The Atom, the Nucleus and Radioactivity Chapter 30
Rutherford's Experiment
Rutherford bombarded a thin Gold Foil with Alpha Particles. -particles are helium nuclei. He knew they were positively charged particles. The -particles could be detected by small flashes of light that they produced on a fluorescent screen. He found that: Most -particles passed straight through the gold foil. Some were deflected through small angles. A very small number were turned back through angles greater than 90°.
Rutherford's Conclusion All the positive charge of the atom is concentrated in a central dense core called the Nucleus. The nucleus is very small compared to the size of the atom. An Atom is Mainly Empty Space. The negatively charged Electrons Orbit the Nucleus in various orbits.
The Bohr Model of the Atom (i) Electrons can only move in certain allowed orbits. The energy of an electron in a given orbit is a fixed value called an Energy Level. If an atom is supplied with energy an electron may absorb some of it and move from its orbit (of energy E1) to an orbit of higher energy (E2). This atom is then said to be in an Excited State.
The Bohr Model of the Atom (ii) After a very short time the electron falls back to its original orbit giving out an amount of energy equal to E2 - E1. This energy is given out as a photon of electromagnetic radiation of frequency f given by: h f = E2 - E1 where h = Planck’s constant.
Continuous Emission Spectrum A Continuous Emission Spectrum is produced by an incandescent solid or liquid. It consists of a continuous spread of changing colours going from red to violet. All visible wavelengths are emitted. Continuous spectra are not characteristic of the material from which they come. E.g. white hot iron produces the same spectrum as white hot tungsten.
Line Emission Spectrum If the atoms of a gaseous element are given sufficient energy they give out coloured light. If this light is passed through a prism (or diffraction grating) a series of bright lines on a dark background is formed. This is an emission line spectrum. Above is a hydrogen line spectrum; below is a carbon line spectrum.
Experiment to show (i) a Line Emission Spectrum (ii) a Continuous Emission Spectrum Use a Gas Discharge Tube as the light source. A number of bright lines, i.e. a Line Emission Spectrum, will be seen in the telescope. Replace the gas discharge tube with an Incandescent Filament Bulb. A Continuous Emission Spectrum will be seen in the telescope.
STRUCTURE OF THE NUCLEUS By 1932 it had been discovered that the nucleus of an atom was itself made up of at least two more particles called Protons and Neutrons. The table shows the main properties of the subatomic particles electrons, protons and neutrons.
What is meant by Atomic Number? The Atomic Number ( Z ) of an element is the number of protons in the nucleus of an atom of that element. The number of protons in the nucleus of an atom tells us what element it is.
What is meant by Mass Number? The total number of protons and neutrons in the nucleus of an atom is called the Mass Number (A) of that atom.
What are Isotopes? Atoms of an element that have the same number of protons but different numbers of neutrons are called Isotopes of that element.
A nucleus has atomic number Z and mass number A A nucleus has atomic number Z and mass number A. How many neutrons are in its nucleus? Number of neutrons = A - Z Number of neutrons = Mass number - Atomic Number
What is Radioactivity? Radioactivity is the disintegration or decay of the nuclei of certain atoms with the emission of one or more types of radiation. Radioactivity was discovered by Becquerel in 1896.
RADIATION FROM THE NUCLEUS The nuclei of many isotopes, particularly those of atomic number greater than 92 are unstable - i.e. they contain excess energy. These nuclei can become stable by getting rid of this energy. The process of getting rid of this energy is the radioactivity discovered by Becquerel. It is also called Nuclear Radiation. The nucleus is said to undergo Radioactive Decay or Radioactive Disintegration.
Name the three kinds of Nuclear Radiation Alpha-radiation ( ) Beta-radiation ( ) Gamma-radiation ( ) Deflection in Electric and Magnetic Fields, Penetrating Power and Ionisation are evidence for the three types of radiation.
Deflection in an Electric field If a narrow beam of radiation from radioactive nuclei is passed through an electric field it is found to split into three components. Some deflects in the way a positively charged particle would (the -rays). Some deflects as a negatively charged particle would (the -rays). Some is not deflected (the -rays).
Deflection in a Magnetic field If a narrow beam of radiation from radioactive nuclei is passed through an Magnetic field it is found to split into three components. Some deflects in the way a positively charged particle would (the -rays). Some deflects as a negatively charged particle would (the -rays). Some is not deflected (the -rays).
Penetrating power of Nuclear Radiation - Radiation is stopped by a sheet of paper. ß - Radiation is stopped by a thin sheet of aluminium. - Radiation is very penetrating and will only be stopped by a thick block of lead or a few feet of concrete.
Ionisation A neutral atom (or molecule) that has lost (or gained) one or more electrons is called an Ion. Ionisation occurs when a neutral atom (or molecule) loses (or gains) one or more electrons. What is an ion? Radioactivity causes ionisation in materials. What is Ionisation?
Experiment to show the Ionising Effect of Radioactivity Charge a gold leaf electroscope. It should remain charged. Bring a radioactive source near to the cap of the electroscope. The leaf will slowly collapse. The radiation ionises air molecules near the cap of the electroscope. Ions with opposite charge to that on the cap are attracted to it and neutralise the charge on it and the leaf collapses.
