1a. The Structure of Atoms - the three fundamental particles

• Atoms are the smallest particles of matter whose properties we study in Chemistry. However from experiments done in the late 19^th and early 20^th century it was deduced that atoms were made up of three fundamental sub-atomic particles (listed below).
• More on the Atomic Structure page and the use of radioactivity 'bullet' experiments to deduce the structure of an atom (Rutherford and Marsden scattering experiment).
• Earlier theories of atomic structure, eg the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superceded by the model described in the picture below.
• It was the only model that could explain the scattering of alpha particles by a small dense and positive atomic centre. Later experiments showed that the out bits could be knocked off atoms and these had a very tiny mass and a negative charge, in other words the electron!
• It should be emphasised right from the start that radioactivity is due to energy changes in the nucleus and the surrounding electrons are not usually involved.

The diagram below gives some idea on the structure of an atom, it also includes some important definitions and notation used to describe atomic structure. The atomic number (Z) is also known as the proton number. the mass number (A) is also known as the nucleon number. The neutron number (N) = mass number (A) - atomic number (Z). Protons and neutrons are the 'nucleons' present in the nucleus and the negative electrons are held by the positive nucleus in 'orbits' called energy levels or shells.

RADIOACTIVITY IS DUE TO CHANGES IN ATOMIC STRUCTURE IN THE NUCLEUS OF AN ATOM