The isotopes in beta-decay stable isobars that are also stable with regards to double beta decay with mass number "A" = 5, "A" = 8, 143 ≤ "A" ≤ 155, 160 ≤ "A" ≤ 162, and "A" ≥ 165 are theorized to undergo alpha decay. All other mass numbers (isobars) have exactly one theoretically stable nuclide). Those with mass 5 decay to helium-4 and a proton or a neutron, and those with mass 8 decay to two helium-4 nuclei; their half-lives (helium-5, lithium-5, and beryllium-8) are very short, unlike the half-lives for all other such nuclides with "A" ≤ 209, which are very long. (Such nuclides with "A" ≤ 209 are primordial nuclides except 146Sm.)