Nucleon & proton number
- The nuclide notation of an element is used to describe the constituents of the nuclei
- An example of this notation for Lithium is:
- Where:
- Li = lithium
- 7 = mass number or nucleon number
- 3 = atomic number or proton number
- The mass number is the number of particles in the nucleus
- The nucleus contains protons plus neutrons
- The atomic number is the number of protons in the atom
- Since atoms are neutral, this is also equal to the number of electrons in the atom
- When given an atomic symbol, you can therefore figure out the number of protons, neutrons and electrons in the atom:
- Protons: The atomic number
- Electrons: Atoms are neutrals, so the number of negative electrons is equal to the number of positive protons. Therefore, this is also the atomic number
- Neutrons: Subtract the proton number from the nucleon number
- For the lithium atom, these numbers would be:
- Protons: 3
- Electrons: 3
- Neutrons: 7 − 3 = 4
- The term nucleon is the used to mean a particle in the nucleus – i.e. a proton or neutron
- The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons
Worked example
The atom is a neutral atom.
State the number of protons, neutrons and electrons in this atom.
Protons | Neutrons | Electrons | |
A | 77 | 192 | 77 |
B | 155 | 77 | 77 |
C | 192 | 77 | 192 |
D | 77 | 115 | 77 |
Answer: D
Step 1: Determine the nucleon and atomic numbers
- Nucleon number = 192
- Atomic number = 77
Step 2: Determine the number of protons
- Number of protons = atomic number
- Number of protons = 77
Step 3: Determine the number of neutrons
- Number of neutrons = nucleon number − atomic number
- Number of neutrons = 192 − 77
- Number of neutrons = 115
Step 3: Determine the number of electrons
- Number of electrons = number of protons
- Number of electrons = 77