Development of the Atomic ModelAQA GCSE Chemistry: Revision notes
Section 1
How did Dalton's solid sphere model begin our understanding of atoms?
John Dalton (1808) proposed the first scientific atomic model, suggesting that all matter is made of tiny, indivisible solid spheres called atoms. His key ideas were:
- Each element consists of identical atoms
- Atoms of different elements have different masses
- Atoms cannot be created, destroyed, or changed into other atoms
- Compounds form when atoms combine in simple whole-number ratios
Dalton's model was revolutionary because it explained chemical reactions as rearrangements of atoms rather than transformations of matter. However, we now know atoms are not solid or indivisible – they have internal structure.
Examiners want you to state that Dalton believed atoms were indivisible solids. This is the key limitation of his model that was later disproven by Thomson's discovery of electrons.
Section 2
What did Thomson discover with the plum pudding model?
J.J. Thomson (1897) discovered that atoms contain negatively charged particles called electrons by studying electrical discharge in gases. This led him to propose the plum pudding model:
- Atoms consist of a positive charge distributed throughout (the 'pudding')
- Electrons are embedded within this positive material (the 'plums')
- Electrons can be removed from atoms
This was the first model to suggest atoms have internal structure. Thomson showed that:
- Atoms are not solid or indivisible
- Electrons are present in all atoms
- Electricity involves the movement of electrons
The plum pudding model was accepted for only about 10 years before new evidence led to its replacement.
The plum pudding model imagined atoms like a Christmas pudding with raisins (electrons) scattered throughout a positive 'pudding' – but this turned out to be completely wrong.
Students often confuse the plum pudding model with the nuclear model. Remember: plum pudding has electrons spread throughout; the nuclear model has electrons orbiting a nucleus.
Section 3
How did Rutherford's alpha particle scattering experiment change atomic theory?
Ernest Rutherford (1909) conducted a crucial experiment that disproved the plum pudding model and revealed the true structure of atoms:
The Experiment:
- Rutherford fired alpha particles (positive, helium nuclei) at a thin gold foil
- He observed how the particles were deflected using a fluorescent screen
- Most alpha particles passed straight through the foil with little or no deflection
- Some particles were deflected at large angles
- A very small number bounced straight back
Key Findings:
- Most of the atom is empty space (explains why most particles pass through)
- The positive charge is concentrated in a tiny, dense centre called the nucleus (explains deflection and bounce-back)
- Electrons must orbit this nucleus in the empty space (explains the atom's overall structure)
Why This Replaced the Plum Pudding Model: The plum pudding model predicted that alpha particles should be only slightly deflected as they pass through distributed positive charge. The observation of large deflections and bounce-backs proved the existence of a concentrated positive nucleus, which is incompatible with the plum pudding idea.
In exam answers, link the three types of observations (straight-through, deflected, bounced-back) to three pieces of evidence (empty space, dense nucleus, concentrated positive charge). This shows full understanding of the experiment.
If asked why some alpha particles bounce back: They approach the nucleus head-on and experience a strong repulsive force from the concentrated positive charge, reversing their direction. This proves the nucleus is very small and very positive.
Section 4
How did Bohr's model explain electron behaviour in shells?
Niels Bohr (1913) modified Rutherford's nuclear model to solve a major problem: classical physics predicted electrons should spiral into the nucleus and collapse. Bohr proposed that:
- Electrons orbit the nucleus in fixed shells (or energy levels) at specific distances
- Electrons in these shells have defined, fixed energies
- Electrons can jump between shells by absorbing or emitting energy
- An electron cannot exist between shells
Key Features of Bohr's Model:
| Feature | Explanation |
|---|---|
| Shells | Circular orbits at specific distances from the nucleus |
| Fixed energy levels | Each shell corresponds to a specific energy |
| Electron jumps | Electrons move between shells by absorbing/emitting energy (not by spiralling gradually) |
| Stability | Electrons don't collapse because they cannot move to lower energies below the first shell |
Bohr's model explained atomic spectra (the colours of light emitted by atoms) and successfully predicted the hydrogen atom's properties. However, later work showed electrons don't orbit like planets – they occupy probability clouds or orbitals (though Bohr's shell concept remains useful in GCSE chemistry).
