Francium (87Fr) has no stable isotopes, so a standard atomic weight cannot be given. Its most stable isotope is 223Fr with a half-life of 22 minutes, occurring in trace quantities in nature as an intermediate decay product of 235U.

Of elements whose most stable isotopes have been identified with certainty, francium is the most unstable, though astatine is the rarest in nature due to its naturally occurring isotopes having even shorter half-lives and even smaller branching ratios.

In the case of all elements with atomic number 106 (seaborgium) and above, no isotopes have been discovered whose half-life exceeds that of francium-223. However, the isotopes of these elements so far discovered are all on the proton-rich side of beta stability; so there is a strong possibility that some undiscovered isotopes of these elements will have longer half-lives, which could exceed that of francium-223.

List of isotopes

|-id=Francium-197 | 197Fr | | style="text-align:right" | 87 | style="text-align:right" | 110 | 197.01101(6) | style="text-align:center" | 2013 | 2.3(19) ms | α | 193At | (7/2−) | |-id=Francium-198 | 198Fr | | style="text-align:right" | 87 | style="text-align:right" | 111 | 198.01028(3) | style="text-align:center" | 2013 | 15(3) ms | α | 194At | 3+# | |-id=Francium-198m | style="text-indent:1em" | 198mFrOrder of ground state and isomer is uncertain. | | colspan="3" style="text-indent:2em" | 0(50) keV | style="text-align:center" | 2013 | 1.1(7) ms | α | 194At | (10−) | |-id=Francium-199 | 199Fr | | style="text-align:right" | 87 | style="text-align:right" | 112 | 199.007269(15) | style="text-align:center" | 1999 | 6.6(22) ms | α | 195At | 1/2+# | |-id=Francium-199m1 | style="text-indent:1em" | 199m1Fr | | colspan="3" style="text-indent:2em" | 45(13) keV | style="text-align:center" | 2013 | 6.5(9) ms | α | 195At | 7/2−# | |-id=Francium-199m2 | style="text-indent:1em" | 199m2Fr | | colspan="3" style="text-indent:2em" | 250(50)# keV | style="text-align:center" | 2013 | 2.2(12) ms | α | 195At | 13/2+# | |-id=Francium-200 | 200Fr | | style="text-align:right" | 87 | style="text-align:right" | 113 | 200.00658(3) | style="text-align:center" | 1995 | 47.5(28) ms | α | 196At | (3+) | |-id=Francium-200m1 | style="text-indent:1em" | 200m1Fr | | colspan="3" style="text-indent:2em" | 50(60) keV | style="text-align:center" | 2014 | 190(120) ms | α | 196At | 10−# | |-id=Francium-200m2 | style="text-indent:1em" | 200m2Fr | | colspan="3" style="text-indent:2em" | 150(50)# keV | style="text-align:center" | (2014) Isomer had only been suggested, not included in discovery database | 790(360) ns

|-id=Francium-201 | 201Fr | | style="text-align:right" | 87 | style="text-align:right" | 114 | 201.003852(10) | style="text-align:center" | 1980 | 62.8(19) ms | α | 197At | 9/2− | |-id=Francium-201m1 | style="text-indent:1em" | 201m1Fr | | colspan="3" style="text-indent:2em" | 129(10) keV | style="text-align:center" | 2005 | 24(6) ms | α | 197At | 1/2+ | |-id=Francium-201m2 | style="text-indent:1em" | 201m2Fr | | colspan="3" style="text-indent:2em" | 289.5(4) keV | style="text-align:center" | 2014 | 720(40) ns | IT | 201Fr | 13/2+ | |-id=Francium-202 | 202Fr | | style="text-align:right" | 87 | style="text-align:right" | 115 | 202.003330(6) | style="text-align:center" | 1980 | 372(12) ms | α | 198At | 3+ | |-id=Francium-202m | rowspan=2 style="text-indent:1em" | 202mFr | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 257(6) keV | rowspan=2 style="text-align:center" | 1992 | rowspan=2|286(13) ms | α | 198At | rowspan=2|10− | rowspan=2| |- | IT (?%) | 202Fr |-id=Francium-203 | 203Fr | | style="text-align:right" | 87 | style="text-align:right" | 116 | 203.000941(7) | style="text-align:center" | 1967 | 550(10) ms | α | 199At | 9/2− | |-id=Francium-203m1 | rowspan=2 style="text-indent:1em" | 203m1Fr | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 361(6) keV | rowspan=2 style="text-align:center" | 2013 | rowspan=2|43(4) ms | IT (80%) | 203Fr | rowspan=2|1/2+ | rowspan=2| |- | α (20%) | 199At |-id=Francium-203m2 | style="text-indent:1em" | 203m2Fr | | colspan="3" style="text-indent:2em" | 426.0(10) keV | style="text-align:center" | 2013 | 370(50) ns | IT | 203Fr | 13/2+

