1.2 ATOMIC STRUCTURE AND RADIOACTIVITY

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You probably heard this millions of times but yes, atoms make up everything. You need to know exacty what atoms are made out of: protons, neutrons and electrons and where to find them.

Just to clarify, neutrons and protons are in the nucleus of an atom and electrons fly around the atom in orbitals. Both protons and neutrons have a mass of 1. In the nucleus protons have a charge of +1 whereas neutrons have no charge. Electrons have a charge of -1. This is the reason electrons fly around atoms. The charges attract.

I am sure you also know that the two numbers next to an atom symbol are mass number (at the top/ shows the number of protons and neutrons combined together) and atomic number (at the bottom/ number of protons in the atom). The atomic number also is the number of electrons however, in the exam they won't accept this explanation.

Not all of the elements are identical. They can exist as isotopes or ions.

First of all, what are isotopes?

Isotopes- atoms that have the same number of protons but different number of neuatrons.

They have the same properties and number of protons, this doesn't change.

Ions- atoms where the number of electrons is not equal to the number of protons.

The number of protons and electrons is always the same (this is the atomic number) however, when atoms lose or gain electrons the number is not equal anymore and the atoms become ions. Every atom wants to be stable and it will do it for example, by becoming ions.

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Now, once we reminded ourselves about the basics, we are going into the more complex stuff, which is radioactivity. Actually, it is not that difficult. We are going through all of this together and trust me, you will catch it.

Radioactivity seems confusing at first because there is a lot of new terms that you have to take in. But we will start slowly and I am sure at the end of this post you will know everything there is to know for your AS-level Chemistry exam.

Radioactivity- when the nucleus splits and sometimes protons and electrons fly out. This happens to unstable isotopes, which means they are radioactive isotopes ( isotope of an element that is radioactive).

When the nucleus splits, there are three types of radiation that can be given out:

1. Alpha- it is a helium nuclei because it has 2 protons and 2 neutrons. It can be stopped by a sheet of paper, so it is the least penetrating.

2. Beta- it is a high energy electron. It can be stopped by a thick sheet of aluminium. It has a medium penetrating power.

3. Gamma- it is an electromagnetic wave. It is the most penetrating and can be stopped by a thick concrete or lead.

When these radiations are created by the splitting of radioactive isotopes they go through matter and knock electrons of the atoms. This is called ionisation (just like ions are created by losing or gaining electrons).

Ionisation- during ionisation energy goes from the radiation to the matter.

Alpha is the largest of the radiations because its mass number is 4 (Helium has a mass number of 4), therefore it is the most ionising. This means that the transfer of energy (ionisation) happens the quickest, so it's the least penetrating (it doesn't travel far because it looses energy on the way).

On the other hand, gamma is the least ionising because it has no mass, it is a ray, an EM wave (it is not a paricle but energy so has no mass and no charge). This means it can travel long distances before all of the energy is transferred to the matter.

What you also need to know is how radiations behave in electric field and magnetic field. On the image above I drawn a diagram to help you visualise my words. You've got a positve plate and a negative plate drawn. Gamma has no charge, so it will just fly through the magneic field. Alpha will be attracted to the the negative plate because it has a positive nucleus and beta will be attracted to the positve plate because it is an electron (negative).

This is similar for a mangetic field. Alpha and beta are deflected in opposite directions, proving that they have opposite charges.

Now, what we are going to do is practice some radioactivity equations. In the exam you will be given the starting atom and then will have to show the equation. They can also give you the equation and just ask to you to fill in the numbers.

1. Alpha decay. During alpha decay you have a staring atom and alpha particle is released on the other side of the equation as shown. Write down all you know (the staring atom and alpha particle on the right hand side) and then balance the equation. As you can see in the example, the top number on the LHS is 228 and because on the RHS the top number is 4, all you do is 228-4= 224 to balance the top numbers. Then you need to do exactly the same with the bottom numbers. Looking at the periodic table the atom with atomic number 88 is Ra. (Remember to look for the atomic number when searching for the missing atom because it will never change).

2. Beta decay. You do exaclty what you did before but this time a beta particle is released instead of alpha. As you know, beta paricle is an electron with no mass number and -1 charge . Just go with the same pattern as in the previous example.

3.Positron emission. First of all positron is the opposite of electron. It still has no mass but the charge is +1. During positron emission a proton is converted to neutron releasing a positron. As the name suggests positron is released, so it is o the RHS .Then, you just need to balance the equation as before.

4. Electron capture. The name says it all but I will explain it to you. An electron is captured by proton forming a neutron. This is because when + and - combine they are neutral and neutron is neutral. It is capture so on the LHS of th equation and then balancing again. I know you love to balance equations.

Although radiations and radioactivities are very important, they are also very dangerous. People working in places where they are exposed to radioactivity can only work certain amount of time before the radiation can mess with their cells. What radioactivity can do to you is:

- damage your DNA;

- cause mutations (which can cause cancerous cells);

Alpha is the most dangerous inside the body, once it is outside the body, it can't pass through our skin because it is stopped by even a sheet of paper. However, when it is inside or body, it quickly ionises the cells and can't get outside, it is trapped.

Gamma is the most dangerous when it is outside of our body.

Uses of radioactivity

-> Radio-dating

carbon-14 --> used to calculate the age of plant and animal remains

-> Medicine

Cobalt-60 --> used in radiotherapy, to treat cancer

-> Analysis

Measuring the thickness of metal strips using a beta emitter.

These are only examples and there are many more uses of radioactivity.

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And now, half-life. This is usually one of the one mark questions in the exam and it is very easy.

Half life - is the time taken for half of the radioactive species to decay.

Now I've got two examples prepared for you to try:

1. Whatever the mass is at the beginning, one half-life is the time taken for it to drop by half. You have three half-lifes to go from 8g to 1g. One half-life is 5years, so you just multiply the half-life by three.

2. This time you know how long it was decaying for and what the half life is. When you divide these numbers by each other, it gives you 4, meaning it has 4 half-lifes. You need to half the mass four times.

Ok, this is all I've got for today. If you want to practice more questions relating to this topic, visit my youtube channel, I am leaving the link at the top of this post. Also, if you would like a copy of past paper questions that I put together, email me, leave a comment or write a message throught the main page. There is a space provided.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!