The first periodic law, as stated by Mendeleev, is that the physical and chemical properties of elements are periodic functions of their atomic weights (atomic masses).
In simpler terms, Mendeleev noticed a repeating pattern (periodicity) when elements were arranged in order of increasing atomic weight. This meant that elements with similar properties would reappear at regular intervals, leading him to organize them into groups (vertical columns) in his periodic table.
Key Aspects of Mendeleev's First Periodic Law:
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Atomic Weight as the Basis: Mendeleev used atomic weight as the primary criterion for arranging elements.
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Periodicity of Properties: The law emphasizes that element properties (both physical and chemical) recur periodically.
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Group Placement: Elements with comparable characteristics were positioned in the same vertical column, or group, on the periodic table.
Example
While not a direct illustration of the law itself, the placement of elements like sodium (Na) and potassium (K) demonstrates its application. Both are highly reactive alkali metals with similar chemical behaviors, and Mendeleev correctly placed them in the same group based on his observed periodicity.
Table Summarizing the First Periodic Law
| Feature | Description |
|---|---|
| Underlying Principle | Periodicity of properties based on atomic weight. |
| Key Observation | Elements with similar properties reappear at regular intervals. |
| Organization | Elements are grouped into vertical columns based on recurring properties. |
