# Isotopes of chlorine

Chlorine ($_{17}$Cl) has 25 isotopes, ranging from $^{28}$Cl to $^{52}$Cl, and two isomers, $^{34m}$Cl and $^{38m}$Cl. There are two stable isotopes, $^{35}$Cl (75.8%) and $^{37}$Cl (24.2%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is $^{36}$Cl, which has a half-life of 301,000 years. All other isotopes have half-lives under 1 hour, many less than one second. The shortest-lived are proton-unbound $^{29}$Cl and $^{30}$Cl, with half-lives less than 10 picoseconds and 30 nanoseconds, respectively; the half-life of $^{28}$Cl is unknown.

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{28}$Cl | 17 | 11 | 28.03035(54)# | | p | $^{27}$S | 1+# | | |
| $^{29}$Cl | 17 | 12 | 29.01505(20)# | 5.4(19) zs | p | $^{28}$S | (1/2+) | | |
| $^{30}$Cl | 17 | 13 | 30.005018(26) | <50 ns | p | $^{29}$S | 3+# | | |
| $^{31}$Cl | 17 | 14 | 30.9924481(37) | 190(1) ms | β$^+$ (97.6%) | $^{31}$S | 3/2+ | | |
| $^{31}$Cl | 17 | 14 | 30.9924481(37) | 190(1) ms | β$^+$, p (2.4%) | $^{30}$P | 3/2+ | | |
| $^{32}$Cl | 17 | 15 | 31.98568461(60) | 298(1) ms | β$^+$ (99.92%) | $^{32}$S | 1+ | | |
| $^{32}$Cl | 17 | 15 | 31.98568461(60) | 298(1) ms | β$^+$, α (0.054%) | $^{28}$Si | 1+ | | |
| $^{32}$Cl | 17 | 15 | 31.98568461(60) | 298(1) ms | β$^+$, p (0.026%) | $^{31}$P | 1+ | | |
| $^{33}$Cl | 17 | 16 | 32.97745199(42) | 2.5038(22) s | β$^+$ | $^{33}$S | 3/2+ | | |
| $^{34}$Cl | 17 | 17 | 33.973762490(52) | 1.5267(4) s | β$^+$ | $^{34}$S | 0+ | | |
| $^{34m}$Cl | 146.360(27) keV | 146.360(27) keV | 146.360(27) keV | 31.99(3) min | β$^+$ (55.4%) | $^{34}$S | 3+ | | |
| $^{34m}$Cl | 146.360(27) keV | 146.360(27) keV | 146.360(27) keV | 31.99(3) min | IT (44.6%) | $^{34}$Cl | 3+ | | |
| $^{35}$Cl | 17 | 18 | 34.968852694(38) | Stable | Stable | Stable | 3/2+ | 0.758(2) | |
| $^{36}$Cl | 17 | 19 | 35.968306822(38) | 3.013(15)×10$^5$ y | β$^−$ (98.1%) | $^{36}$Ar | 2+ | 7×10$^{−13}$ | |
| $^{36}$Cl | 17 | 19 | 35.968306822(38) | 3.013(15)×10$^5$ y | β$^+$ (1.9%) | $^{36}$S | 2+ | 7×10$^{−13}$ | |
| $^{37}$Cl | 17 | 20 | 36.965902573(55) | Stable | Stable | Stable | 3/2+ | 0.242(2) | |
| $^{38}$Cl | 17 | 21 | 37.96801041(11) | 37.230(14) min | β$^−$ | $^{38}$Ar | 2− | | |
| $^{38m}$Cl | 671.365(8) keV | 671.365(8) keV | 671.365(8) keV | 715(3) ms | IT | $^{38}$Cl | 5− | | |
| $^{39}$Cl | 17 | 22 | 38.9680082(19) | 56.2(6) min | β$^−$ | $^{39}$Ar | 3/2+ | | |
| $^{40}$Cl | 17 | 23 | 39.970415(34) | 1.35(3) min | β$^−$ | $^{40}$Ar | 2− | | |
| $^{41}$Cl | 17 | 24 | 40.970685(74) | 38.4(8) s | β$^−$ | $^{41}$Ar | (1/2+) | | |
| $^{42}$Cl | 17 | 25 | 41.973342(64) | 6.8(3) s | β$^−$ | $^{42}$Ar | (2−) | | |
| $^{42}$Cl | 17 | 25 | 41.973342(64) | 6.8(3) s | β$^−$, n? | $^{41}$Ar | (2−) | | |
| $^{43}$Cl | 17 | 26 | 42.974064(66) | 3.13(9) s | β$^−$ | $^{43}$Ar | (3/2+) | | |
| $^{43}$Cl | 17 | 26 | 42.974064(66) | 3.13(9) s | β$^−$, n? | $^{42}$Ar | (3/2+) | | |
| $^{44}$Cl | 17 | 27 | 43.978015(92) | 0.56(11) s | β$^−$ (>92%) | $^{44}$Ar | (2-) | | |
| $^{44}$Cl | 17 | 27 | 43.978015(92) | 0.56(11) s | β$^−$, n? (<8%) | $^{43}$Ar | (2-) | | |
| $^{45}$Cl | 17 | 28 | 44.98039(15) | 513(36) ms | β$^−$ (76%) | $^{45}$Ar | (3/2+) | | |
| $^{45}$Cl | 17 | 28 | 44.98039(15) | 513(36) ms | β$^−$, n (24%) | $^{44}$Ar | (3/2+) | | |
| $^{46}$Cl | 17 | 29 | 45.98525(10) | 232(2) ms | β$^−$, n (60%) | $^{45}$Ar | 2-# | | |
| $^{46}$Cl | 17 | 29 | 45.98525(10) | 232(2) ms | β$^−$ (40%) | $^{46}$Ar | 2-# | | |
| $^{46}$Cl | 17 | 29 | 45.98525(10) | 232(2) ms | β$^−$, 2n? | $^{44}$Ar | 2-# | | |
| $^{47}$Cl | 17 | 30 | 46.98972(22)# | 101(5) ms | β$^−$ (>97%) | $^{47}$Ar | 3/2+# | | |
| $^{47}$Cl | 17 | 30 | 46.98972(22)# | 101(5) ms | β$^−$, n? (<3%) | $^{46}$Ar | 3/2+# | | |
| $^{47}$Cl | 17 | 30 | 46.98972(22)# | 101(5) ms | β$^−$, 2n? | $^{45}$Ar | 3/2+# | | |
| $^{48}$Cl | 17 | 31 | 47.99541(54)# | 30# ms [>200 ns] | β$^−$? | $^{48}$Ar | | | |
| $^{48}$Cl | 17 | 31 | 47.99541(54)# | 30# ms [>200 ns] | β$^−$, n? | $^{47}$Ar | | | |
| $^{48}$Cl | 17 | 31 | 47.99541(54)# | 30# ms [>200 ns] | β$^−$, 2n? | $^{46}$Ar | | | |
| $^{49}$Cl | 17 | 32 | 49.00079(43)# | 35# ms [>200 ns] | β$^−$? | $^{49}$Ar | 3/2+# | | |
| $^{49}$Cl | 17 | 32 | 49.00079(43)# | 35# ms [>200 ns] | β$^−$, n? | $^{48}$Ar | 3/2+# | | |
| $^{49}$Cl | 17 | 32 | 49.00079(43)# | 35# ms [>200 ns] | β$^−$, 2n? | $^{47}$Ar | 3/2+# | | |
| $^{50}$Cl | 17 | 33 | 50.00827(43)# | 10# ms [>620 ns] | β$^−$ | $^{50}$Ar | | | |
| $^{50}$Cl | 17 | 33 | 50.00827(43)# | 10# ms [>620 ns] | β$^−$, n? | $^{49}$Ar | | | |
| $^{50}$Cl | 17 | 33 | 50.00827(43)# | 10# ms [>620 ns] | β$^−$, 2n? | $^{48}$Ar | | | |
| $^{51}$Cl | 17 | 34 | 51.01534(75)# | 5# ms [>200 ns] | β$^−$? | $^{51}$Ar | 3/2+# | | |
| $^{51}$Cl | 17 | 34 | 51.01534(75)# | 5# ms [>200 ns] | β$^−$, n? | $^{50}$Ar | 3/2+# | | |
| $^{51}$Cl | 17 | 34 | 51.01534(75)# | 5# ms [>200 ns] | β$^−$, 2n? | $^{49}$Ar | 3/2+# | | |
| $^{52}$Cl | 17 | 35 | 52.02400(75)# | 2# ms [>400 ns] | β$^−$? | $^{52}$Ar | | | |
| $^{52}$Cl | 17 | 35 | 52.02400(75)# | 2# ms [>400 ns] | β$^−$, n? | $^{51}$Ar | | | |
| $^{52}$Cl | 17 | 35 | 52.02400(75)# | 2# ms [>400 ns] | β$^−$, 2n? | $^{50}$Ar | | | |
| This table header & footer: view ||||||||||
* ↑ $^m$Cl – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Modes of decay: IT:Isomeric transition n:Neutron emission p:Proton emission
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ Used in radiodating water
* ↑ Cosmogenic nuclide

