Isotopes of chlorine

Chlorine (₁₇Cl) has two stable isotopes, ³⁵Cl (75.8%) and ³⁷Cl (24.2%), giving chlorine a standard atomic weight of 35.45. Artificical radioisotopes are known ranging from ²⁸Cl to ⁵²Cl, and there are also two isomers, ^34mCl and ^38mCl. The longest-lived radioactive isotope is ³⁶Cl, which has a half-life of 301,000 years. All other isotopes and isomers have half-lives under an hour, and most under 10 seconds.

Isotopes of chlorine (₁₇Cl)

Main isotopes[1] Decay Isotope abun­dance half-life (t1/2) mode pro­duct 35Cl 75.8% stable 36Cl trace 3.01×105 y β− 36Ar ε 36S 37Cl 24.2% stable
Standard atomic weight Ar°(Cl)
	[35.446, 35.457][2] 35.45±0.01 (abridged)[3]
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List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[4] [n 2][n 3]	Half-life[1] [n 4]	Decay mode[1] [n 5]	Daughter isotope [n 6]	Spin and parity[1] [n 7][n 4]	Natural abundance (mole fraction)
	Excitation energy							Normal proportion[1]	Range of variation
28Cl[5]	17	11	28.03035(54)#		p	27S	1+#
29Cl	17	12	29.01505(20)#	5.4(19) zs	p	28S	(1/2+)
30Cl	17	13	30.005018(26)	<50 ns[5]	p	29S	3+#
31Cl	17	14	30.9924481(37)	190(1) ms	β+ (97.6%)	31S	3/2+
					β+, p (2.4%)	30P
32Cl	17	15	31.98568461(60)	298(1) ms	β+ (99.92%)	32S	1+
					β+, α (0.054%)	28Si
					β+, p (0.026%)	31P
33Cl	17	16	32.97745199(42)	2.5038(22) s	β+	33S	3/2+
34Cl	17	17	33.973762490(52)	1.5267(4) s	β+	34S	0+
34mCl	146.360(27) keV			31.99(3) min	β+ (55.4%)	34S	3+
					IT (44.6%)	34Cl
35Cl	17	18	34.968852694(38)	Stable			3/2+	0.758(2)
36Cl[n 8]	17	19	35.968306822(38)	3.013(15)×105 y	β− (98.1%)	36Ar	2+	7×10−13[6][7][n 9]
					β+ (1.9%)	36S
37Cl	17	20	36.965902573(55)	Stable			3/2+	0.242(2)
38Cl	17	21	37.96801041(11)	37.230(14) min	β−	38Ar	2−
38mCl	671.365(8) keV			715(3) ms	IT	38Cl	5−
39Cl	17	22	38.9680082(19)	56.2(6) min	β−	39Ar	3/2+
40Cl	17	23	39.970415(34)	1.35(3) min	β−	40Ar	2−
41Cl	17	24	40.970685(74)	38.4(8) s	β−	41Ar	(1/2+)
42Cl	17	25	41.973342(64)	6.8(3) s	β−	42Ar	(2−)
					β−, n?	41Ar
43Cl	17	26	42.974064(66)	3.13(9) s	β−	43Ar	(3/2+)
					β−, n?	42Ar
44Cl	17	27	43.978015(92)	0.56(11) s	β− (>92%)	44Ar	(2-)
					β−, n? (<8%)	43Ar
45Cl	17	28	44.98039(15)	513(36) ms[8]	β− (76%)	45Ar	(3/2+)
					β−, n (24%)	44Ar
46Cl	17	29	45.98525(10)	232(2) ms	β−, n (60%)	45Ar	2-#
					β− (40%)	46Ar
					β−, 2n?	44Ar
47Cl	17	30	46.98972(22)#	101(5) ms	β− (>97%)	47Ar	3/2+#
					β−, n? (<3%)	46Ar
					β−, 2n?	45Ar
48Cl	17	31	47.99541(54)#	30# ms [>200 ns]	β−?	48Ar
					β−, n?	47Ar
					β−, 2n?	46Ar
49Cl	17	32	49.00079(43)#	35# ms [>200 ns]	β−?	49Ar	3/2+#
					β−, n?	48Ar
					β−, 2n?	47Ar
50Cl	17	33	50.00827(43)#	10# ms [>620 ns]	β−	50Ar
					β−, n?	49Ar
					β−, 2n?	48Ar
51Cl	17	34	51.01534(75)#	5# ms [>200 ns]	β−?	51Ar	3/2+#
					β−, n?	50Ar
					β−, 2n?	49Ar
52Cl	17	35	52.02400(75)#	2# ms [>400 ns]	β−?	52Ar
					β−, n?	51Ar
					β−, 2n?	50Ar
This table header & footer: view

1. ^mCl – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
5. Modes of decay:

   IT:	Isomeric transition
   n:	Neutron emission
   p:	Proton emission

6. Bold symbol as daughter – Daughter product is stable.
7. ( ) spin value – Indicates spin with weak assignment arguments.
8. Used in radiodating water
9. Cosmogenic nuclide

Stable isotope analysis

The representative terrestrial abundance of chlorine contains about is 24.2% of ³⁷Cl, with a normal range of 23.9–24.5% of chlorine atoms.^[2] When measuring deviations in isotopic composition, the usual reference point is "Standard Mean Ocean Chloride" (SMOC), although a NIST Standard Reference Material (975a) also exists. SMOC is known to have a ³⁷Cl/³⁵Cl ratio of 0.319627 ± 0.000199 (24.221% +/- 0.0015% chlorine-37)^[9] and to have an atomic weight of 35.4525.

There is known variation in the isotopic abundance of chlorine. This heavier isotope tends to be more prevalent in chloride minerals than in aqueous solutions such as seawater, although the isotopic composition of organochlorine compounds can vary in either direction from the SMOC standard in the range of several parts per thousand.^[2]

Chlorine-36

Main article: Chlorine-36

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about 7×10⁻¹³ to 1 with stable isotopes. ³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the subsurface environment, ³⁶Cl is generated primarily as a result of neutron capture by ³⁵Cl or muon capture by ⁴⁰Ca. ³⁶Cl decays to either ³⁶S (1.9%) or to ³⁶Ar (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 ³⁶Cl were produced by neutron irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years before the present. ³⁶Cl 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

Main article: Chlorine-37

Besides being a component of natural stable chlorine, the chief notability of this isotope is its use to detect solar neutrinos through inverse electron capture (producing the gas ³⁷Ar). This was used in the first detection at the Homestake experiment. Subsequently gallium-71 was found more suitable for this purpose, and used in GALLEX/GNO and SAGE.

See also

Daughter products other than chlorine

• Isotopes of argon
• Isotopes of sulfur
• Isotopes of phosphorus
• Isotopes of silicon