Knowledge Organiser: Development of the Periodic Table
Key Scientists
- Döbereiner (1817): Triads — groups of 3 by mass
- Newlands (1866): Octaves — every 8th element similar
- Mendeleev (1869): Gaps, predictions, swaps
- Moseley (1913): Ordered by atomic number
Must-Know Facts
- Early tables: ordered by atomic mass (wrong basis)
- Mendeleev left gaps for undiscovered elements
- Mendeleev predicted properties of missing elements
- Predictions confirmed by discovering gallium, germanium, scandium
- Modern table: ordered by atomic number (protons)
- Noble gases added in 1890s as Group 0
Why Mendeleev Succeeded (4-mark answer)
- Left gaps for undiscovered elements (rather than forcing elements into wrong groups)
- Made predictions about the properties of missing elements (e.g. eka-silicon)
- Swapped elements out of strict mass order when properties demanded it
- Predictions were confirmed when gallium (1875) and germanium (1886) were discovered — this provided evidence for his table
Common Mistakes
- Saying Mendeleev's table was ordered by atomic number: He used atomic mass — Moseley introduced atomic number ordering in 1913
- Forgetting Mendeleev swapped some elements: He placed tellurium before iodine despite iodine's lower mass, because properties mattered more than strict mass order
- Saying Newlands was accepted: His Law of Octaves was ridiculed by the Chemical Society — it broke down at element 21 (scandium) where the pattern of repeating properties every 8 elements failed
- Noble gases "missing" from Mendeleev's table: They hadn't been discovered yet (argon found 1894) — this was not an error, it was a gap in human knowledge
Memory Aid
- "Mendeleev's Magic Gaps" — The key thing that made Mendeleev's table special was that he left gaps. All other mnemonic detail flows from this: gaps → predictions → discovered elements → proof.
- Timeline order: D-N-M-Mo — Döbereiner (1817), Newlands (1866), Mendeleev (1869), Moseley (1913). "Did Newton Meet Moses?" helps remember the order.
- Why Mendeleev beat Newlands: "Mendeleev Left Gaps and Predicted" (the four key words: Left, Gaps, Predicted, Swapped).