Introduction to Health Physics Chapter 3 Atomic and Nuclear structure

1. Introduction to Health PhysicsChapter 3 Atomic and Nuclear structure

2. ATOMIC STRUCTURE • Rutherford's Nuclear Atom • The British physicist Rutherford had postulated, in 1911, that the positive charge in an atom was concentrated in a central massive point called the nucleus and that the negative electrons were situated at some remote points, about one angstrom unit distant from the nucleus

3. ATOMIC STRUCTURE • Rutherford's Nuclear Atom

4. ATOMIC STRUCTURE • Rutherford's Nuclear Atom • Detailed analyses of many experimental data later showed the radius of the nucleus to be as follows: • r =1.2xl0-15 A1/3 m • The outer periphery of the atom, at a distance of about 5x10-11m from the nucleus, was thought to be formed by electrons, equal in number to the protons within the nucleus, distributed around the nucleus

5. ATOMIC STRUCTURE • Rutherford's Nuclear Atom • questions : • how are the electrons held in place outside the nucleus despite the attractive electrostatic forces, • what holds the positive charges in the nucleus together in the face of the repulsive electro static forces?

6. ATOMIC STRUCTURE • Bohr's Atomic Model • Classical electromagnetic theory, however, predicted that such an atom is unstable. • The electrons revolving in their orbits undergo continuous radial acceleration. • electrons should eventually spiral into the nucleus as they lose their kinetic energy by radiation. (bremsstrahlang )

7. ATOMIC STRUCTURE • Bohr's atomic model : • The orbital electrons can revolve around the nucleus only in certain fixed radii, called stationary states mvr = nh/2p • A photon is emitted only when an electron falls from one orbit to another orbit of lower energy

8. ATOMIC STRUCTURE • Bohr's atomic model :

9. ATOMIC STRUCTURE • Bohr's atomic model : • When the electron revolves around the nucleus

10. ATOMIC STRUCTURE • Excitation and lonization • when a sufficient amount of energy is impaired to raise the electron to an infinitely great orbit, that is, to remove it from the electrical field of the nucleus of the atom is said to be ionized, and the negative electron together with the remaining positively charged atom are called an ion pair. This process is called ionization

11. ATOMIC STRUCTURE • modifications of the Bohr Atom • The simple Bohr theory described above is inadequate • Examination of the spectral lines of hydrogen with spectroscopes of very high resolving power shows the lines to have a fine structure • To describe an atom completely, it is necessary to specify four quantum numbers

12. THE NUCLEUS • The Neutron and Nuclear Force • in 1932, by the British physicist Chadwick, of the third basic building block in nature: the neutron. • (Chadwick won the Nobel prize in 1935 for this discovery.) • This particle, whose mass is about the same as that of a proton, 1.67474 X 10-27kg. is electrically neutral • it also supplies the cohesive force that holds the nucleus together. This force is called the nuclear force. It is thought to act over an extremely short range about 2 to 3 X 10 -15 m

13. THE NUCLEUS • Isotopes • the number of neutrons within the nucleus is not constant • Isotopes of an element are atoms that contain the same number of positive nuclear charges and have the same extranuclear electronic structure but differ in the number of neutrons • Most elements contain several isotopes. The atomic weight of an element is the weighted average of the weights of the different isotopes of which the element is composed

14. THE NUCLEUS • The Atomic Mass Unit • Since C-12 was assigned an atomic weight of 12.0000, one amu is

15. THE NUCLEUS • Binding Energy • Mass defect • The difference between the atomic weight and the sum of the weights of the parts • = W - M W = ZmH + ( A-Z )mn M = atomic weight BE = ( W – M )amu × 931 MeV/amu

16. THE NUCLEUS • Binding Energy

17. THE NUCLEUS • Nuclear Stability

18. homework • 3.1, 3.2, 3.10, 3.14, 3.15, 3.17, 3.21