Editing Curie (unit) (section)

==As a measure of quantity==
Units of activity (the curie and the becquerel) also refer to a quantity of radioactive atoms. Because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular [[radionuclide]], a predictable number will decay in a given time.  The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the [[specific activity]] of that radionuclide.

The activity of a sample decreases with time because of decay.

The rules of [[radioactive decay]] may be used to convert activity to an actual number of atoms. They state that 1&nbsp;Ci of radioactive atoms would follow the expression
: ''N'' (atoms) × ''λ'' (s{{sup|−1}}) = 1 Ci = 3.7 × 10{{sup|10}} Bq,
and so
: ''N'' = 3.7 × 10{{sup|10}} Bq / ''λ'',
where ''λ'' is the [[exponential decay|decay constant]] in s<sup>−1</sup>.

Here are some examples, ordered by half-life:
{| class="wikitable"
! Nuclide !! [[Atomic mass|Isotopic mass]] ([[Dalton (unit)|Da]]){{AME2020 II|ref}} !! Number of atoms in 1 gram !! Half-life{{NUBASE2020|ref}}{{efn|Note that NUBASE2020 uses the <em>tropical</em> year to convert between years and other units of time, not the [[Gregorian year]]. The relationship between years and other time units in NUBASE2020 is as follows: {{nowrap|1=1 y = 365.2422 d = 31 556 926 s}} }} !! Specific activity (Ci/g) !! Mass of 1 curie
|-
| [[bismuth-209|<sup>209</sup>Bi]] || 208.9803986 || 2.8816773×10<sup>21</sup> || 2.01×10<sup>19</sup> years || 8.51×10<sup>−17</sup> || 11.7 billion tonnes
|-
| [[platinum-190|<sup>190</sup>Pt]]|| 189.9599498 || 3.1702160×10<sup>21</sup> || 4.83×10<sup>11</sup> years || 3.90×10<sup>−9</sup> || 257 tonnes
|-
| [[samarium-147|<sup>147</sup>Sm]]|| 146.9149044 || 4.0990673×10<sup>21</sup> || 1.066×10<sup>11</sup> years || 2.28×10<sup>−8</sup> || 43.8 tonnes
|-
| [[thorium-232|<sup>232</sup>Th]] || 232.0380536 || 2.5953246×10<sup>21</sup> || 1.405×10<sup>10</sup> years || 1.10×10<sup>−7</sup> (0.110 μCi/g) || 9.12 tonnes
|-
| [[uranium-238|<sup>238</sup>U]] || 238.0507876 || 2.5297714×10<sup>21</sup> || 4.468×10<sup>9</sup> years || 3.36×10<sup>−7</sup> (0.336 μCi/g) || 2.98 tonnes
|-
| [[potassium-40|<sup>40</sup>K]] || 39.96399817 || 1.50689146×10<sup>22</sup> || 1.248×10<sup>9</sup> years || 7.18×10<sup>−6</sup> (7.17 μCi/g) || 140 kg
|-
| [[uranium-235|<sup>235</sup>U]] || 235.0439281 || 2.5621342×10<sup>21</sup> || 7.038×10<sup>8</sup> years || 2.16×10<sup>−6</sup> (2.16 μCi/g) || 463 kg
|-
| [[iodine-129|<sup>129</sup>I]] || 128.9049836 || 4.6717672×10<sup>21</sup> || 1.614×10<sup>7</sup> years || 1.72×10<sup>−4</sup> (172 μCi/g) || 5.82 kg
|-
| [[technetium-99|<sup>99</sup>Tc]] || 98.90624968 || 6.0887363×10<sup>21</sup> || 2.111×10<sup>5</sup> years || 1.71×10<sup>−4</sup> || 58.4 g
|-
| [[plutonium-239|<sup>239</sup>Pu]] || 239.0521616 || 2.5191744×10<sup>21</sup> || 2.411×10<sup>4</sup> years || 6.20×10<sup>−2</sup> || 16.1 g
|-
| [[plutonium-240|<sup>240</sup>Pu]] || 240.0538117 || 2.5086628×10<sup>21</sup> || 6561 years || 0.227 || 4.41 g
|-
| [[carbon-14|<sup>14</sup>C]] || 14.00324199 || 4.30053323×10<sup>22</sup> || 5700 years || 4.48 || 223 mg
|-
| [[radium-226|<sup>226</sup>Ra]] || 226.0254082 || 2.6643645×10<sup>21</sup> || 1600 years || 0.989 || 1.01 g
|-
| [[americium-241|<sup>241</sup>Am]] || 241.0568273 || 2.4982245×10<sup>21</sup> || 432.6 years || 3.43 || 292 mg
|-
| [[plutonium-238|<sup>238</sup>Pu]] || 238.0495582 || 2.5297845×10<sup>21</sup> || 87.7 years || 17.1 || 58.4 mg
|-
| [[caesium-137|<sup>137</sup>Cs]] || 136.9070893 || 4.3987063×10<sup>21</sup> || 30.04 years || 86.9 || 11.5 mg
|-
| [[strontium-90|<sup>90</sup>Sr]] || 89.9077279 || 6.6981347×10<sup>21</sup> || 28.91 years || 138 || 7.27 mg
|-
| [[plutonium-241|<sup>241</sup>Pu]] || 241.0568497 || 2.4982243×10<sup>21</sup> || 14.329 years || 104 || 9.66 mg
|-
| [[tritium|<sup>3</sup>H]] || 3.016049281320 || 1.996698393×10<sup>23</sup> || 12.32 years || 9.62×10<sup>3</sup> || 104 μg
|-
| [[radium-228|<sup>228</sup>Ra]] || 228.0310686 || 2.6409299×10<sup>21</sup> || 5.75 years || 273 || 3.67 mg
|-
| [[cobalt-60|<sup>60</sup>Co]] || 59.93381554 || 1.00479849×10<sup>22</sup> || 5.2714 years || 1.13×10<sup>3</sup> || 884 μg
|-
| [[polonium-210|<sup>210</sup>Po]] || 209.9828737 || 2.8679200×10<sup>21</sup> || 138.376 days || 4.49×10<sup>3</sup> || 223 μg
|-
| [[iodine-131|<sup>131</sup>I]] || 130.9061264 || 4.6003506×10<sup>21</sup> || 8.0249 days || 1.24×10<sup>5</sup> || 8.05 μg
|-
| [[iodine-123|<sup>123</sup>I]] || 122.9055898 || 4.8998103×10<sup>21</sup> || 13.2232 hours || 1.93×10<sup>6</sup> || 519 ng
|-
| [[lead-212|<sup>212</sup>Pb]] || 211.9918959 || 2.8407410×10<sup>21</sup> || 10.627 hours || 1.39×10<sup>6</sup> || 719 ng
|-
| [[francium-223|<sup>223</sup>Fr]] || 223.0197342 || 2.7002726×10<sup>21</sup> || 22.00 minutes || 3.83×10<sup>7</sup> || 26.1 ng
|-
| [[polonium-212|<sup>212</sup>Po]] || 211.9888680 || 2.8407816×10<sup>21</sup> || 294.4 nanoseconds || 1.81×10<sup>17</sup> || 5.53 ag
|}