- 1). Determine the ratio of neutrons and protons (neutrons/protons) and check for how close the answer is to 1, noting that the smaller the atomic number of the element, the closer to 1 the answer needs to be. Mark anything higher than 1.518 automatically as being unstable as that's the ratio of the heaviest known isotope.
- 2). Check whether the number of both protons and neutrons are even or if they are both odd. Atoms with even numbers of protons and neutrons are more stable than atoms with odd numbers of protons and neutrons.
- 3). Determine if the number of protons and/or neutrons equals one of the so-called "magic numbers" of 2, 8, 20, 28, 50, 82 or 126. An atom with protons and neutrons equal to magic numbers are especially stable while those with only the protons or neutrons equal to a magic number are less stable (but still stable).
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