P1bL6 Radioactivity

Key Words

Atomic number - number of protons in the atom.

Electrons - tiny particles of negative charge.  Mass of an electron is 1/1800 th the mass of a proton

Mass number - the total number of protons and neutrons.

Neutrons - particles in the nucleus that carry zero charge.

Nuclear energy - energy obtained by splitting atomic nuclei.

Plutonium - a radioactive metal used as a fuel in a nuclear reactor.

Proton - particles in the nucleus that carry a positive charge.

Radiation - energy coming from the nucleus in the form of particles or electromagnetic waves.

Radioactive decay - unstable nuclei decay to form other nuclei.

Radioactivity - studying the way in which nuclei decay.

Unstable - the nucleus is likely undergo radioactive decay.

Uranium - a heavy metal that is used as a fuel in nuclear power stations.

Test Yourself

Homework
Physics GCSE
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Grade E

Atoms consist of a nucleus, which contains protons and neutrons.  There are electrons orbiting the nucleus.

  • Protons are positively charged;
  • Neutrons carry no charge;
  • Electrons are negatively charged.

This atom is neutral.  It has the same number of electrons (3) as protons.  If an electron is removed, the atom becomes a positive ion.  If an electron is added, the atom becomes a negative ion.  All chemical reactions between elements involve the electrons.

Here is a picture of carbon 12.

It has 6 protons, and 6 neutrons.  Its atomic number is 6 because it has 6 protons.  Its mass number is 12, because there are 6 protons and 6 neutrons.  Now look at carbon 14:

This atom still has an atomic number of 6, but has a mass number of 14, because there are 6 protons and 8 neutrons.  It is an isotope of carbon.  An isotope has the same number of protons and a different number of neutrons.

This isotope has too many neutrons and is unstable; it decays radioactively, giving out a beta particle.  One of the neutrons turns into a proton and a high speed electron (beta particle) is kicked out of the nucleus.

carbon-14 ® nitrogen-14 + beta particle.

                                                                                   

Grade C

Atoms are often represented like this:

The atomic number is the number of protons

The mass number is the number of protons and neutrons.

Therefore:

Number of neutrons = mass number - atomic number

Remember that if the atomic number stays the same, but the mass number changes, we have an isotope.  However, if the atomic number changes, the element changes, even if the mass number stays the same.

In lighter elements the number of neutrons generally balances the number of protons, up to about atomic number 20.

For heavy elements there are many more neutrons than protons:

Element Mass Number Atomic Number Neutrons
Uranium 238 238 92 146
Uranium 235 235 92 143
Plutonium 233 94 139

Both of these heavy elements decay by alpha decay.  Uranium-235 and plutonium are fissile, which means that the nuclei can break up when they capture a neutron fired at them.  The splitting, or fission, releases a lot of energy, so both of these are used as nuclear fuels.

The process of fission is NOTHING whatever to do with radioactive decay.   While radioactive decay does release energy, the energy for nuclear fuels is the result of fission.

Nuclei with atomic numbers greater than 82 (lead) are unstable.  The nuclei are just too big to hold together permanently.  Most decay by alpha decay.  Uranium decays this way, but its half life is very long indeed - 4500 million years.  Some elements and isotopes have a half life of a tiny fraction of a second.

Many isotopes of elements of atomic number below 82 are unstable, because there is an imbalance of neutrons.  Cobalt has a mass number of 59.  Cobalt-60 decays giving off gamma rays.

Grade A

Uranium is the most common radioactive element found.  Its most common isotope is U-238, which is radioactive, but not fissile.  U-235 is fissile, and this isotope needs to be separated and concentrated to make the fuel.  This is done by reacting the uranium with fluorine to give uranium hexafluoride, UF6, a gas.  The U-235 is separated from the heavier isotope in a gas centrifuge.  The U-235 is pelleted and put into the fuel rods.

One of the many waste products is plutonium.  It and other highly radioactive elements are separated from the remaining U-235 during reprocessing.  The high level nuclear waste is cooled, mixed in with glass, and stored in deep underground vaults.  The main works that does this is at Sellafield in Cumbria.