The atom consists of a small, dense, positively charged nucleus surrounded by negatively charged electrons in orbits (energy levels).
Structure
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (negligible) | Orbits around the nucleus |
Key numbers
- Proton number (atomic number, ): the number of protons in the nucleus. This defines the element.
- Nucleon number (mass number, ): the total number of protons and neutrons in the nucleus.
where is the number of neutrons.
Size of the atom
- The nucleus is tiny compared to the atom (about 1/10,000 of the atom’s diameter).
- Most of the atom is empty space.
- Almost all the mass is concentrated in the nucleus.
The Rutherford scattering experiment
Alpha particles fired at thin gold foil showed:
- Most passed straight through (atom is mostly empty space).
- Some were deflected (nucleus is positively charged).
- A very few bounced back (nucleus is small, dense and contains most of the mass).
This replaced the earlier “plum pudding” model with the nuclear model.
Common errors and how to correct them
Saying electrons are in the nucleus. Electrons orbit the nucleus. Only protons and neutrons are inside the nucleus.
Confusing proton number and nucleon number. Proton number = protons only. Nucleon number = protons + neutrons.
How to apply this in an exam
State the locations and properties of all three particles. Know that the proton number defines the element and that the nucleus is small, dense and positive. Use the Rutherford experiment to justify the nuclear model.
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