Short Answer

Modern periodic law states that the properties of elements are a periodic function of their atomic numbers (Z). This means that when elements are arranged in order of increasing atomic number, elements with similar chemical and physical properties appear at regular intervals.

Unlike Mendeleev’s law, which was based on atomic mass, modern periodic law explains the arrangement of all elements including isotopes. It forms the basis of the modern periodic table, allowing chemists to predict trends in reactivity, valency, and other chemical properties.

Detailed Explanation :

Definition of Modern Periodic Law

Modern periodic law, proposed after the discovery of the atomic number by Henry Moseley in 1913, states:

“The properties of elements are a periodic function of their atomic numbers.”

• Atomic number (Z): Number of protons in the nucleus of an atom
• Periodic function: Elements with similar chemical properties appear at regular intervals when arranged by Z

This law corrected inconsistencies in Mendeleev’s table, such as the placement of iodine and tellurium.

Key Features of Modern Periodic Law

1. Arrangement by Atomic Number:
   Elements are arranged in increasing order of atomic number, not atomic mass.
2. Periodic Recurrence:
   Properties of elements repeat periodically in a predictable pattern.
3. Grouping of Elements:
   Elements with similar chemical and physical properties are placed in columns (groups).
4. Explanation of Isotopes:
   Atomic number is unique for each element, so isotopes (same Z, different A) are correctly placed in the table without disrupting periodicity.

Structure of Modern Periodic Table

• Periods (Rows): Horizontal rows correspond to energy levels or shells.
• Groups (Columns): Vertical columns contain elements with similar valence electron configuration and chemical behavior.
• Blocks: s, p, d, f blocks indicate subshells being filled.

Advantages over Mendeleev’s Law

1. No Inconsistencies: Elements like iodine and tellurium are correctly placed.
2. Inclusion of All Elements: Includes noble gases and isotopes naturally.
3. Predictive Power: Can predict chemical reactivity, valency, and atomic structure trends.
4. Scientific Basis: Based on atomic number, which is precise and measurable.
5. Foundation for Modern Chemistry: Explains electron configuration, periodic trends, and chemical periodicity.

Examples of Periodic Trends Explained

1. Atomic Radius: Decreases across a period, increases down a group.
2. Ionization Energy: Increases across a period, decreases down a group.
3. Electronegativity: Increases across a period, decreases down a group.
4. Metallic and Non-metallic Properties: Metals are on the left, non-metals on the right.

Significance of Modern Periodic Law

1. Provides a rational basis for arranging elements.
2. Explains periodic trends in physical and chemical properties.
3. Accounts for isotopes and atomic structure.
4. Useful for predicting properties of unknown elements.
5. Helps in teaching, research, and industrial chemistry.

Historical Context

• Moseley’s Experiment (1913): Showed that each element has a unique atomic number using X-ray spectroscopy.
• Corrected limitations of Mendeleev’s periodic law, which was based on atomic mass.
• Led to the creation of the modern periodic table used today.

Applications

1. Chemical Research: Predicts behavior of new or unknown elements.
2. Industrial Chemistry: Guides the selection of elements for manufacturing processes.
3. Education: Explains trends in valency, reactivity, and chemical properties.
4. Atomic Structure Studies: Connects electron configuration to periodic trends.
5. Isotope Chemistry: Provides consistent placement of isotopes in the table.

Conclusion

Modern periodic law states that the properties of elements are periodic functions of their atomic numbers, correcting limitations of Mendeleev’s law. It provides a logical and scientific basis for the arrangement of all elements in the modern periodic table, explains chemical and physical periodic trends, accommodates isotopes, and serves as a foundation for modern chemistry, research, and industry.