The Nucleus

Alpha particle scattering experiment

Early in the 20th century scientists realised that all matter is made up of atoms. As a result of an experiment carried out by his assistants, Hans Geiger and Ernest Marsden, Ernest Rutherford suggested a model for the atom.

The experiment is known as Rutherford's alpha particle scattering experiment. In it, a beam of alpha particles is aimed at a thin foil of gold. In the experiment:

• about 99% of the alpha particles pass straight through the foil
• some of the alpha particles are deflected through small angles
• a very small number of the alpha particles rebound off the gold foil

Because of the results of this experiment, Rutherford showed that:

• the nucleus of an atom has a relatively small diameter compared with that of the atom
• most of the mass of the atom is concentrated in the nucleus

Nuclear arithmetic

Atoms are made up of protons and neutrons, collectively called nucleons, surrounded by electrons.

Things you need to know:

• The atomic number of an element (Z) is the number of protons in a nucleus. This determines what the element is.
• The mass number of an element (A) is the number of nucleons in a nucleus. The number of neutrons in a nucleus is A-Z.
• Atoms of elements can contain different numbers of neutrons. Different forms are known as isotopes. The nucleus of an isotope is called a nuclide.
• Radioisotopes and radionuclides refer to radioactive forms of elements.
• A particular isotope of an element is defined by giving the name of the element and the mass number of the isotope. An example of this would be uranium-235.

The symbol is used for the nucleus of an element that has the chemical symbol X, a mass number A, and atomic number Z.

You can view a periodic table in Patterns in the Periodic Table in Bitesize Higher Chemistry.

Radioisotopes of elements are unstable and can decay in one of three ways into a different element, or a more stable isotope of the same element.

• Alpha decay is when an alpha particle consisting of two protons and two neutrons is emitted from a nucleus. An alpha particle is the same as a helium nucleus so it has the symbol .
• Beta decay is when a fast moving electron () is emitted from a nucleus as a result of a neutron splitting into a proton and an electron.
• Gamma decay is when a burst of electromagnetic radiation is given out from a nucleus due to a rearrangement within the nucleus

Nuclear reactions can be written in symbolic form. Mass number is conserved during a nuclear reaction so the numbers in the top line must add up. Atomic number is also conserved so the numbers in the bottom line must add up.

• An isotope of polonium decays to an isotope of lead by emitting an alpha particle.
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• An isotope of lead decays to an isotope of bismuth by emitting a beta particle.
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• Gamma decay does not show up in a decay equation because no new isotope is formed.