In the following simulation, you are going to build a few atomic nuclei. For the purposes of this lesson, don't worry about the number of electrons.
In order to describe an element, and the proper isotope of that element, physicists have adopted a particular nomenclature that occurs very frequently when discussing nuclear physics. Whenever an atomic symbol is written, such as that for uranium (whose atomic symbol is a capital “U”), there are two numbers written on the left hand side of the symbol. The lower number is the number of protons in the nucleus of the atom, which tells what atom it is. The upper number is the total mass of the atom, which is the sum of the number of protons and neutrons in its nucleus. For example, a uranium 235 nucleus is represented as 235 92 U. This atom has 92 protons and 143 neutrons (235 – 92 = 143) in its nucleus.
In the following simulation, you are going to build a few atomic nuclei. For the purposes of this lesson, don't worry about the number of electrons.
| Click to Run |
Source: Build an atom, University of Colorado-Physics
Click on the plus sign to open the "symbol" window in the simulation for the first atom, then add two protons and two neutrons.
What atom have you created?
Interactive popup. Assistance may be required.
Helium-4 4 2 He
Is it stable or unstable?
Click Reset All
Now add 6 protons and 8 neutrons.
What atom have you created?
Is it stable or unstable?
In the simulation, you developed a stable and an unstable atom. How do you think nuclear stability and mass-energy equivalence are related?
In the next two sections, you will look more closely at two processes related to mass-energy equivalence, fission, and fusion.