Atomic Physics

Introduction

Modern atomic and nuclear physics is among the most impressive scientific achievements of the last century. Both the theories and techniques of atomic and nuclear physics have played an important role in the life sciences. The theories provided a solid foundation for understanding the structure and interaction of organic molecules, and the techniques provided many tools for both experimental and clinical work. Contributions from this field have been so numerous and influential that it is impossible to do them justice in a single chapter. We will present a brief description of the atom and the nucleus, which will lead into a discussion of the applications of atomic and nuclear physics to the life sciences.

1) Early Models of the Atom

By 1912, through the work of J. J. Thompson, E. Rutherford, and their colleagues, a number of important facts had been discovered about atoms which make up matter:

·   Atoms contain small negatively charged electrons and relatively heavier positively  
   charged protons.

·   The proton is about 2000 times heavier than the electron, but the magnitude of the
   charge on the two is the same.

·   There are as many positively charged protons in an atom as negatively charged
   electrons. The atom as a whole is, therefore, electrically neutral.

·   The identity of an atom is determined by the number of protons it has. For example,
   hydrogen has 1 proton, carbon has 6 protons, and silver has 47 protons.

Through a series of ingenious experiments, Rutherford explained his astonishing results by developing a new atomic model which was presented as follows:  

·   The positive charge in the atom was concentrated in a region that was small relative to
   the size of the atom. He called this concentration of positive charge the nucleus of the
   atom.

·   Any electrons belonging to the atom were assumed to be in the relative large volume
   outside the nucleus. To explain why these electrons where not pulled into the nucleus
   by the attractive electric force, Rutherford modeled them as moving in orbits around
   the nucleus in the same manner as the planets orbit the Sun. For this reason, this model
   is often referred to as the planetary model of the atom (Figure 1).

·   It was subsequently discovered that the nucleus also contains another particle, the
   neutron, which has approximately the same mass as the proton but is electrically
   neutral.

·   Although the nucleus contains most of the atomic mass, it occupies only a small part of
   the total atomic volume. The diameter of the whole atom is on the order of 10⁻⁸ cm,
   but the diameter of the nucleus is only about 10⁻¹³ cm.