Atoms and Radioactivity

1. Radioactivity Atoms and Radioactivity Introduction Types of Radiation Using Radioactive Decay Carbon Dating Other methods Example Glossary By Mark Varley 11SY

2. Proton Neutron Atoms and Radioactivity Isotopes of some elements have unstable nuclei In order to become stable the make break apart or decay This decay causes the nucleus to emit ionising radiation, and an atom of a different element is formed These isotopes are called radioisotopes or radionuclides Example – using Carbon A normal carbon nucleus looks like this, it has 6 protons and 6 neutrons. This is perfectly stable.

3. Electron Proton Neutron This forms a nitrogen nucleus with 7 protons and 7 neutrons, that is a stable nucleus. However, another isotope of carbon, (carbon 14) has 6 protons and 8 neutrons, causing the nucleus to be unstable. The carbon 14 decays by beta radiation, a neutron turns into a proton emitting a beta particle.

4. The alpha radiation, a helium nucleus (2 protons and 2 neutrons). Different types of radiation Alpha radiation It is actually the nucleus of a helium atom consisting of 2 protons and 2 neutrons, held tightly together. It comes directly from the nucleus, so only big atoms tend to emit it. This is a massive particle, with 2 positive charge. It strongly interacts with matter around it, smashing past atoms and ionising them. This quickly absorbs their energy so they can only penetrate short distances through air.

5. Beta radiation, (a high speed electron) Gamma Radiation, (a short electromagnetic wave) Beta Radiation Is a high speedelectron, given off when a neutron turns into a proton They are many times smaller than alpha radiation They only carry 1 single negative charge They interact much less strongly with matter So the radiation is much less quickly absorbed, so they can travel through thin sheets of aluminium. Gamma Radiation Sometimes a nucleus that is still unstable after a beta or alpha decay is still unstable so may emit gamma radiation to become more stable. This is an electromagnetic wave with a very short wavelength It does not change the number of protons or neutrons in the nucleus It is not strongly absorbed by matter at all It is most penetrating and needs thick lead 10cm plus to absorb most of it.

6. Using Radioactive Decay As radioactive substances decay, the amount of radiation they emit decreases, this means it can be used to date things. Carbon Dating High in the earths atmosphere, carbon atoms are bombarded by radiation from space, this converts some stable carbon atoms into radioactive carbon atoms. Many of these molecules are taken up by plants in photosynthesis (CO2) and incorporated into matter. That can then end up in animals due to them eating plants. When a plant or creature dies, the amount of radioactive carbon in it decreases through radioactive decay. The age of the remains of the plant or animal can be calculated from the amount of radioactive carbon in it.

7. Two other ways to remember: Uranium-Lead method This uses the fact that uranium decays through several short lived radioisotopes to produce a stable isotope of lead. By measuring the relative proportion of uranium to lead in a rock, an age can be calculated. Potassium-40 – Argon Potassium-40 decays to argon and no further This decay can date many rocks as many rocks have potassium atoms in them. The age is determined by measuring the relative amounts or argon gas trapped and potassium atoms. This relies on the gas being trapped therefore meaning it only works for igneous rocks that are impermeable to gas.

8. Example A sample of rock is examined. Inside the rock, argon gas is found, formed through the radioactive decay of potassium-40 ions. For every 3 atoms of argon there is is 1 atom of potassium. Estimate the age of the rock, with the half life of potassium being 1300 million years. When the rocks formed There are no argon atoms present so for every 4 potassium atoms there are 0 argon atoms Ratio 4 : 0 After 1 half life Half the potassium atoms have decayed to form argon atoms, so for every 4 potassium atoms in the rock when it was formed, 2 will have decayed to produce argon atoms. Ratio 2 : 2 (= 1 : 1) After 2 half lives A further half the potassium atoms have decayed to form argon atoms, so for every 2 potassium atoms after 1 half life, one will have decayed to form an argon atom. This means 3 out of the 4 potassium atoms have decayed to argon. Ratio 1 : 3 This means the age of the rock is 2 half lives old. As each half life is 1300 million years Age = 2 X 1300 million = 2600 million years old

9. Glossary Isotopes The same element (same number of protons) but with different numbers of neutrons Nuclei The centre of an atom where almost all the mass is held, (protons and neutrons) Decay The process of the atom changing into something else by releasing radiation Ionising Radiation Radiation that can ionise another atom, if it hits Proton A positive particle with a mass of 1, a charge of +1 and found in the nucleus Neutron A neutral particle with a mass of 1, no charge and found in the nucleus. Electron A negative particle with a mass of 1/1840, a charge of –1 and found orbiting the nucleus Half Life The time it takes for half the atoms in a substance to decay.