List any three observations which posed a challenge to Mendeleev's Periodic Law.

Explanation

### Step-by-Step Solution:

1. **Understanding Mendeleev's Periodic Law**:
- Mendeleev's Periodic Law states that the properties of elements are a periodic function of their atomic masses. This implies that when elements are arranged in order of increasing atomic mass, elements with similar properties appear at regular intervals.

2. **Identifying Observations that Challenge Mendeleev's Law**:
- Several observations posed challenges to Mendeleev's Periodic Law. Here are three significant ones:

3. **First Observation - Position of Hydrogen**:
- Hydrogen's placement in the periodic table is ambiguous. It shares properties with both alkali metals (Group 1) and halogens (Group 17). Mendeleev could not definitively place hydrogen in either group, leading to confusion about its classification.

4. **Second Observation - Position of Isotopes**:
- Isotopes are variants of the same element that have the same atomic number but different atomic masses. Mendeleev's periodic table was based on atomic mass, which meant that isotopes would need to occupy different positions in the table. However, since they are the same element, this created a contradiction in Mendeleev's arrangement.

5. **Third Observation - Inconsistency in Atomic Mass Order**:
- Mendeleev's periodic table did not consistently follow the order of atomic masses. In some instances, elements with higher atomic masses were placed before those with lower atomic masses to group similar elements together. This inconsistency undermined the reliability of the periodic law as it did not adhere to its own principle.

### Summary of Observations:
1. The ambiguous position of hydrogen in the periodic table.
2. The challenge posed by isotopes not fitting into Mendeleev's arrangement.
3. The inconsistency in the order of atomic masses within the table.