Sub-atomic particles, isotopes, electronic configuration in sub-shells, ionisation energies and the evidence they give for shell structure.
Sub-atomic particles
An atom has a tiny dense nucleus of protons and neutrons, surrounded by electrons in shells. The relative charges and masses are:
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | (≈ 0) | −1 |
The proton (atomic) number defines the element; the nucleon (mass) number is protons + neutrons. The number of neutrons . In a neutral atom, protons = electrons.
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