Isotopes of caesium

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Although caesium (Cs) has multiple isotopes, only one of these isotopes is stable; as such, it is considered a monoisotopic element.
Standard atomic mass: 132.9054519(2) u

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life nuclear
spin
representative
isotopic
composition
(mole fraction)
range of natural
variation
(mole fraction)
excitation energy
112Cs 55 57 111.95030(33)# 500(100) µs 1+#
113Cs 55 58 112.94449(11) 16.7(7) µs 5/2+#
114Cs 55 59 113.94145(33)# 0.57(2) s (1+)
115Cs 55 60 114.93591(32)# 1.4(8) s 9/2+#
116Cs 55 61 115.93337(11)# 0.70(4) s (1+)
116mCs 100(60)# keV 3.85(13) s 4+,5,6
117Cs 55 62 116.92867(7) 8.4(6) s (9/2+)#
117mCs 150(80)# keV 6.5(4) s 3/2+#
118Cs 55 63 117.926559(14) 14(2) s 2
118mCs 100(60)# keV 17(3) s (7-)
119Cs 55 64 118.922377(15) 43.0(2) s 9/2+
119mCs 50(30)# keV 30.4(1) s 3/2(+)
120Cs 55 65 119.920677(11) 61.2(18) s 2(-#)
120mCs 100(60)# keV 57(6) s (7-)
121Cs 55 66 120.917229(15) 155(4) s 3/2(+)
121mCs 68.5(3) keV 122(3) s 9/2(+)
122Cs 55 67 121.91611(3) 21.18(19) s 1+
122m1Cs 45.8 keV >1 µs (3)+
122m2Cs 140(30) keV 3.70(11) min 8-
122m3Cs 127.0(5) keV 360(20) ms (5)-
123Cs 55 68 122.912996(13) 5.88(3) min 1/2+
123m1Cs 156.27(5) keV 1.64(12) s (11/2)-
123m2Cs 231.63+X keV 114(5) ns (9/2+)
124Cs 55 69 123.912258(9) 30.9(4) s 1+
124mCs 462.55(17) keV 6.3(2) s (7)+
125Cs 55 70 124.909728(8) 46.7(1) min 1/2(+)
125mCs 266.6(11) keV 900(30) ms (11/2-)
126Cs 55 71 125.909452(13) 1.64(2) min 1+
126m1Cs 273.0(7) keV >1 µs
126m2Cs 596.1(11) keV 171(14) µs
127Cs 55 72 126.907418(6) 6.25(10) h 1/2+
127mCs 452.23(21) keV 55(3) µs (11/2)-
128Cs 55 73 127.907749(6) 3.640(14) min 1+
129Cs 55 74 128.906064(5) 32.06(6) h 1/2+
130Cs 55 75 129.906709(9) 29.21(4) min 1+
130mCs 163.25(11) keV 3.46(6) min 5-
131Cs 55 76 130.905464(5) 9.689(16) d 5/2+
132Cs 55 77 131.9064343(20) 6.480(6) d 2+
133Cs 55 78 132.905451933(24) STABLE 7/2+ 1.0000
134Cs 55 79 133.906718475(28) 2.0652(4) a 4+
134mCs 138.7441(26) keV 2.912(2) h 8-
135Cs 55 80 134.9059770(11) 2.3(3)E+6 a 7/2+
135mCs 1632.9(15) keV 53(2) min 19/2-
136Cs 55 81 135.9073116(20) 13.16(3) d 5+
136mCs 518(5) keV 19(2) s 8-
137Cs 55 82 136.9070895(5) 30.1671(13) a 7/2+
138Cs 55 83 137.911017(10) 33.41(18) min 3-
138mCs 79.9(3) keV 2.91(8) min 6-
139Cs 55 84 138.913364(3) 9.27(5) min 7/2+
140Cs 55 85 139.917282(9) 63.7(3) s 1-
141Cs 55 86 140.920046(11) 24.84(16) s 7/2+
142Cs 55 87 141.924299(11) 1.689(11) s 0-
143Cs 55 88 142.927352(25) 1.791(7) s 3/2+
144Cs 55 89 143.932077(28) 994(4) ms 1(-#)
144mCs 300(200)# keV <1 s (>3)
145Cs 55 90 144.935526(12) 582(6) ms 3/2+
146Cs 55 91 145.94029(8) 0.321(2) s 1-
147Cs 55 92 146.94416(6) 0.235(3) s (3/2+)
148Cs 55 93 147.94922(62) 146(6) ms
149Cs 55 94 148.95293(21)# 150# ms [>50 ms] 3/2+#
150Cs 55 95 149.95817(32)# 100# ms [>50 ms]
151Cs 55 96 150.96219(54)# 60# ms [>50 ms] 3/2+#

  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.


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