isotopes were not included in Mendeleev's periodic table.(T/F)

Explanation

To determine whether the statement "isotopes were not included in Mendeleev's periodic table" is true or false, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Isotopes**:
- Isotopes are atoms of the same element that have the same atomic number (number of protons) but different mass numbers (number of neutrons).

2. **Mendeleev's Periodic Table**:
- Mendeleev's periodic table was constructed based on the atomic masses of elements. He arranged the elements in order of increasing atomic mass and grouped them according to similar chemical properties.

3. **Position of Isotopes**:
- Since isotopes of an element have the same atomic number but different mass numbers, they would not have distinct positions in Mendeleev's table. Mendeleev’s arrangement did not account for variations in mass number since he focused on atomic mass.

4. **Historical Context**:
- Isotopes were discovered after Mendeleev's time. Mendeleev published his periodic table in 1869, while the concept of isotopes was introduced later in the early 20th century.

5. **Conclusion**:
- Given that isotopes were not recognized during Mendeleev's time and that his periodic table did not provide for their inclusion, the statement "isotopes were not included in Mendeleev's periodic table" is indeed true.

### Final Answer:
The statement is **True**.

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