Chemistry
From elements to molecules
Follow how substances became elements, atoms, molecules, reactions, and materials.
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1789
Elements and mass anchor quantitative chemistry
Antoine Lavoisier · Marie-Anne Paulze Lavoisier
The Traite elementaire organized chemistry around weighing, conservation of mass, and a practical list of substances not yet decomposed.
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1797
Definite proportions constrain compounds
Joseph Proust
Proust argued that a chemical compound contains its constituent elements in a fixed proportion by mass.
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1808
Atomic theory explains combining ratios
John Dalton
Dalton's chemical atomic theory treated each element as having characteristic atoms and explained compounds through whole-number combinations.
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1811
Atoms and molecules are distinguished
Amedeo Avogadro
Avogadro proposed that equal gas volumes under the same conditions contain equal numbers of molecules and that elemental gases can be molecular.
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1813–1814
Symbols and atomic weights standardize formulas
Jons Jacob Berzelius
Berzelius developed letter-based chemical symbols and a systematic program of relative atomic-weight measurements.
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1858
Avogadro's hypothesis clarifies atomic weights
Stanislao Cannizzaro
Cannizzaro used Avogadro's molecular hypothesis to distinguish atomic from molecular weights and circulated the argument at Karlsruhe.
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1869
Periodic table leaves predictive gaps
Dmitri Mendeleev · Julius Lothar Meyer
Mendeleev published an atomic-weight-based periodic system that left gaps and predicted properties; Meyer independently developed a closely related classification.
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1897
The electron enters chemical explanation
J. J. Thomson
Cathode-ray measurements identified a universal negatively charged particle smaller than atoms.
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1913
Atomic number replaces atomic weight order
Henry Moseley
Moseley's X-ray measurements tied each element to an integer nuclear charge and reordered the periodic table by atomic number.
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1913
Quantized shells connect atoms to spectra
Niels Bohr
Bohr proposed quantized electron states that explained hydrogen spectra and suggested a shell-based basis for chemical periodicity.
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1926
Wave mechanics defines atomic orbitals
Erwin Schrodinger
Schrodinger's wave equation described electrons through quantum states whose solutions became atomic orbitals.
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1916
Electron pairs explain covalent bonds
Gilbert N. Lewis · Walther Kossel · Irving Langmuir
Lewis proposed shared electron pairs and octets for covalent bonds, alongside Kossel's ionic picture and Langmuir's elaboration.
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1927
Quantum mechanics explains a covalent bond
Walter Heitler · Fritz London
Heitler and London applied quantum mechanics to the hydrogen molecule and showed how electron exchange stabilizes a covalent bond.
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1939
Bonding theories become a chemical language
Linus Pauling
Pauling synthesized quantum bonding, resonance, hybridization, electronegativity, and ionic-covalent character into a practical chemical framework.
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1928–1932
Molecular orbitals extend across whole molecules
Robert Mulliken · Friedrich Hund
Mulliken and Hund developed a molecular-orbital description in which electrons occupy states extending over all nuclei in a molecule.
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