Mendeleev's Periodic Law and Periodic Table

Statement: 'The physical and chemical properties of elements are a periodic function of their atomic masses.'

This means that when elements are arranged in increasing order of their atomic masses, elements with similar properties recur at regular intervals (periodically).

Mendeleev arranged 63 known elements in rows (Periods) and columns (Groups) based on atomic mass, grouping elements with similar chemical properties in the same column.

What made Mendeleev's table extraordinary was his boldness in predicting undiscovered elements. He left blank spaces in his table for elements not yet discovered and predicted their properties:

• Eka-Boron → Later discovered as Scandium (Sc)
• Eka-Aluminium → Later discovered as Gallium (Ga)
• Eka-Silicon → Later discovered as Germanium (Ge) The close match between his predictions and actual properties stunned the scientific world.

1. Anomalous pairs: Some elements with higher atomic mass were placed before elements with lower atomic mass (e.g., Argon 39.9 before Potassium 39.1, Cobalt before Nickel).
2. Position of Hydrogen: Hydrogen's position (Group 1 with alkali metals OR Group 17 with halogens) was unclear.
3. Isotopes: The same element's isotopes would have to be placed in different positions.
4. Modern law: Moseley (1913) showed properties are a function of atomic number, not atomic mass, leading to the Modern Periodic Law.