Atomic Structure 3

One of the biggest hurdles to understanding chemistry is the idea of scale. With atomic structure, we are dealing with processes and objects that we cannot see directly which is why it was not until well into the 20^th century that protons and neutrons were identified. Our knowledge of what lies inside an atom has been discovered by using indirect observations, for example, the scattering of alpha particles as they passed through a thin piece of gold foil demonstrated that the mass of an atom was concentrated into a very small area at the center - the nucleus.

The first of the sub-atomic particles that were identified was the electron. Experiments using 'cathode rays' in 1897 by J.J. Thompson showed that they were the same no matter what element they came from. He guessed that they were some kind of fundamental particle that was a structural part of all atoms. They had a negative charge and in 1904 he suggested that an atom consisted of these 'corpuscles' whizzing round in a 'sea' of positive charge. We now know them as electrons which do move round in atoms but they do so in well defined areas known as 'shells' or 'energy levels'. They remain part of the atom because of the attraction of the nucleus. We also now appreciate that each of the 'shells' contains a specific maximum number of electrons. You only need to know how to work out the arrangement of electrons in each of the first 20 elements and their ions for high school.

For the exam, you need to be able to recognize elements from their electron arrangements and state what happens to electrons and the electron structure of atoms when they bond with other atoms, including the formation of ions.