Nucleon Number, Proton Number & Nuclide Notation
Nucleon Number, Proton Number & Nuclide Notation
- To describe a nucleus we count two things and write them in a standard symbol.
Protons, neutrons and electrons
- The nucleus is made of protons (positive) and neutrons (no charge). Together these are called , because they are the particles inside the nucleus.
- The electrons (negative) move in the space around the nucleus. They spread through almost the whole of the atom but have almost none of its mass. A proton or neutron is about 2000 times heavier than an electron.
- The is the number of protons. It fixes which element the atom is: 6 protons is always carbon, 79 is always gold. It also fixes how many electrons a neutral atom has.
- The is the total number of protons and neutrons. So the number of neutrons is .
| particle | relative charge | relative mass |
|---|---|---|
| proton | +1e | 1 |
| neutron | 0 | 1 |
| electron | −1e | ≈ 1/2000 |
| alpha-particle | +2e | 4 |
- These masses are so small that we do not measure them in kilograms. Instead we use the (). One is one twelfth of the mass of a carbon-12 atom.
- In this unit . A proton and a neutron each have a mass of about , so the nucleon number is also roughly the mass of the nucleus in .
Writing a nuclide
Symbols
- = chemical symbol of the element ((no unit))
- = nucleon number (protons + neutrons) ((no unit))
- = proton number (protons only) ((no unit))
- The bottom number is the smaller one (protons only). The top number counts every nucleon. A is one exact combination of protons and neutrons.
- Gold has 79 protons and 118 neutrons, so its 197 nucleons are written .
- Learn this symbol well now. Every decay equation later in the chapter is just these numbers kept equal on both sides.
Worked example
Reading a nuclide
How many protons, neutrons and electrons are in a neutral atom of ?
- Protons = = 19. A neutral atom has the same number of electrons, so electrons = 19.
- Neutrons = = = 21.
Answer
Your turn— tap to reveal the worked answer (9702/21/O/N/25 Q6(a))
Tritium is . How many protons, neutrons and electrons does a neutral tritium atom have? (9702/21/O/N/25 Q6(a))
Answer: protons = = 1, so electrons = 1. Neutrons = = 2. So 1 proton, 2 neutrons, 1 electron.
Just how empty is the atom
The nucleus is about m across, while the whole atom is about m. That is a hundred thousand times bigger. So the nucleus is a tiny dot at the centre, and almost all of the atom is empty space. It is the same empty space that let the alpha-particles pass through the gold foil.