 ﻿Physics Primer

Nuclear Reactions

Let's start with a very basic recap of chemistry and atomic physics:

atoms are made out of a positively charged nucleus (protons + neutrons) and a negatively charged electron cloud (electrons) 原子由带正电的原子核（质子+中子）和带负电荷的电子云（电子）组成。

a chemical element is identified by its number of protons which we generally refer to as the atomic number (Z). E.g. Hydrogen has one proton, and ; Oxygen has eight protons, and ; Uranium has 92 protons, and the sum of the number of protons and neutrons in a nucleus is generally called nucleon number or atomic mass number (A). Because the proton and neutron masses are very close to each other ( ), and the electron mass is negligible compared to the proton mass ( ), the total mass of an atom can be approximated to the atomic mass number x the mass of one proton/neutron.

a useful quantity is the unified atomic mass unit (u) that is approximately equal to the mass of one proton/neutron: . Then the mass of some element X is going to be mX=AX×u. E.g. . As we will see, this is not quite true. The mass of an element is not just the mass of its components; instead one must also include the mass equivalent for the binding energy between the protons and neutrons. 

two elements with the same number of protons, but different number of neutrons are called isotopes. E.g. U-238 and U-235 have 92 protons, but 146 and respectively 143 neutrons.

we are going to write when referring to an element X with atomic mass number A and atomic number Z. E.g.  .

we are often going to write down nuclear reactions such as: For each nuclear reaction we can write down two equalities: ， .

These two equalities impose the conservation of mass number ( ) and conservation of charge ( ）.

E.g.: In this nuclear reaction a neutron decays into a proton and two extra particles (an electron and a neutrino). The overall charge and atomic mass number are conserved. Uranium-238 decays into Thorium plus an alpha-particle (2 protons and 2 electrons). 铀-238衰变为钍加一个α粒子（2个质子和2个电子）。

for a nuclear reaction to be correct, atomic mass number and charge must be conserved. However, not all correct nuclear reactions are possible or likely. Factors like binding energy, half-life, stability, kinetic energy etc. can play an essential role in favoring some reactions over others.