1. A known and constant decay rate: The isotope should decay at a predictable and unchanging rate, allowing scientists to accurately calculate the time elapsed since the organism died or the rock was formed.

2. A sufficiently long half-life: The half-life should be long enough to allow for dating of the desired time scale. For example, carbon-14 is used for dating relatively recent samples (up to 50,000 years old), while uranium-238 is used for dating much older samples (billions of years old).

3. A measurable abundance in the sample: The isotope should be present in a detectable amount in the sample being dated. This ensures that the dating method is reliable.

4. A known initial abundance: The initial abundance of the isotope in the system being dated should be known or estimated. This allows for accurate calculation of the age.

5. No contamination: The sample should not be contaminated with other isotopes or materials that could interfere with the dating process.

6. Stability of the daughter product: The daughter product of the radioactive decay should be stable and not undergo further decay.

7. Suitable for the material being dated: The isotope should be appropriate for the specific material being dated. For example, carbon-14 is used for dating organic materials, while uranium-lead dating is used for dating rocks.

Examples of isotopes used for dating:

* Carbon-14 (14C): Used for dating organic materials up to about 50,000 years old.

* Potassium-40 (40K): Used for dating rocks and minerals up to billions of years old.

* Uranium-238 (238U): Used for dating rocks and minerals up to billions of years old.

* Rubidium-87 (87Rb): Used for dating rocks and minerals up to billions of years old.

By using isotopes with these characteristics, scientists can accurately determine the ages of ancient artifacts, fossils, and geological formations.