CK-12-Physics - Intermediate

http://www.ck12.org Chapter 25. Nuclear Physics

• Gamma decayis most penetrating, requiring four or more centimeters of lead to stop it. It has no electric
  charge, but always occurs in coordination with other decay.

Detailed study of these and other radiation types over a period of twenty years led to a detailed picture of the
nucleus. The key came when British physicist James Chadwick (1891-1974) identified a rare type of radiation as
particles he namedneutronssince they were electrically neutral (i.e. had no charge). This lead to a coherent view
of atomic nuclei.

• All nuclei are made of a set of protons and neutrons. These two particles are collectively callednucleons.


• Theatomic numberis the number of protons in the nucleus, expressed symbolically asZ. This uniquely
  defines the chemical symbol of the element. All hydrogen nuclei have one proton, which means atomic
  numberZ=1. All carbon nuclei have six protons, which means atomic numberZ=6.


• Theatomic mass numberis the total number of all nucleons in the nucleus, expressed symbolically as
  A. Nuclei of a given element may have different number of neutrons and hence differentA. Mass number is
  an integer that is not the same as the exact mass.


• The different masses of nuclei of an element are calledisotopesof that element, and are referred to by the
  element name followed by mass, like hydrogen-1 (one proton), hydrogen-2 (one proton and one neutron), or
  carbon-12 (six protons and six electrons).


• The nucleus is represented symbolically asAZX, whereXis the chemical symbol of the element. For example,
  a nitrogen nucleus might be^147 Nand oxygen-17 (an isotope of oxygen) is^178 O.


• Mass deficit: The difference in mass of an atomic nucleus and the sum of the masses of the atomic nucleus.


• Nuclear fission: The splitting of an atom with a large atomic mass into at least two relatively equal parts, with
  an accompanying release of energy.


• Nuclear fusion: The process in which the nuclei of lighter elements combine and form heavier elements, with
  an accompanying release of energy.

Important Constants

The proton, neutron, and electron can be measured in kilograms, but are more commonly measured in terms of

unified atomic mass units, abbreviatedu. One unified a.m.u. is defined as one-twelfth the mass of a carbon-12

nucleus.

Proton massmp= 1. 6726 × 10 −^27 kg= 1. 007287 u

Neutron mass mn= 1. 6749 × 10 −^27 kg= 1. 008665 u

Electron mass me= 9. 11 × 10 −^31 kg= 0. 000549 u

Check Your Understanding

a. How many neutrons are in the iron nucleus^5626 Fe?

Answer: The atomic mass numberAis equal to the atomic number (number of protons)Zplus the number of
neutrons in the nucleus.

Therefore, the number of neutrons in the nucleus must be 56− 26 =30 neutrons.

b. How many electrons are there in the^5626 Featom?

Answer:Since atoms are neutral, they must have an equal number of protons and electrons. Thus, there are 26
electrons in the atom.

2. True or False: The nuclei^147 Nand^157 Nare isotopes of the same element.