TABLE OF CONTENTS Matter has distinct levels of structure. For example, atoms, once considered the ultimate building blocks, are themselves composed of nuclei and electrons. The nucleus, in turn, consists of protons and neutrons, which we now believe are made of quarks and gluons. Gaining an understanding of the fundamental structure of matter has not been an easy achievement, primarily because the dimensions of the constituents are so small. For example, the typical size of an atom is about 10 ?8cm, the average nucleus is about 10 ?12cm in diameter, neutrons and protons have radii of about 10 ?13cm, while electrons and quarks are believed to be without structure down to distances of at least 10 ?16cm (namely, they behave as particles of 
10 ?16cm in size).
The study of the structure of matter presents formidable challenges both experimentally and theoretically, simply because we are dealing with the sub-microscopic domain, where much of our classical intuition regarding the behavior of objects fails us. Experimental investigations of atomic spectra provided our first insights into atomic structure. These studies ultimately led to the birth of quantum mechanics, which beautifully explained, both qualitatively and quantitatively, not only the observed spectra and the structure of the atom, but also clarified the nature of chemical bonding, and a host of phenomena in condensed matter. The remarkable success of quantum theory in explaining atomic phenomena was mainly due to two reasons. First, the interaction responsible for holding the atom together...
