IB Physics Sub-topic E3 Notes

Nuclides

In Topic D.1, you learned about atomic notation. In this, it is important to understand that mass number and charge regularly change in chemical reactions, giving rise to isotopes and ions, respectively. These are collectively termed nuclides.

1. Ions are nuclides that have lost or gained electrons, affecting their charge number
2. Isotopes are nuclides with different numbers of neutrons, affecting their mass number. Understandably, this affects their stability.

Nuclear reactions

The number of neutrons in a nucleus ultimately determines the stability of the nucleus and whether it will participate in nuclear reactions. There are two types of nuclear reactions:

1. Fusion - the joining of two nuclei to form a larger nucleus, releasing lots of energy. 

   An example is the fusion of H-2 and H-3 to form He-4 and one neutron. This is the process that occurs in the sun.

2. Fission - the splitting of a large nucleus into two smaller nuclei, releasing less energy, although still a large amount. 

   An example is the fission of a U-235 nucleus by a neutron into Ba-141, Kr-92 and 3 neutrons. This is the process that occurs in nuclear reactors.

Mass-energy equivalence

How the fission or fusion reaction produces energy is dictated by mass-energy equivalence. Mass-energy equivalence is a concept proposed by Einstein, which proposes that mass and energy are one and the same. This means if an object’s mass changes, its energy also changes. Mass and energy are related by the famous formula:

$\Delta E = \Delta mc^{2}$

So, just like energy is released when bonds form between atoms, mass is converted to energy and released when bonds form between nucleons. As a result, the mass of a nucleus is lower than the sum of the masses of the nucleons.

This is called the mass defect, defined as the difference in mass of a nucleus and the sum of its constituents. A consequence of mass equivalence is that atomic particle masses can also now be expressed in terms of their energy value.

In order to calculate this, we start by revisiting the masses of nuclear particles. All nuclear masses are based off Carbon-12, which has 6 protons and 6 neutrons and weighs 12.00 g mol^-1. Since writing proton and neutron masses out every time is tedious, their masses were redefined to be simpler to work with, the unified atomic mass unit (amu). Here, 1 amu = 1/12th the mass of a Carbon-12 nucleus (1.661 x 10^-27 kg). Then, 1 amu is associated with 1 u of energy = 931.5 MeV c^-2.

The table from Topic 7.1 can thus be updated to show these two added units: