This property of nuclei to decay by themselves is called radioactivity. Radioactive nuclei decay at constant rates regardless of temperature, pressure, chemical combination, or physical state. The process goes on no matter what happens to the atom. In other words, the activity inside the nucleus is in no way affected by what happens to the ELECTRONS circling around it. (Only in very special cases can outside disturbances affect the radioactivity of a nucleus and then only slightly. For all practical purposes, rates of radioactive decay are constant.) Most radioactive nuclides have rapid rates of decay (and lose their radioactivity in a few days, or a few years, at most); most of these are known today only because they are produced artificially. Some of them may have been present at the time the solar system was formed, but they have since decayed to such insignificant fractions of their original amounts that they can no longer be detected. Only a few radioactive nuclides decay slowly enough to have been preserved to this day, and so are present in nature. They are listed in Table II. Table II RADIOACTIVE NUCLIDES WITH HALF-LIVES LARGE ENOUGH TO BE STILL PRESENT IN USEFUL AMOUNTS ON THE EARTH[6] PARENT Element DAUGHTER HALF-LIFE (years) Type of Decay Product Potassium-40 Argon-40 1.3 × 10⁹ (total) ELECTRON CAPTURE Calcium-40 BETA DECAY Vanadium-50 Titanium-50 ~6 × 10¹⁵ (total) Electron capture Chromium-50 Beta decay Rubidium-87 Strontium-87 4.7 × 10¹⁰ Beta decay Indium-115 Tin-115 5 × 10¹⁴ Beta decay Tellurium-123 Antimony-123 1.2 × 10¹³ Electron capture Lanthanum-138 Barium-138 1.1 × 10¹¹ (total) Electron capture Cerium-138 Beta decay Cerium-142 Barium-138 5 × 10¹⁵ ALPHA DECAY Neodymium-144 Cerium-140 2.4 × 10¹⁵ Alpha decay Samarium-147 Neodymium-143 1.06 × 10¹¹ Alpha decay Samarium-148 Neodymium-144 1.2 × 10¹³ Alpha decay Samarium-149 Neodymium-145 ~4 × 10¹⁴? Alpha decay Gadolinium-152 Samarium-148 1.1 × 10¹⁴ Alpha decay Dysprosium-156 Gadolinium-152 2 × 10¹⁴ Alpha decay Hafnium-174 Ytterbium-170 4.3 × 10¹⁵ Alpha decay Lutetium-176 Hafnium-176 2.2 × 10¹⁰ Beta decay Rhenium-187 Osmium-187 4 × 10¹⁰ Beta decay Platinum-190 Osmium-186 7 × 10¹¹ Alpha decay Lead-204 Mercury-200 1.4 × 10¹⁷ Alpha decay Thorium-232 Lead-208 1.41 × 10¹⁰ 6 Alpha + 4 beta[7]