To Demonstrate the Penetrating Power of , and radiations (i) Measure the Background Count Rate. Place an -source about 1 cm from the GM tube. Read the count rate. Place a sheet of paper between the source and the GM tube and read the count rate. It will read the background count rate. Remove the paper and slowly move the GM tube back from the source. The count will fall and reach the background count within a few cm. CONCLUSION -rays are stopped by a sheet of paper or a few cm of air.
To Demonstrate the Penetrating Power of , and radiations (ii) Repeat using a -source. Use aluminium sheets and the paper as stoppers. CONCLUSION -rays are not stopped by air or a sheet of paper, but will be stopped by a sheet of aluminium a few mm thick. Repeat using a -ray source. CONCLUSION It takes a sheet of lead a few cm thick to completely block the -rays.
What is the nature of an Alpha-Particle? An Alpha-Particle is a helium nucleus, i.e. a bundle of two protons and two neutrons, emitted from the nucleus of a radioactive atom. An alpha-particle
Alpha-Particle Emission When a nucleus emits an alpha particle ( 4He2 ): Its atomic number decreases by 2 ( it loses 2 protons ) Its mass number decreases by 4 (it loses 2 protons and 2 neutrons) Thus the daughter nucleus is two places to the left of the parent nucleus on the periodic table. E.g. Radium Radon + -particle
General Equation for Alpha-Emission Parent Nucleus → Daughter Nucleus + Alpha-Particle
What is the nature of an Beta-Particle? A Beta-Particle is a high speed electron emitted from the nucleus of a radioactive atom.
Beta-Particle Emission (i) When a nucleus emits a beta particle a neutron in the nucleus splits up and becomes a proton and an electron. Neutron Proton + Electron (ß-particle) The proton remains in the nucleus and the electron is ejected out at high velocity (the ß-particle). Since the mass of the beta particle is very small there is almost no change in the mass of the parent nucleus.
Beta-Particle Emission (ii) When a nucleus emits a beta-particle there is now one more proton in the nucleus. The atomic number of the daughter nucleus is one greater than that of the parent. i.e. the daughter is one place to the right of the parent on the periodic table, e.g. Radium Actinium + -particle
General Equation for Beta-Emission Parent Nucleus → Daughter Nucleus + Beta-Particle
What is the nature of Gamma-Rays? Gamma-Rays are high frequency electromagnetic radiation emitted from the nucleus of a radioactive atom. When gamma rays are emitted from a nucleus the structure of the nucleus remains the same. The nucleus loses energy and becomes more stable. Gamma rays are normally emitted from a nucleus that has already emitted an alpha or beta particle.
Properties of Alpha, Beta and Gamma Rays
Detecting Ionising Radiation: The GM Tube Radiation enters through the mica window. It ionises Argon atoms producing positive Argon ions and electrons. The electrons pick up high speed in the strong electric field near the wire anode. They produce further ions and electrons by collision with more Argon atoms. An avalanche of electrons is produced. The electrons reach the anode and a pulse of current flows in the external circuit. The pulses can be counted electronically to measure the activity of a source.
Detecting Radiation: The Solid State Detector A solid state detector consists of a reverse biased p-n junction connected to some form of counting device such as a ratemeter or a scaler. When nuclear radiation strikes the depletion layer some electron-hole pairs are formed there. These charge carriers move under the influence of the voltage across it and so a pulse of current is formed. This is amplified before being passed to a pulse counter.
Uses of Radioisotopes Medical Imaging. Small amounts of short lived isotopes are placed in a living organ. An image of the organ can be made from the emitted radiation. Medical Therapy. Radiation kills cancer cells more easily than healthy cells. Food Irradiation. Gamma rays can be used to sterilise food. Radioactive Tracers are used in medicine and agriculture to trace the movements of various substances in living matter. Carbon Dating. The age of archaeological specimens can be determined by the activity of the isotope Carbon-14 contained in them. In Industry to check the thickness of objects, the fullness of containers, to find leaks, to detect wear in components. Smoke Detectors.
What is meant by the Activity of a Radioactive Source? The Activity (A) of a radioactive source is the number of disintegrations occurring per second.
What is the SI Unit of Activity? The SI Unit of Activity is the becquerel (Bq). 1 becquerel = 1 disintegration per second.
State the Law of Radioactive Decay The Number of Nuclei Decaying Per Second (i.e. the activity) is directly proportional to the number of nuclei undecayed. i.e. Rate of decay Number of Nuclei Undecayed Rate of decay = λ N λ is a constant and N is the number of nuclei undecayed.
What is the Decay Constant? The number λ in the equation: Rate of decay = λ N is called the decay constant. The SI Unit of the Decay Constant is the per second ( s-1 )
What is meant by the Half-Life of a radioactive Isotope? The Half-Life (T½ ) of a radioactive isotope is the time taken for half of the radioactive nuclei in a sample of that isotope to decay.
Because the activity of a radioactive source is directly proportional to the number of nuclei undecayed: The half life (T½ ) of a radioactive isotope is also the time taken for its activity to decrease by half.
What is the relationship between the Half-Life (T½ ) and the Decay Constant ( λ ) for a given radioactive isotope? OR