Beta decay of an even–even nucleus produces an odd–odd nucleus, and vice versa. An even number of protons or of neutrons are more stable (higher binding energy) because of pairing effects, so even–even nuclei are much more stable than odd–odd. One effect is that there are few stable odd–odd nuclides, but another effect is to prevent beta decay of many even–even nuclei into another even–even nucleus of the same mass number but lower energy, because decay proceeding one step at a time would have to pass through an odd–odd nucleus of higher energy. Double beta decay directly from even–even to even–even skipping over an odd–odd nuclide is only occasionally possible, and even then with a half-life greater than a billion times the age of the universe. For example, the double beta emitter Cd has a half-life of 2.9 × 10 years. This makes for a larger number of stable even–even nuclides, with some mass numbers having two stable nuclides, and some elements (atomic numbers) having as many as seven.