Answer
The six elements have Z = 2, 8, 20, 28, 50, and 82. Z=126 would also qualify, but that nuclide has not been observed in nature.
Work Step by Step
This is discussed on page 1450. The numbers result because of an exclusion principle for nuclei. Such elements have a greater number of stable isotopes than average. They have a higher binding energy per nucleon than neighboring elements.
