鍅沒有穩定的同位素，因此無法得到其標準原子量，但鍅存在有兩個天然放射性同位素。目前已知的有 35 種鍅的同位素，原子量從 199 到 233，其中最穩定的同位素是鍅 223，半衰期為 22 分鐘，由於是鈾 235的衰變產物，因此在地殼中微量存在，而另一個自然同位素是，半衰期較短，只有 49 分鐘，目前已知穩定性最差的鍅同位素是，半衰期為 86 奈秒。 鍅在前 105 個元素之中最不穩定。106 號元素𨭎和之後的元素雖然比鍅更不穩定，但因為目前發現的同位素數量不多，所以以後可能會發現更長壽的同位素。 圖表 199Fr styletextalignright 87 styletextalignright 112 19900726 4 16 7 ms 12 200Fr styletextalignright 87 styletextalignright 113 20000657 8 24 10 ms 196At 3 rowspan2 styletextindent1em 200mFr rowspan2 rowspan2 colspan3 styletextindent2em 60 110 keV rowspan2650 210 ms 196At rowspan210 rowspan2 IT （不常見） 200Fr rowspan2201Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 114 rowspan220100386 8 rowspan267 3 ms 99 197At rowspan292 rowspan2 1 201Rn rowspan2202Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 115 rowspan220200337 5 rowspan2290 30 ms 97 198At rowspan23 rowspan2 3 202Rn rowspan2 styletextindent1em 202mFr rowspan2 rowspan2 colspan3 styletextindent2em 330 90 keV rowspan2340 40 ms 97 198At rowspan210 rowspan2 3 202Rn rowspan2203Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 116 rowspan2203000925 17 rowspan2055 2 s 95 199At rowspan292 rowspan2 5 203Rn rowspan2204Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 117 rowspan2204000653 26 rowspan217 3 s 96 200At rowspan23 rowspan2 4 204Rn rowspan2 styletextindent1em 204m1Fr rowspan2 rowspan2 colspan3 styletextindent2em 50 4 keV rowspan226 3 s 90 200At rowspan27 rowspan2 10 204Rn styletextindent1em 204m2Fr colspan3 styletextindent2em 326 4 keV 17 6 s 10 rowspan2205Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 118 rowspan2204998594 8 rowspan2380 3 s 99 201At rowspan292 rowspan2 1 205Rn rowspan2206Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 119 rowspan220599867 3 rowspan216 s 58 206Rn rowspan223 rowspan2 42 202At styletextindent1em 206m1Fr colspan3 styletextindent2em 190 40 keV 159 1 s 7 styletextindent1em 206m2Fr colspan3 styletextindent2em 730 40 keV 700 100 ms 10 rowspan2207Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 120 rowspan220699695 5 rowspan2148 1 s 95 203At rowspan292 rowspan2 5 207Rn rowspan2208Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 121 rowspan220799714 5 rowspan2591 3 s 90 204At rowspan27 rowspan2 10 208Rn rowspan2209Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 122 rowspan2208995954 16 rowspan2500 3 s 89 205At rowspan292 rowspan2 11 209Rn rowspan2210Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 123 rowspan2209996408 24 rowspan2318 6 min 60 206At rowspan26 rowspan2 40 210Rn rowspan2211Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 124 rowspan2210995537 23 rowspan2310 2 min 80 207At rowspan292 rowspan2 20 211Rn rowspan2212Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 125 rowspan2211996202 28 rowspan2200 6 min 57 212Rn rowspan25 rowspan2 43 208At rowspan2213Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 126 rowspan2212996189 8 rowspan2346 3 s 9945 209At rowspan292 rowspan2 55 213Rn 214Fr styletextalignright 87 styletextalignright 127 213998971 9 50 2 ms 210At 1 styletextindent1em 214m1Fr colspan3 styletextindent2em 123 6 keV 335 5 ms 210At 8 styletextindent1em 214m2Fr colspan3 styletextindent2em 638 6 keV 103 4 ns 11 styletextindent1em 214m3Fr colspan3 styletextindent2em 6477Y keV 108 7 ns 33 215Fr styletextalignright 87 styletextalignright 128 215000341 8 86 5 ns 211At 92 rowspan2216Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 129 rowspan2216003198 15 rowspan2070 2 µs 212At rowspan21 rowspan2 2107 216Rn 217Fr styletextalignright 87 styletextalignright 130 217004632 7 168 19 µs 213At 92 218Fr styletextalignright 87 styletextalignright 131 218007578 5 10 6 ms 214At 1 rowspan2 styletextindent1em 218m1Fr rowspan2 rowspan2 colspan3 styletextindent2em 86 4 keV rowspan2220 5 ms 214At rowspan2 rowspan2 IT （不常見） 218Fr styletextindent1em 218m2Fr colspan3 styletextindent2em 200 150 keV high 219Fr styletextalignright 87 styletextalignright 132 219009252 8 20 2 ms 215At 92 rowspan2220Fr rowspan2 rowspan2 styletextalignright 87 rowspan2 styletextalignright 133 rowspan2220012327 4 rowspan2274 3 s 9965 216At rowspan21 rowspan2 35 220Ra rowspan3221Fr rowspan3 rowspan3 styletextalignright 87 rowspan3 styletextalignright 134 rowspan3221014255 5 rowspan349 2 min 999 217At rowspan352 rowspan3TraceIntermediate decay product of 237Np 1 221Ra 8791011 207Tl 222Fr styletextalignright 87 styletextalignright 135 222017552 23 142 3 min 222Ra 2 rowspan2223Fr rowspan2Actinium K rowspan2 styletextalignright 87 rowspan2 styletextalignright 136 rowspan22230197359 26 rowspan22200 7 min 9999 223Ra rowspan232 rowspan2TraceIntermediate decay product of 235U 006 219At 224Fr styletextalignright 87 styletextalignright 137 22402325 5 333 10 min 224Ra 1 225Fr styletextalignright 87 styletextalignright 138 22502557 3 40 2 min 225Ra 32 226Fr styletextalignright 87 styletextalignright 139 22602939 11 49 1 s 226Ra 1 227Fr styletextalignright 87 styletextalignright 140 22703184 11 247 3 min 227Ra 12 228Fr styletextalignright 87 styletextalignright 141 22803573 22 38 1 s 228Ra 2 229Fr styletextalignright 87 styletextalignright 142 22903845 4 502 4 s 229Ra 12 230Fr styletextalignright 87 styletextalignright 143 23004251 48 191 5 s 230Ra 231Fr styletextalignright 87 styletextalignright 144 23104544 50 176 6 s 231Ra 12 232Fr styletextalignright 87 styletextalignright 145 23204977 69 5 1 s 232Ra 233Fr styletextalignright 87 styletextalignright 146 233052518 21 900 100 ms 233Ra 12 參考文獻 Isotope masses from Ame2003 Atomic Mass Evaluation by G Audi AH Wapstra C Thibault J Blachot and O Bersillon in Nuclear Physics A729 2003 Halflife spin and isomer data selected from these sources Editing notes on this articles talk page Audi Bersillon Blachot Wapstra The Nubase2003 evaluation of nuclear and decay properties Nuc Phys A 729 pp 3128 2003 National Nuclear Data Center Brookhaven National Laboratory Information extracted from the NuDat 21 database retrieved Sept 2005 David R Lide ed Norman E Holden in CRC Handbook of Chemistry and Physics 85th Edition online version CRC Press Boca Raton Florida 2005 Section 11 Table of the Isotopes Fr