What are the limitations of Mendeleev's periodic
table?

Explanation

### Step-by-Step Text Solution:

1. **Anomalous Position of Hydrogen**:
- Hydrogen is placed in Group 1A (alkali metals) but it shares properties with both alkali metals and halogens (Group 7A). This duality makes its position in the periodic table controversial and not fixed.

2. **Anomalous Pairs of Elements**:
- In Mendeleev's table, some elements with higher atomic weights are placed before those with lower atomic weights. For example, argon (atomic weight 39.9) is placed before potassium (atomic weight 39.1), and cobalt (atomic weight 58.9) precedes nickel (atomic weight 58.7). This violates the principle of arranging elements in order of increasing atomic weight.

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