Structure

You need to know about the structure of an atom and the particles that make it up.

All atoms are made up of the same 3 basic particles:

• protons
• electrons
• neutrons.

The only difference between one atom and the next is the number of these particles in the atom. That is enough to make things as different as gold and oxygen.

Neutrons and protons are heavy in comparison to electrons. In fact, a neutron or a proton weighs about 2000 times as much as an electron!

The other thing to remember is that protons have a positive charge, electrons have a negative charge and neutrons have no charge at all.

How are these particles aranged?

An atom is not a solid thing. In fact, quite the opposite. Atoms are nearly completely empty.

Protons and neutrons are tightly clumped together in the middle, in the nucleus, while electrons spin around them.

To give you an idea of the proportions, imagine a full size football stadium. The nucleus would be equivalent to the size of an ant in the middle, with the electrons whizzing around the outskirts.

The central part of the atom is called the nucleus. That's where you find all the protons and neutrons.

As we said above, protons and neutrons are heavy compared to electrons, so you can see that all the mass is concentrated in the middle of the atom. Also, as all the protons are in the nucleus of the atom, the nucleus has a positive charge.

The electrons (negatively charged) orbit around the outside of the atom.

Scientists used to think that atoms were solid. They thought that they were a bit like plum puddings - a light sponge pudding with bits of plum mixed into it.

This was a good model because:

1. Atoms occupied the correct amount of space (it was the space filled by the sponge).
2. The mass of the atom was about right (because although the sponge filled the whole of the atom, it was very low density).
3. The charge on the positive sponge was balanced by the negative plums so that overall the atom was neutral.

Then a man called Ernest Rutherford performed a now famous experiment. He fired an alpha particle (α - particle) at a thin sheet of gold foil.

α - particles are fast moving, small, dense, positively charged particles, and all predictions said that they should smash through the soft spongy 'plum pudding' atoms almost unaffected.

And most of them did! So far so good.

But Rutherford noticed that some α - particles were deflected through big angles. Some even bounced straight back.

That seemed impossible - there shouldn't be anything dense enough in the 'plum pudding' atom to make the α - particle bounce back.

So this model was scientifically proven to be incorrect

These observations allowed Rutherford to work out what was inside the atom.

The Table below shows each of the observations Rutherford made to determine the structure of the atom:

Observation Deduction Alpha particle bounces back Shows there must be small concentrated masses in the atom that were dense enough to make the alpha particles rebound. This is the evidence for the nucleus. Alpha particle changing direction The alpha particle is deflected by the gold nucleus as they both have a positive charge. This showed that the nucleus of an atom is positively charged. Alpha partcles pass straight through Many alpha particles passed straight through the gold showing that most of the atom is made up of space.

The model of the atom that Rutherford came up with is called the 'nuclear model'. It is the model of the atom that we still use today.

To help us to describe atoms and nuclei, we use numbers and letters. Here is an example:

The atom in the diagram is described by the numbers and letters shown next to it. All atoms of a certain element will have the same number of protons in the nucleus.

The top number is called the mass number or the nucleon number. It tells you how many particles are in the nucleus, i.e. how many protons and neutrons.

The bottom number is called the proton number or the atomic number. It tells you how many protons there are in the nucleus.

The letters give you a clue as to the name of the atom. This is an atom of 'helium', He.

The number of electrons in an atom is the same as the number of protons. That makes the atom neutral overall (neither negative nor positive).

If the numbers are not equal, the atom becomes a charged particle. We call these charged particles ions.

You can also write equations for nuclear reactions. In these equations you will need to make sure that:

1. The total number of protons is the same before and after the reaction.
2. The total number of nucleons is the same before and after the reaction.

In this reaction, carbon and helium is combined to form oxygen.

Notice that if you add up the numbers at the top of each atom (the nucleon numbers) on the left hand side you get the same as the total for the numbers on the right hand side.

The same is true for the proton numbers.

The number of protons is the thing that decides how an atom is going to behave and therefore what element the atom belongs to. If you change the number of protons in an atom, you change the type of atom (it becomes an atom of another element.)

However, you can change the number of neutrons in an atom without changing the type of atom.

