CDS Chemistry · Atomic Structure and Periodic Classification
Atomic Models: Dalton to Bohr
What Dalton's theory explained and could not, what Rutherford's gold-foil experiment found, and the one problem Bohr's model fixed.
Why this matters
Six CDS questions, three of them in a single 2025 (I) paper. Three ask what the gold-foil experiment discovered (the nucleus), and two ask what a model could NOT explain.
Concept 1 of 2: Dalton's atomic theory and its limits
Dalton pictured each atom as a tiny, solid, indivisible ball. That picture was enough to explain why elements combine in fixed ratios. It could say nothing about electrons, protons or neutrons, because for Dalton there was nothing inside an atom.
Definition
Dalton's theory (1808):
- Matter is made of indivisible atoms; atoms of one element are identical in mass and properties.
- Atoms combine in simple whole-number ratios and are neither created nor destroyed in a reaction.
- So it explains the laws of conservation of mass, constant composition and multiple proportions.
- It does not explain sub-atomic particles or isotopes, because it treats the atom as indivisible.
- Dalton was also the first to use symbols for elements (circles with marks). The letter symbols used today (H, O, Na) were proposed later by Berzelius.
| Statement | Explained by Dalton's theory? |
|---|---|
| Law of conservation of mass | Yes |
| Law of constant composition | Yes |
| Law of multiple proportions | Yes |
| Atoms contain electrons, protons and neutrons | No — the atom is indivisible in this theoryQ CDS 2025 (I): the presence of sub-atomic particles is what Dalton's theory does not explain. |
Practice this concept4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q58Moderate
Example 1 · Atomic Structure and Periodic Classification · Atomic Models
Which one of the following is not explained by the Dalton's atomic theory ?
Dalton's symbols versus today's symbols
A question asking who introduced the symbol of an element usually wants Dalton, who first used symbols. The one- and two-letter symbols in use today came later, from Berzelius. Read the stem for which one it means.
Concept 2 of 2: Rutherford's gold-foil experiment and its limit
Rutherford fired alpha particles at a very thin gold foil. Most went straight through, so an atom is mostly empty space. A very few bounced back, so all the positive charge and almost all the mass sit in a tiny centre: the nucleus. But an electron circling a nucleus should lose energy and spiral in, and this model had no answer to that.
Definition
The experiment and the models around it:
- Thomson (plum pudding): electrons sit in a sphere of positive charge. Thomson also discovered the electron.
- Rutherford (gold foil, alpha particles): most pass straight through, a few are deflected through large angles, about 1 in 20,000 bounces back. Conclusion: a small, dense, positively charged nucleus at the centre.
- What Rutherford's model could not explain: the stability of the atom. A circling electron is accelerating, so it should radiate energy and fall into the nucleus.
- Bohr fixed this: electrons move only in fixed orbits (energy levels) and do not radiate while in one.
- Chadwick discovered the neutron (1932); Goldstein's canal rays led to the proton.
| Scientist | Key result |
|---|---|
| J. J. Thomson | Discovered the electron; plum-pudding model |
| Ernest Rutherford | Gold-foil experiment: discovered the atomic nucleusQ Asked three times: CDS 2019 (II), 2020 (I) and 2025 (I). The answer is always the nucleus, never the proton or neutron. |
| Niels Bohr | Fixed orbits; explains why the atom is stableQ CDS 2025 (I): Rutherford's model could not explain the stability of the atom. |
| James Chadwick | Discovered the neutron |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q1Easy
Example 2 · Atomic Structure and Periodic Classification · Atomic Models
Scattering of -particles by a thin gold foil suggests the presence of
Gold foil found the nucleus, not the proton
The large deflections showed a positively charged nucleus. Options naming the electron, proton, neutron or 'isotopes of gold' are distractors.
Rutherford's weakness is stability
Rutherford's model does explain the nucleus, its positive charge and its small size compared with the atom. It does not explain why the circling electron does not fall in: the stability of the atom.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Dalton's atomic theory and its limits4 rows
| Statement | Explained by Dalton's theory? |
|---|---|
| Law of conservation of mass | Yes |
| Law of constant composition | Yes |
| Law of multiple proportions | Yes |
| Atoms contain electrons, protons and neutrons | No — the atom is indivisible in this theoryQ CDS 2025 (I): the presence of sub-atomic particles is what Dalton's theory does not explain. |
Rutherford's gold-foil experiment and its limit4 rows
| Scientist | Key result |
|---|---|
| J. J. Thomson | Discovered the electron; plum-pudding model |
| Ernest Rutherford | Gold-foil experiment: discovered the atomic nucleusQ Asked three times: CDS 2019 (II), 2020 (I) and 2025 (I). The answer is always the nucleus, never the proton or neutron. |
| Niels Bohr | Fixed orbits; explains why the atom is stableQ CDS 2025 (I): Rutherford's model could not explain the stability of the atom. |
| James Chadwick | Discovered the neutron |
Watch out for (3)
- Dalton's symbols versus today's symbols→ Dalton's atomic theory and its limits
- Gold foil found the nucleus, not the proton→ Rutherford's gold-foil experiment and its limit
- Rutherford's weakness is stability→ Rutherford's gold-foil experiment and its limit
Test yourself on Atomic Structure and Periodic Classification
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.