ABSTRACT
This chapter describes the behaviour of atoms, beginning with hydrogen and the hydrogen-like ions of helium and lithium. The energy levels of hydrogen-like ions are shown to depend on the square of the atomic number of the ion. Energy levels for atoms and ions with two electrons are shown to be more complex, because one of the electrons screens the charge of the nucleus to some extent from the other. The Pauli exclusion principle is described and shown to determine how quantum states are filled with electrons in heavier atoms. The resulting electron configuration gives rise to the structure seen in the periodic table of the elements and so gives rise to chemistry. Examples are presented of the outer shell electron configurations in alkali metals, noble gases, and halogens.