For example, hydrogen is an atom that contains only 1 proton. Atoms with more or less neutrons in them are called isotopes. The picture below shows three isotopes of hydrogen. So all the atoms below are hydrogen, except one. Which one?

Some isotopes are radioactive. Radioisotopes are radioactive isotopes of an element.

What is Radioactivity?

Some isotopes of atoms can be unstable.

They may have:

a) Too much energy or b) The wrong number of particles in the nucleus.

We call these radioisotopes.

To make themselves more stable, they throw out particles and/or energy from the nucleus. We call this process 'radioactive decay'. The atom is also said to disintegrate.

The atom left behind (the daughter) is different from the original atom (the parent). It is an atom of a new element. For example uranium breaks down to radon which in turn breaks down into other elements.

The particles and energy given out are what we call 'radiation' or 'radioactive emissions'.

Background Radiation

There is a certain amount of radiation around us (and even inside us) all the time. There always has been - since the beginning of the Earth. It is called Background radiation.

Background radiation comes from a huge number of sources.

In most areas, Background radiation is safe. It is at such a low level that it doesn't harm you. You need to be exposed to many times the normal background level before you notice any symptoms.

However, some areas of the country have a higher level of background radiation than others because the rocks near the surface contain more radioactive isotopes (for example, Cornwall).

Look at this example:

You use a radiation detector to record that a sample of rock produces 100 decays per minute. You then remove the rock and record the background radiation in the room. It is 7 decays per minute.

Ionisation

The radiation emitted by radioactive substances has a huge amount of energy, which is why it is so dangerous. The energetic radiation causes ionisation.

When radiation hits a neutral atom, some of the energy from the radiation is passed to the atom. This energy can cause an electron from the atom to escape, leaving the atom with a positive charge. This positively charged atom is called an ion, so the process is called ionisation.

As the radiation travels along it ionises atoms that are close enough. The more atoms the radiation ionises the more energy the radiation gives away, until eventually there is no energy left. The radiation is then said to have been absorbed.

Detecting Radiation

You will see in the following quick learn that there is more than one type of radiation, but each sort causes ionisation. This is how we are able to detect radiation.

It is hard to detect the actual particles or waves emitted by radioactive substances, but it is easy to detect the positive and negative ions produced by the ionisation they cause. A device called a Geiger-Muller tube collects the charged ions and can measure the amount of ionisation that is taking place in a certain time. The greater the amount of ionisation the more radiation there must be.

Why is Radiation Harmful? It is this process of ionisation that makes radioactive substances so dangerous. Living cells can be fatally damaged if molecules in the cell are ionised. This damage can kill cells or cause cancers to form. The greater the dose of radiation the more likely it is that cancer will occur.

Alpha, Beta and Gamma

There are three main types of radiation that can be emitted by radioactive particles. They are called alpha, beta, and gamma. All three types of radiation come from the nucleus of the atom. All three types of radiation will cause ionisation, but they behave slightly differently, because of the way they are made up.

Type of radiation Greek symbol What is it? Charge Alpha α Particle - A highly energetic helium nucleus, containing 2 protons and 2 neutrons. Positive 2+ Beta β Particle - A highly energetic electron, released from inside a nucleus. It has negligible mass. When a beta particle is produced a neutron in the nucleus divides into a proton and an electron. It is the electron that is rejected from the nucleus at high speed that is the beta particle. Negative 1- Gamma γ Wave - from the high frequency end of the electromagnetic spectrum. Waves have no mass. No charge

The Absorption of Alpha, Beta and Gamma

The Table below explains why different types of radiation are absorbed by different things:

Beta These are small particles with a negative charge. They can ionise fairly easily so can only travel through thin materials before they are absorbed. Gamma This is a wave that carries a huge amount of energy, but waves are not as good at ionising atoms as particles are. It is therefore really difficult to absorb them and they can even travel through thin lead and thick concrete. Alpha These are large particles with a positive charge. They can ionise atoms really easily so quickly lose their energy by ionising nearby atoms. This means they can be absorbed by just a few centimetres of air, a sheet of paper or by skin.

Each type of radiation that can be emitted can be absorbed by different materials and ionises different amounts. They are equally dangerous but for different reasons.

