Radioactivity

• Particle theory (or kinetic theory)

  A model that helps explain the properties of solids, liquids and gases. The particles are usually represented as spheres
• We can explain some of the properties of different elements by thinking about the particles that each contains. We call these particles atoms
• Chemical reactions occur when the different atoms in substances become joined in different ways
• In 1897 JJ. Thomson (1856-1940) carried out some experiments that showed that atoms contain much smaller subatomic particles called electrons. These had a negative charge and hardly any mass
• Plum pudding model


  A new model for atoms that described the atom as a 'pudding' made of positively charged material, with negatively charged electrons (the 'plums) scattered through it
• Between 1909 and 1913, a team of scientists led by Ernest Rutherford (1871-1937) carried out a series of experiments that involved studying what happened when positively charged subatomic particles, called alpha particles, passed through various substances (such as gold foil)
• Most of the alpha particles passed through the gold foil, but a few bounced back. The plum pudding model could not explain this result
• Rutherford's model of the atom


  Atoms were mostly empty space, with most of their mass in a tiny central nucleus with a positive charge and electrons moving around the nucleus
• Today, we know that the radius of a nucleus is about 1 × 10-15 m (0.000 000 000 000 001 m). The radius of an atom is about 1 x 10-10 m (0.000 000 000 1 m). So the atom itself is 100000 times bigger than the nucleus inside it
• Proton


  Subatomic particle in the nucleus with a positive charge and a relative mass of 1
• Neutron


  Subatomic particle in the nucleus with no charge and a relative mass of 1
• Electron


  Subatomic particle around the nucleus with a negative charge and a negligible relative mass
• Atomic number (or proton number)


  The number of protons in an atom of an element
• Mass number (or nucleon number)


  The total number of protons and neutrons in the nucleus of an atom
• Atoms of different elements have different numbers of protons and so have different atomic numbers
• Atoms have the same number of electrons as protons and so atoms are always electrically neutral (they have no overall charge)
• The number of neutrons in an atom can vary
• Atoms of a single element that have different numbers of neutrons are called isotopes
• Carbon can occur naturally as carbon-12, carbon-13 or carbon-14. The number in the name is the mass number of the isotope
• An atom of carbon-14 has 6 protons and 8 neutrons in its nucleus
• Electron shells (or orbits)

  The different energy levels that electrons can occupy around the nucleus of an atom
• When an atom absorbs energy, an electron can move to a higher orbit. When an electron returns to a lower orbit the atom emits energy as visible light of a particular wavelength
• Each different electron transition produces a different wavelength of light
• Neon tubes glow orange because the neon gas emits light at orange wavelengths when electrons return to lower energy levels
• Gases can also absorb energy transferred by electromagnetic radiation, such as visible light. The bottom part of photo C shows the parts of the visible spectrum that neon gas absorbs when light passes through it. This is the absorption spectrum for neon
• The wavelengths of light that neon gas absorbs are the same wavelengths that it emits
• Bohr's model of the atom

  Electrons can only be in certain fixed orbits (electron shells) around the nucleus. They cannot be part-way between two orbits. This model could explain the lines in emission and absorption spectra
• Ionisation


  When an atom gains so much energy that one or more of the electrons can escape from the atom altogether. An atom that has lost or gained electrons is called an ion
• Radiation that causes electrons to escape is called ionising radiation
• If an atom loses an electron, it then has one more proton than it has electrons. It has an overall positive charge and is called a positive ion
• We are constantly being exposed to ionising radiation at a low level, from space and from naturally radioactive substances in the environment. This is called background radiation
• The main source of background radiation is radon gas. This radioactive gas is produced by rocks that contain small amounts of uranium
• Approximately 95% of background radiation in the UK comes from natural sources
• Radioactivity can be detected using photographic film, which becomes darker and darker as more radiation reaches it
• People who work with radiation often wear film badges (called dosimeters) to check how much radiation they have been exposed to
• Newer dosimeters use materials that change colour without needing to be developed
• Radiation passing through a Geiger-Müller (GM) tube ionises gas inside it and allows a short pulse of current to flow
• A GM tube can be connected to a counter, to count the pulses of current, or the GM tube may give a click each time radiation is detected
• When scientists measure the radioactivity of a source, they need to measure the background radiation first by taking several readings and finding the mean. This mean value is then subtracted from measurements
• GM tube


  Gives a click each time radiation is detected