8.1 Atomic Structure
Our understanding of the atom has changed over time:
- Dalton (~1800): Thought atoms were tiny, solid spheres that could not be divided.
- Thomson (1897): Discovered the electron. He proposed the "Plum Pudding Model," where the atom was a ball of positive charge with negative electrons dotted inside it.
- Rutherford (1909): Conducted the gold foil experiment. He fired alpha particles at a thin sheet of gold.
His results were:
- Most alpha particles passed straight through. (Conclusion: The atom is mostly empty space).
- Some were deflected. (Conclusion: The centre is positively charged, repelling the positive alpha particles).
- A few bounced back. (Conclusion: The centre (the nucleus) is tiny, dense, and contains most of the mass).
This led to the nuclear model: a tiny, positive nucleus surrounded by orbiting electrons.
4. Bohr (1913): Refined this by suggesting electrons orbit in fixed energy levels or shells.
An atom is made of three subatomic particles:
| Particle | Location | Relative Mass | Relative Charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells (orbiting) | Very small (~1/1840) | -1 |
The Atomic Number tells you the number of protons. This defines what element it is.
The Mass Number tells you the total number of protons + neutrons.
In a neutral atom, the number of electrons = the number of protons.
An ion is an atom that has gained or lost electrons, giving it an overall charge.