8.1 Atomic Structure
Our understanding of the atom has evolved over time as scientists made new discoveries.
• Each new model explained observations better than the last
• The current model may still be refined in the future...
~1800 Dalton's Model
Atoms were thought to be tiny, solid spheres that could not be divided. Like tiny billiard balls - the simplest possible model.
1897 Thomson's Plum Pudding Model
J.J. Thomson discovered the electron. He proposed atoms were a ball of positive charge with negative electrons dotted inside - like plums in a pudding.
1909 Rutherford's Nuclear Model
The famous gold foil experiment proved atoms are mostly empty space with a tiny, dense, positive nucleus at the centre.
1913 Bohr's Model
Niels Bohr refined the model - electrons orbit the nucleus in fixed energy levels (shells), not randomly.
In 1909, Ernest Rutherford and his team fired alpha particles at a thin sheet of gold foil. The results were unexpected.
🔬 The Gold Foil Experiment
📊 Most passed through
Conclusion: The atom is mostly empty space
↗️ Some were deflected
Conclusion: The centre is positively charged
⬅️ A few bounced back
Conclusion: The nucleus is tiny, dense, and massive
The fact that some bounced back was "as if you fired a cannon shell at tissue paper and it came back and hit you" - Rutherford
• A tiny, dense nucleus at the centre
• Contains most of the atom's mass
• Has a positive charge
• Electrons orbit around it
• Most of the atom is empty space
An atom is made of three subatomic particles: protons, neutrons, and electrons.
Proton
Location: Nucleus
Mass: 1
Charge: +1
Neutron
Location: Nucleus
Mass: 1
Charge: 0
Electron
Location: Shells
Mass: ~1/1840
Charge: -1
Structure of an Atom
| Particle | Location | Relative Mass | Relative Charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells (orbiting) | Very small (~1/1840) | -1 |
• Electrons orbit in shells (energy levels)
• Protons and neutrons have roughly the same mass
• Electrons have almost no mass compared to protons/neutrons
• The positive protons and negative electrons balance out in a neutral atom
Every element has two important numbers that tell us about its atomic structure.
• Mass Number (A): Number of protons + neutrons
To find neutrons: Neutrons = Mass Number − Atomic Number
Element Notation
Carbon-12 has 6 protons, 6 neutrons (12-6=6), and 6 electrons
Step 1: Protons = Atomic Number = 11
Step 2: Neutrons = Mass Number − Atomic Number
Neutrons = 23 − 11 = 12
Step 3: In a neutral atom, Electrons = Protons = 11
Answer: Sodium has 11 protons, 12 neutrons, and 11 electrons
🔧 Atom Builder
Your Atom:
This is a neutral Carbon atom.
An ion is an atom that has gained or lost electrons, giving it an overall electric charge.
• Gain electrons → Negative ion (more electrons than protons)
Remember: Protons don't move - only electrons are transferred.
Positive Ion (+)
Lost electrons
More protons than electrons
Neutral Atom
Equal protons & electrons
No overall charge
Negative Ion (−)
Gained electrons
More electrons than protons
When it loses 1 electron:
• Protons: 11 (unchanged)
• Electrons: 10 (lost one)
• Overall charge: 11+ and 10− = +1
This is written as Na⁺ (sodium ion)
When it gains 1 electron:
• Protons: 17 (unchanged)
• Electrons: 18 (gained one)
• Overall charge: 17+ and 18− = −1
This is written as Cl⁻ (chloride ion)