Examiners want you to emphasise that Bohr proposed electrons orbit at specific distances and have defined energies. This explains why atoms don't collapse and why they emit/absorb light in discrete colours.
Think of electron shells like the floors of a building: electrons can be on floor 1, 2, or 3, but not between floors. When they jump up or down, they must travel the whole distance at once.
Section 5
How did Chadwick confirm the existence of neutrons?
James Chadwick (1932) discovered the neutron, completing our understanding of atomic structure.
The Problem: Scientists knew that:
- An atom's mass is much greater than the mass of its protons and electrons combined
- The nucleus contains protons (positive particles) but cannot contain electrons (negative)
- There must be something else in the nucleus to account for the missing mass
Chadwick's Experiment:
- He bombarded beryllium with alpha particles
- He observed radiation that was not deflected by electric or magnetic fields
- This proved the existence of a neutral particle in the nucleus – the neutron
Significance:
- Neutrons have similar mass to protons but carry no electrical charge
- Neutrons explain why some atoms of the same element have different masses (isotopes)
- The nuclear model was now complete: a nucleus containing protons and neutrons, surrounded by orbiting electrons
Chadwick's discovery allowed scientists to fully explain atomic mass, isotopes, and nuclear stability.
Chadwick's evidence for neutrons was that the radiation was not deflected by electric or magnetic fields, proving it was electrically neutral. This is the key observation examiners want to see.
Section 6
How have scientific models changed with new evidence?
The development of atomic models demonstrates a fundamental principle of science: models change when new experimental evidence becomes available.
Timeline of Changes:
| Year | Scientist | Model | Key Change | New Evidence |
|---|---|---|---|---|
| 1808 | Dalton | Solid sphere | Atoms are indivisible | Simple ratios in compounds |
| 1897 | Thomson | Plum pudding | Atoms have internal structure with electrons | Electrical discharge experiments |
| 1909 | Rutherford | Nuclear model | Nucleus exists; atom is mostly empty space | Alpha particle scattering |
| 1913 | Bohr | Shells model | Electrons orbit in fixed shells | Atomic spectra |
| 1932 | Chadwick | Proton-neutron nucleus | Nucleus contains neutrons | Bombardment experiments |
Why Models Change:
- Old evidence was incomplete: Dalton had no way to detect electrons
- New experiments provided contradictions: Plum pudding model couldn't explain Rutherford's results
- Technology improved: Better equipment allowed closer observation of atomic behaviour
- Understanding deepened: Each model answered previous questions but raised new ones
Modern Atomic Model: Today, atoms are understood to contain:
- A nucleus (protons + neutrons) at the centre
- Electrons in orbitals (not fixed orbits) around the nucleus
- Mostly empty space
This shows that scientific knowledge is provisional – models are the best current explanation, not absolute truth.
Examiners specifically ask students to 'explain how models change with evidence'. Always connect a specific piece of experimental evidence to a specific change in the model – don't just list the models.
Must Know
- Dalton (1808): Atoms are indivisible solid spheres – disproven by Thomson's discovery of electrons
- Thomson (1897): Proposed the plum pudding model with electrons embedded in positive charge – disproven by Rutherford's experiment
- Rutherford (1909): Alpha particle scattering experiment showed that atoms have a tiny, dense, positive nucleus surrounded by empty space – most alpha particles pass straight through, some deflect at large angles, very few bounce back
- Bohr (1913): Electrons orbit the nucleus in fixed shells at specific distances with defined energies – explained atomic spectra and solved the collapse problem
- Chadwick (1932): Discovered the neutron (neutral particle in nucleus) using bombardment – completed the nuclear model
- Key Principle: Scientific models change when new experimental evidence becomes available; models are provisional, not absolute truth
That's the notes covered.
Carry on to the next subtopic.