Atoms Seen!
STM image of a silicon surface; each bright dot is a single atom.
Modern evidence of atoms
Scanning-tunnelling microscopy (STM) resolves individual silicon atoms as bright dots.
Seeing atoms turns a once-theoretical idea into observable fact.
Key Points:
- Quiz: Estimate how many atoms could fit across a human hair.
John Dalton
Portrait of John Dalton (1766–1844)
Father of atomic theory
Early-19th-century chemists needed a model to explain why elements combine in fixed mass ratios.
In 1808, English scientist John Dalton proposed that each element is made of tiny, indivisible atoms unique to it, laying the foundation for modern atomic theory.
Key Points:
- Explained the Law of Constant Proportions using atoms.
- Introduced the Law of Multiple Proportions, linking mass data to atomic ratios.
- First coherent scientific atomic theory—central to modern chemistry.
Early Symbols
Dalton’s 1808 table of elemental symbols — hover to see modern letters.
From Sketches to Letters
Dalton used patterned circles to depict the symbolism of elements.
Interpret each pattern by pairing it with today’s one- or two-letter symbol.
Key Points:
- Creative visuals made elements recognisable in small lists.
- System became cumbersome when reactions involved many symbols.
- Modern letters (H, O, Na…) are faster, universal, and easy to type.
Relative Mass Idea
Dalton's balance-scale analogy
Why compare, not weigh?
Atoms are far too light to place on a real balance. Instead, scientists compare one atom's mass with another, just as we compare equal fruit slices.
This comparison gives a relative atomic mass, expressed in atomic mass units (u), making calculations quick and meaningful.
Key Points:
- 1 u is defined as one-twelfth the mass of a carbon-12 atom.
- Relative masses let us rank atoms without knowing their tiny absolute masses.
- Dalton's fruit-slice analogy links laboratory weighing to everyday experience.