## Chlorine-36

Trace amounts of radioactive $^{36}$Cl exist in the environment, in a ratio of about 7×10$^{−13}$ to 1 with stable isotopes. $^{36}$Cl is produced in the atmosphere by spallation of $^{36}$Ar by interactions with cosmic ray protons. In the subsurface environment, $^{36}$Cl is generated primarily as a result of neutron capture by $^{35}$Cl or muon capture by $^{40}$Ca. $^{36}$Cl decays to either $^{36}$S (1.9%) or to $^{36}$Ar (98.1%), with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of $^{36}$Cl were produced by neutron irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of $^{36}$Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, $^{36}$Cl is also useful for dating waters less than 50 years before the present. $^{36}$Cl has seen use in other areas of the geological sciences, forecasts, and elements. In chloride-based molten salt reactors the production of $^{36}$ Cl by neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing $^{35}$ Cl). This produces a long lived radioactive product which has to be stored or disposed off. Isotope separation to produce pure $^{37}$ Cl can vastly reduce $^{36}$ Cl production, but a small amount might still be produced by (n,2n) reactions involving fast neutrons.

## Chlorine-37

Stable chlorine-37 makes up about 24.23% of the naturally occurring chlorine on earth. Variation occurs as chloride mineral deposits have a slightly elevated chlorine-37 balance over the average found in sea water and halite deposits.