Finally, a nucleus may be altered by electron capture. In a nucleus with a low ratio of neutrons to protons, the nucleus captures one of its own orbital electrons. This immediately combines with a proton to form a new neutron and emit a neutrino (a high-energy particle with neither mass nor charge). The process increases the neutron-to-proton ratio of the nucleus; the daughter has the same mass number as the parent, but has an atomic number one less than the parent. There are three series by which naturally radioactive nuclei decay to stable ones: The Uranium Series, the Thorium Series, and the Actinium Series. Man-made radioactive nuclei decay similarly, with bismuth as the end product, via the Neptunium Series. These can be illustrated in tabular form and diagrammatically. The Actinium Series (Uranium-235 Series), for example, proceeds like this: THE URANIUM-235 SERIES Element Symbol Radiation Emitted Half-life Uranium ²³⁵U α 7.13 × 10⁸ years Thorium ²³¹Th β 25.6 hours Protactinium ²³¹Pa α 3.25 × 10⁴ years Actinium[16] ²²⁷Ac β (98.8%) and α (1.2%) 21.2 years Thorium ²²⁷Th α 18.17 days Francium ²²³Fr β 22 minutes Radium ²²³Ra α 11.7 days Radon ²¹⁹Rn α 4.0 seconds Polonium[16] ²¹⁵Po α (~100%) and β (~5 × 1.83 × 10⁻³ second 10⁻⁴%) Lead ²¹¹Pb β 36.1 minutes Astatine ²¹⁵At α ~10⁻⁴ second Bismuth[16] ²¹¹Bi α (99.7%) and β (0.3%) 2.15 minutes Polonium ²¹¹Po α 0.25 second Thallium ²⁰⁷Tl β 4.78 minutes Lead ²⁰⁷Pb — Stable [Illustration: The Uranium-235 Series] The Uranium (Uranium-238) Series proceeds like this: