Overview
This lecture reviews the history of classifying chemical elements, from early attempts to the development of the modern periodic table, highlighting key scientists and classification systems.
Early Classification Attempts
- Johann Dobereiner (1829) identified triads—groups of three elements with similar properties.
- In Dobereiner’s triads, the atomic mass of the middle element was roughly the average of the other two.
- This system was limited, as only a few triads fit the pattern.
Newlands’ Octaves
- John Newlands arranged 56 known elements by increasing atomic mass.
- Observed that every eighth element had similar properties, known as Newlands’ Octaves.
- System worked only up to calcium; did not fit newly discovered elements.
Mendeleev’s Periodic Table
- Dmitri Mendeleev (1869) arranged 63 elements by increasing atomic mass and similar properties.
- Mendeleev’s Law: properties of elements are a periodic function of their atomic masses.
- Table organized in horizontal rows (periods, 7 total) and vertical columns (groups, 8 total).
- Left blank spaces for undiscovered elements like eka-boron, eka-aluminium, and eka-silicon, predicting their properties.
- Noble gases were added later without rearranging other elements.
Limitations of Mendeleev’s Table
- No fixed position for hydrogen, as it shared traits with alkali metals and halogens.
- Some elements with higher atomic mass (e.g., cobalt) placed before those with lower mass (e.g., nickel).
- Grouped elements sometimes had different properties (e.g., manganese with halogens).
Modern Periodic Table and Law
- In 1913, Henry Moseley identified atomic number as the fundamental property for classification.
- Modern Periodic Law: properties of elements are a periodic function of their atomic number.
- The long form of the modern periodic table is most widely used today.
Key Terms & Definitions
- Dobereiner’s Triads — Groups of three elements with similar properties; the second has an atomic mass roughly equal to the average of the others.
- Newlands’ Octaves — Arrangement where every eighth element shares similar properties, analogous to musical octaves.
- Mendeleev’s Periodic Law — Properties of elements are a periodic function of their atomic masses.
- Periods — Horizontal rows in the periodic table (7 in total).
- Groups — Vertical columns in the periodic table (8 in Mendeleev’s version).
- Modern Periodic Law — Properties of elements are a periodic function of their atomic numbers.
- Atomic Number — The number of protons in an atom, defining the element.
Action Items / Next Steps
- Study details of the long form of the modern periodic table in upcoming lessons.