Write the drawbacks in Mendeleev's periodic table that let to its modification.

Explanation

**Step-by-Step Solution:**

1. **Introduction to Mendeleev's Periodic Table:**
- Mendeleev's periodic table was developed based on the atomic mass of elements. While it was a significant advancement in the classification of elements, it had several drawbacks that necessitated modifications.

2. **Drawback 1: Position of Isotopes**
- One major drawback was the placement of isotopes. Isotopes are atoms of the same element that have different atomic masses but the same atomic number. Mendeleev's table could not accommodate this because it classified elements solely based on atomic mass, leading to confusion as isotopes could not be placed separately.

3. **Drawback 2: Anomalous Pairs**
- Another significant issue was the existence of anomalous pairs. For example, Mendeleev placed argon (atomic mass 39.9) before potassium (atomic mass 39.1) to align with their chemical properties, which contradicted the atomic mass arrangement. Similarly, cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6), creating inconsistencies in the table.

4. **Drawback 3: Cause of Periodicity**
- Mendeleev defined periodicity as the repetition of similar properties among elements but failed to explain the underlying reason for this periodicity. He attributed it to atomic mass without a clear scientific basis, which was a limitation of his table.

5. **Drawback 4: Position of Lanthanides and Actinides**
- The placement of lanthanides and actinides was another drawback. In Mendeleev's periodic table, these series were not distinctly positioned, whereas modern periodic tables provide a separate section for these elements, reflecting their unique properties.

6. **Drawback 5: Convenience of the Periodic Table**
- Lastly, the convenience of using atomic masses was a drawback. Atomic masses often include decimal values, making them harder to remember compared to whole numbers of atomic numbers. This made it less practical for students and chemists to use Mendeleev's table effectively.

7. **Conclusion:**
- These drawbacks led to the development of the modern periodic table, which is based on atomic numbers rather than atomic masses, addressing the issues found in Mendeleev's original classification.

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