LI 1 Revision of Atomic Structure and Nuclide Notations

Transcription

1 Topic 13 National 5 Chemistry Summary Notes Nuclear Chemistry In this topic you will learn about radioactive elements whose atoms are so unstable that they have to release radiation in order to become more stable. This release of radiation often results in the formation of new substances. We will also learn about the different types of radiation and some of the important uses of radioactive elements. LI 1 Revision of Atomic Structure and Nuclide Notations Atoms consist of a dense central nucleus containing protons and neutrons with electrons orbiting around the outside: Each element has its own unique number of protons (atomic number) but the mass number can vary depending on how many neutrons there are. Isotopes are atoms with the same atomic number but different mass numbers. Nuclide Notations We can use a special diagram to illustrate the different isotopes. It is called a nuclide notation: 1

2 LI 2 Radioactive Isotopes and Radioactive Emissions The atoms of most elements have isotopes. Some of these isotopes have nuclei which are unstable and give out different types of particles or rays. This release (emission) of particles and rays is what we call radiation and helps to make the atom more stable. Radioactivity is all around us. We are constantly being bombarded by particles and rays, this is often described as background radiation. Types of Radiation There are three types of nuclear radiation that we will be looking at: 1. Alpha Particles, These are the heaviest particles. They are made up of a Helium nuclei, He 2+, containing two protons and two neutrons and have their own nuclide notation: 2. Beta Particles,β These light weight particles are made up of a single, very high energy (fast) electron. Nuclide notation: 2

3 3. Gamma Waves, These are not particles. They are a form of electromagnetic radiation, waves and they come from the nucleus. These very high energy rays can travel very long distances and penetrate deep into even very dense substances. LI 3 Properties of alpha, beta and gamma radiation When the three different types of radiation are passed through an electric field they are affected in different ways: 1. Beta particles are negatively charged so they are attracted (deflected) to the positive side. 2. Alpha particles have a positive charge so they are attracted (deflected) to the negative side. 3. Gamma ways are not charged so pass through unaffected. 3

4 The radioactive emissions also have different penetrating powers: 1. Alpha particles are the biggest and they can be stopped by a thin piece of paper. 2. The smaller beta particles can travel a few metres through the air but will be stopped by a few millimetres of aluminium. 3. Gamma ways have no mass and charge and can travel long distances. They are only stopped by thick lead and even thicker concrete. LI4 Nuclear Equations Another word to describe the breakdown of unstable nuclei is decay. An atom can often go through a series of changes by emission of radiation in order to become stable. We can represent radioactive decay using nuclear equations: Alpha Emission mass number atomic number Nuclear equations show the mass number, atomic number and symbols of all the particles involved. In other words the nuclide notations we revised earlier. When an atom loses an alpha particle it loses 4 mass units and turns into a new element with an atomic number 2 less than the original. 4

5 Beta Emission When an atom decays by losing a beta particle it gains a proton but its mass is unchanged. A neutron has been changed into a proton and an electron. Gamma emission often happens at the same time but has no effect on the mass number or atomic number. It is not included in nuclear equations. Balancing the Numbers! In nuclear equations the top numbers (mass numbers) must add up to the same number on both sides of the equation. The bottom numbers, the atomic numbers must also add up to the same number on both sides of the equation. This means the nuclear equation is balanced. If you know what sort of particle an atom is emitting then you should be able to work out the identity of the new atom produced: Example 1: What new atom is formed when an atom of Bismuth-211 loses an alpha particle? +? The numbers must balance so the new atom must have an atomic number of 83-2=81 and a mass number of 211-4=207: Answer: Example 2: What new atom is formed when an atom of Sodium-24 decays by beta emission? +? 24-0= 24 so the mass number of the new atom stays at 24, 11 (-1) = 12 so the atomic number of the new atom is now 12 (a neutron has changed to a proton) Answer: 5

6 LI 5 Half Life You have probably heard that substances can remain radioactive for a long time. It can in fact take anything from seconds to millions of years for different elements to completely decay and lose their radioactivity. Scientists use the amount of time it takes for an element to lose half of its radioactivity as a useful measure. Each radioisotope has a unique half-life: Radioisotope Half-life Cobalt years Americium years Iodine days A graph can be drawn showing the curve you always get when plotting radioactivity (often in counts per minute) against time. In the graph below you can see that the sample halves its radioactivity every five days so its half-life is 5 days. Half-life Calculations 1. Strontium-90 has a half-life of 28 years. How many years would it take for a sample of it to reduce to 1/16 th of its original radioactivity? Answer 1/16 th is a half of a half of a half of a half i.e. 4 half-lives so it must have taken 4 x 28 = 112 years. 6

7 2. The half-life of Caesium-137 is 30 years. What mass of caesium-137 would be left after 90 years if 100g was there at the start? Answer 90 years is 90/30 = 3 half-lives g left The following table helps you to match fractions and percentages of a sample left to numbers of half-lives that have passed: % of sample left Fraction of sample left Number of Half-lives LI6 1. Dating Uses of Radioisotopes Since every radioisotope has a unique and constant half-life that fact can be used to find out how old an object containing that substance is. The radioactive element Carbon-14 is found in any object made of once living things. Carbon-14 is made in the upper atmosphere at a constant rate due to a type of cosmic radiation (high speed neutrons) colliding with nitrogen atoms: This radioactive form of carbon is quickly oxidised to carbon dioxide, trapped by plants in photosynthesis and then passed into the food chain. All livings contain a small but constant amount of Carbon-14 until they die. At that point no more new carbon-14 can 7

8 be trapped so the amount of carbon-14 will drop, the older the object made of a dead thing the less radioactive it will be. The half-life of carbon-14 is 5730 years and very old objects up to around 50,000 years can be dated accurately this way. 2. Other Uses of Radioisotopes There are many radioisotopes used in medicine and industry. They can be used to help diagnose diseases, kill tumours and even sterilise medical equipment. They can be used to test metals and welds for cracks, calculate (gauge) the thickness of materials to name just a few. The following table shows some examples with space left to add your own: Radioisotope Use Cobalt-60 Caesium-137 Produces gamma radiation to kill cancerous cells Thickness gauging Iodine-131 Diagnosing and treating thyroid problems Americium-241 Smoke detectors 8

9 Topic 13 Pupil Self Evaluation Nuclear Chemistry Nat 5 Number Learning Intention Success Criteria 1 I will revise atomic structure and nuclide notations 2 I will find out most elements have isotopes and some of these are unstable and produce different types of radiation 3 I will find out about the properties of alpha, beta and gamma radiation 4 I will find out about how to write nuclear equations 5 I will find out that Half Life is a useful term used to measure how long an isotope takes to lose half its radioactivity 6 I will find out some of the uses of radioisotopes I can draw nuclide notations given the name and mass of an atom. I can: Describe the three different types of radiation State that we are being constantly bombarded by particles and rays known as background radiation I can describe how and why the three different types of radiation are affected by an electric field. I can describe the penetrating powers of alpha, beta and gamma radiation. I can write equations for beta and alpha emission I can interpret Half Life graphs and do simple calculations involving half life and radioactivity I can give example and describe some of the uses as: Radiocarbon dating Medical uses Industrial uses 9

10 10