Isotopes of an element are placed at the same place in the long form of the periodic table.

Answer

### Step-by-Step Solution:

1. **Understanding Isotopes**:
- Isotopes are variants of the same element that have the same atomic number but different mass numbers. This means they have the same number of protons but a different number of neutrons.
- For example, chlorine has isotopes like Chlorine-35 and Chlorine-37. Both have an atomic number of 17 (indicating they are chlorine), but their mass numbers differ due to the varying number of neutrons.

2. **Atomic Number and the Periodic Table**:
- The long form of the periodic table is organized based on the atomic number (Z) of elements. The atomic number is the number of protons in the nucleus of an atom and determines the identity of the element.
- According to the modern periodic law, the physical and chemical properties of elements are periodic functions of their atomic numbers.

3. **Placement of Isotopes in the Periodic Table**:
- Since isotopes of an element have the same atomic number, they occupy the same position in the periodic table. There is only one place for each atomic number in the periodic table.
- Therefore, regardless of the number of isotopes (like Protium, Deuterium, and Tritium for hydrogen), they will all be placed at the same location corresponding to their atomic number.

4. **Conclusion**:
- Isotopes are placed at the same position in the long form of the periodic table because they have the same atomic number, which is the basis for the arrangement of elements in the periodic table.