Short Answer

Mendeleev’s periodic law states that the properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in order of increasing atomic mass, elements with similar chemical and physical properties appear at regular intervals.

Using this law, Mendeleev was able to organize the first periodic table, group elements with similar properties together, and predict the existence and properties of undiscovered elements, such as gallium and germanium.

Detailed Explanation :

Definition and Concept

Mendeleev, a Russian chemist, formulated his periodic law in 1869 while studying the known chemical elements. He noticed that when elements are arranged by increasing atomic mass, their chemical properties repeat periodically.

• This periodicity allowed Mendeleev to identify patterns in elemental properties.
• Elements with similar chemical behavior were grouped in columns (groups), forming the early structure of the periodic table.

Key Features of Mendeleev’s Periodic Law

1. Arrangement by Atomic Mass: Elements are arranged in ascending order of atomic mass.
2. Periodic Recurrence: Elements with similar chemical properties occur at regular intervals.
3. Group Similarities: Elements with analogous chemical and physical properties were placed in the same vertical column.
4. Gaps for Unknown Elements: Mendeleev left spaces in his table for elements that were not yet discovered, predicting their existence and properties.

Examples of Mendeleev’s Predictions

1. Gallium (eka-aluminium): Predicted density, melting point, and chemical behavior before discovery.
2. Germanium (eka-silicon): Predicted atomic mass, density, and oxide formula.
3. Scandium (eka-boron): Predicted properties matched the discovered element.

These predictions demonstrated the power and accuracy of his periodic law.

Significance of the Law

1. Systematic Classification: Provided a rational system to organize elements instead of arbitrary grouping.
2. Prediction of Elements: Allowed Mendeleev to foresee elements not yet discovered, proving the law’s usefulness.
3. Chemical Relationships: Showed a clear relationship between atomic mass and chemical properties.
4. Foundation for Modern Periodic Table: Laid the groundwork for the modern periodic law based on atomic number.
5. Educational Importance: Teaches the concept of periodicity in elements, which is fundamental in chemistry.

Limitations of Mendeleev’s Periodic Law

1. Based on Atomic Mass: Some elements, like iodine and tellurium, had to be swapped to maintain proper groupings.
2. Inaccuracy for Modern Understanding: The modern periodic law uses atomic number instead of atomic mass for precise arrangement.
3. Does Not Explain Isotopes: Cannot explain why isotopes have the same chemical properties but different atomic masses.
4. Exceptions: Some elements did not fit perfectly due to variation in physical properties.

Comparison with Modern Periodic Law

• Mendeleev’s Law: Properties vary periodically with atomic mass.
• Modern Periodic Law: Properties vary periodically with atomic number (Z).
• Modern law resolves inconsistencies present in Mendeleev’s arrangement and explains isotopes and atomic structure.

Applications of Mendeleev’s Periodic Law

1. Predicting New Elements: Allowed discovery of missing elements with predicted properties.
2. Chemical Research: Guided chemists to study unknown elements based on periodic patterns.
3. Understanding Periodicity: Helped explain trends in melting point, density, and valency.
4. Foundation for Periodic Table: Provided a framework for grouping elements in modern chemistry.
5. Industrial Applications: Helped in metal extraction and chemical synthesis using knowledge of periodic relationships.

Conclusion

Mendeleev’s periodic law states that the properties of elements are a periodic function of their atomic masses, allowing elements to be arranged in a systematic table. It revealed patterns in chemical behavior, enabled prediction of undiscovered elements, and formed the basis for the modern periodic table arranged by atomic number. Mendeleev’s work remains a cornerstone in chemistry, element classification, and understanding periodicity.