Alpha particles: Although alpha particles cannot penetrate the skin, if it gets into the body it can ionise many atoms in a short distance. This makes it potentially extremely dangerous. A radioactive substance that emits just alpha particles can therefore be handled with rubber gloves, but it must not be inhaled, eaten, or allowed near open cuts or the eyes.

Beta particles:Beta particles are much more penetrating and can travel easily through skin. Sources that emit beta particles must be held with long handled tongs and pointed away from the body. Inside of the body beta particles do not ionise as much as alpha particles but it is much harder to prevent them entering the body.

Gamma waves: These waves are very penetrating and it is almost impossible to absorb them completely. Sources of gamma waves must also be held with long handled tongs and pointed away from the body. Lead lined clothing can reduce the amount of waves reaching the body. Gamma waves are the least ionising of the three types of radiation but it is extremely difficult to prevent them entering the body.

Decay Equations

When atoms disintegrate by radioactive decay, new daughter atoms are produced. We can work out which elements will be produced using decay equations. These are like the equations you may have used for chemical reactions. Each type of radiation has a chemical symbol that is used in the equation.

Note: the equations must always balance, so there are the same number of protons and neutrons on each side of the equation.

These equations are not likely to happen in real life, as usually a combination of alpha, beta and gamma are released rather than just one.

Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically. As they decay the atoms change to daughter atoms, until eventually there won't be any of the original atoms left.

Different substances decay at different rates and so will last for different lengths of time. We use the half-life of a substance to tell us which substances decay the quickest.

Half-life - is the time it takes for half of the radioactive particles to decay.

It is also the time it takes for the count-rate of a substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time.

The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph.

Using Radioacticity

Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are the main uses you will be expected to know about:

1. Uses in medicine to kill cancer - radiation damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body. Sources of radiation that are put in the body need to have a high count-rate and a short half life so that they are effective, but only stay in the body for a short period of time. If the radiation source is outside of the body it must be able to penetrate to the required depth in the body. (Alpha radiation can't travel through the skin remember!)

2. Uses in industry - one of the main uses for radioactivity in industry is to detect the thickness of materials. The thicker a material is the less the amount of radiation that will be able to penetrate it.

3. Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness. Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal.

They can even use this idea to detect when toothpaste tubes are full of toothpaste!

4. Photographic radiation detectors - these make use of the fact that radiation can change the colour of photographic film. The more radiation that is absorbed by the film the darker the colour it will go when it is developed. This is useful for people working with radiation, they wear radiation badges to show them how much radiation they are being exposed to.

5. Dating materials - The older a radioactive substance is the less radiation it will release. This can be used to find out how old things are. The half-life of the radioactive substance can be used to find the age of an object containing that substance.

There are three main examples of this:

i) Carbon dating - many natural substances contain two isotopes of Carbon. Carbon-12 is stable and doesn't disintegrate. Carbon-14 is radioactive. Over time Carbon-14 will slowly decay. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object.

ii) Uranium decays by a series of disintegrations that eventually produces a stable isotope of lead. Types of rock (igneous) contain this type of uranium so can be dated, by comparing the amount of uranium and lead in the rock sample.

iii) Igneous rocks also contain potassium-40, which decays to a stable form of Argon. Argon is a gas but if it can't escape from the rock then the amount of trapped argon can be used to date the rock.

Nuclear Fission

Nuclear power produces energy that is converted into electrical energy in nuclear power stations. Nuclear fuel does not burn. Instead a process called nuclear fission takes place.

During nuclear fission a neutron is fired at a Uranium atom. The neutron is absorbed, which makes the atom extremely unstable so it splits into two smaller atoms, releasing more neutrons and a huge amount of energy at the same time.

The neutrons that are released can go on to collide with other uranium atoms causing more fission and more neutrons to be released. This is called a chain reaction.

The new atoms that are formed are radioactive.

The amount of energy released during nuclear fission is much larger than the energy released when substances react chemically. For instance, 1 kg of uranium undergoing fission can release the same energy as 10 000kg of coal burning!

Uranium is not the only element that can be used in nuclear power. Plutonium is an alternative fuel.

Note: Don't confuse this with nuclear fusion, which is what happens in stars. Two hydrogen atoms are pushed together to fuse and make a helium atom. This also releases massive amounts of energy!