Phys141 Principles of Physical Science Chapter 10 Nuclear Physics Instructor: Li Ma Office: NBC 126 Phone: (713) Webpage: Department of Computer Science & Physics Texas Southern University, Houston Nov. 3, 2004

Topics To Be Discussed Symbols of the Elements The Atomic Nucleus Skip §10.3 to §10.7

About Atomic Nucleus The atomic nucleus and its properties have important impact on our society: –Advantage: Archeological dating Diagnosis & treatment diseases, esp. cancer Generation of electricity by nuclear energy Formation of new elements Shinning of the Sun and other stars Smoke detector Etc.

About Atomic Nucleus (cont) The atomic nucleus and its properties have important impact on our society (cont): –Disadvantage: Radiation damage Nuclear bomb Disposal of nuclear waste Etc.

Symbols of the Elements A brief history of how the concept of element arose and how elements are expressed in symbols –About 600 to 200 B.C., Greek philosophers speculated the basic substance or substances that make up matter Aristotle: 4 “elements”: earth, air, fire & water – wrong –Discovery and properties of the true elements are discussed in Chemical Elements (Chapter 11)

Symbols of the Elements (cont) How are these true elements symbolized? –Berzelius (in the early 1800s) started to use symbol notation for elements First one or two letters of the Latin name for each element –Since Berzelius’ time, most elements have been symbolized by the first one or two letters of the English name: C for carbon; O for oxygen; Ca for calcium First letter is always in upper case, second in lower case

Symbols of the Elements (cont) Periodic table: –Show the positions, names, and symbols of the 114 elements presently known Table 10.2 on page 228

The Atomic Nucleus All matter is made of atoms An atom is composed of negatively charged electrons that surround a nucleus The nucleus is the central core of an atom. It consists of positively charged protons and neutrons, which are electrically neutral

The Atomic Nucleus (cont) An electron and a proton have the same magnitude of electric charge, but opposite signed A proton and a neutron have almost the same mass, and are about 2000 times more than an electron Nuclear protons and neutrons are collectively called nucleons

The Atomic Nucleus (cont) Rutherford’s alpha-scattering experiment showed –The diameter of a nucleus is about m –The diameter of an atom (i.e. the orbits of the atom’s outer electrons) is about m Electron orbits determine the size of atoms The nucleus contributes 99.97% of the mass

The Atomic Nucleus (cont) Charge is due to moving of electrons, so it seems electron is a truly fundamental particle of matter However, further investigation have revealed that there still exist smaller particles called quarks Theoretically, six types of quarks exist (verified experimentally as well)

Numbers for An Element An element is defined as a substance in which all the atoms have the same number of protons The atomic number (Z) is the number of protons in the nucleus of each atom of that element The atomic number also represents the number of electrons in a neutral atom

Numbers for An Element (cont) Electrons may be gained or lost by an atom => ion of that same element –A sodium atom (Na) becomes a sodium ion (Na + ) by losing an electron The neutron number (N) is the number of neutrons in the nucleus The mass number (A) is the number of protons and neutrons in the nucleus, i.e. it’s the total number of nucleons

Numbers for An Element (cont) Atom of the same element can be different because of different numbers of neutrons in their nuclei Forms of atoms that have same number of protons but differ in their numbers of neutrons are known as the isotopes of that element –The isotopes of the same element have same Z but different N and different A

Numbers for An Element (cont) The general designation for a specific nucleus (for example, for element X): X A Z chemical symbol mass number atomic number N = A – Z

Numbers for An Element (cont) Determine the composition of an atom: –Use N = A – Z –Ex. The neutron number (N) for the following element is 10 The isotopes of an element have the same chemical properties but different physical properties F 19 9

The Strong Nuclear Force Two fundamental forces of nature: electromagnetic and gravitational The electromagnetic force between a proton and an electron is about times greater than the corresponding gravitational force The electromagnetic force is the only important force on the electrons in an atom

The Strong Nuclear Force (cont) In an atom, the protons and neutrons are packed together in the nucleus and electrons circulate around the nucleus in am empty space Coulomb’s law: like charges repel, unlike charges attract A strong force must exist in nucleus to hold the nucleus together

The Strong Nuclear Force (cont) The third fundamental force of nature strong nuclear force (or just strong force or nuclear force) acts on neutrons The strong nuclear force is a short range force (distance less than m) A weak nuclear force also exists –A short range force –Stronger than gravitational force, but very much weaker than strong nuclear force

Assignment Homework Assignment –Review Questions (page 255): 1,2,5,6,7,15 –Exercises (page 257): 2 –It’s due Monday, 11/15/04 Reading Assignment –Chapter 15: §15.5 – §15.7