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

Explanation

### Step-by-Step Text Solution:

1. **Introduction to Mendeleev's Periodic Table:**
- Mendeleev's periodic table was one of the first systematic attempts to organize the known elements based on their atomic masses and properties. However, it had several drawbacks that led to the development of a more refined periodic table.

2. **Drawback 1: Position of Hydrogen:**
- One of the significant drawbacks of Mendeleev's periodic table was his inability to justify the position of hydrogen. Hydrogen shares properties with both alkali metals and halogens, making it difficult to place it in a specific group. Mendeleev did not provide a clear rationale for hydrogen's placement, which created confusion.

3. **Drawback 2: Irregularities in Atomic Mass:**
- Mendeleev arranged elements in order of increasing atomic mass, but this led to inconsistencies. For example, cobalt (Co) and nickel (Ni) have atomic masses that are not in a straightforward increasing order. Cobalt has a higher atomic mass than nickel, yet nickel is placed before cobalt in the periodic table. This irregularity in the order of atomic masses resulted in elements being grouped incorrectly.

4. **Conclusion:**
- Due to these drawbacks, particularly the ambiguous placement of hydrogen and the irregularities in atomic mass, Mendeleev's periodic table required modifications. The modern periodic table, which arranges elements according to their atomic number rather than atomic mass, resolved these issues and provided a more accurate representation of elemental properties.