|-id=Francium-204 | rowspan=2|204Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 117 | rowspan=2|204.000652(26) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|1.75(26) s | α (96%) | 200At | rowspan=2|3+ | rowspan=2| |- | β+ (4%) | 204Rn |-id=Francium-204m1 | rowspan=2 style="text-indent:1em" | 204m1Fr | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 50(4) keV | rowspan=2 style="text-align:center" | 1967 | rowspan=2|2.41(19) s | α (90%) | 200At | rowspan=2|7+ | rowspan=2| |- | β+ (10%) | 204Rn |-id=Francium-204m2 | rowspan=2 style="text-indent:1em" | 204m2Fr | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 326(4) keV | rowspan=2 style="text-align:center" | 1992 | rowspan=2|1.65(15) s | α (53%) | 200At | rowspan=2|10− | rowspan=2| |- | IT (47%) | 204Fr |-id=Francium-205 | rowspan=2|205Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 118 | rowspan=2|204.998594(8) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|3.90(7) s | α (98.5%) | 201At | rowspan=2|9/2− | rowspan=2| |- | β+ (1.5%) | 205Rn |-id=Francium-205m1 | style="text-indent:1em" | 205m1Fr | | colspan="3" style="text-indent:2em" | 544.0(10) keV | style="text-align:center" | 2012 | 80(20) ns | IT | 205Fr | 13/2+ | |-id=Francium-205m2 | style="text-indent:1em" | 205m2Fr | | colspan="3" style="text-indent:2em" | 609(6) keV | style="text-align:center" | 2012 | 1.15(4) ms | IT | 205Fr | (1/2+)

|-id=Francium-206 | rowspan=2|206Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 119 | rowspan=2|205.998661(30) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|~16 s | α (88.4%) | 202At | rowspan=2|3+ | rowspan=2| |- | β+ (11.6%) | 206Rn |-id=Francium-206m1 | rowspan=3 style="text-indent:1em" | 206m1Fr | rowspan=3| | rowspan=3 colspan="3" style="text-indent:2em" | 200(40) keV | rowspan=3 style="text-align:center" | 2015 | rowspan=3|~16 s | α (84.7%) | 202At | rowspan=3|7+ | rowspan=3| |- | β+ (?%) | 206Rn |- | IT (?%) | 206Fr |-id=Francium-206m2 | rowspan=2 style="text-indent:1em" | 206m2Fr | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 730(40) keV | rowspan=2 style="text-align:center" | 1981 | rowspan=2|700(100) ms | IT (87%) | 206Fr | rowspan=2|10− | rowspan=2| |- | α (13%) | 202At |-id=Francium-207 | rowspan=2|207Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 120 | rowspan=2|206.996941(19) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|14.8(1) s | α (95%) | 203At | rowspan=2|9/2− | rowspan=2| |- | β+ (5%) | 207Rn |-id=Francium-208 | rowspan=2|208Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 121 | rowspan=2|207.997139(13) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|59.1(3) s | α (89%) | 204At | rowspan=2|7+ | rowspan=2| |- | β+ (11%) | 208Rn |-id=Francium-208m | style="text-indent:1em" | 208mFr | | colspan="3" style="text-indent:2em" | 826.3(5) keV | style="text-align:center" | 2009 | 432(11) ns | IT | 208Fr | 10−

|-id=Francium-209 | rowspan=2|209Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 122 | rowspan=2|208.995940(12) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|50.5(7) s | α (89%) | 205At | rowspan=2|9/2− | rowspan=2| |- | β+ (11%) | 209Rn |-id=Francium-209m | style="text-indent:1em" | 209mFr | | colspan="3" style="text-indent:2em" | 4659.8(7) keV | style="text-align:center" | 2006 | 420(18) ns | IT | 209Fr | 45/2−

|-id=Francium-210 | rowspan=2|210Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 123 | rowspan=2|209.996411(14) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|3.18(6) min | α (71%) | 206At | rowspan=2|6+ | rowspan=2| |- | β+ (29%) | 210Rn |-id=Francium-210m | style="text-indent:1em" | 210mFr | | colspan="3" style="text-indent:2em" | 4417.2(10) keV | style="text-align:center" | 2016 | 475(6) ns | IT | 210Fr | (23)+

|-id=Francium-211 | rowspan=2|211Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 124 | rowspan=2|210.995555(13) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|3.10(2) min | α (87%) | 207At | rowspan=2|9/2− | rowspan=2| |- | β+ (13%) | 211Rn |-id=Francium-211m1 | style="text-indent:1em" | 211m1Fr | | colspan="3" style="text-indent:2em" | 2423.16(24) keV | style="text-align:center" | 1986 | 146(14) ns | IT | 211Fr | 29/2+ | |-id=Francium-211m2 | style="text-indent:1em" | 211m2Fr | | colspan="3" style="text-indent:2em" | 4657.3(4) keV | style="text-align:center" | 1986 | 124.5(12) ns | IT | 211Fr | 45/2− | |-id=Francium-212 | rowspan=2|212Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 125 | rowspan=2|211.996225(9) | rowspan=2 style="text-align:center" | 1950 | rowspan=2|20.0(6) min | β+ (57%) | 212Rn | rowspan=2|5+ | rowspan=2| |- | α (43%) | 208At |-id=Francium-212m1 | style="text-indent:1em" | 212m1Fr | | colspan="3" style="text-indent:2em" | 1551.4(3) keV | style="text-align:center" | 1990 | 31.9(7) μs | IT | 212Fr | 11+ | |-id=Francium-212m2 | style="text-indent:1em" | 212m2Fr | | colspan="3" style="text-indent:2em" | 2492.2(4) keV | style="text-align:center" | 1977 | 604(28) ns | IT | 212Fr | 15− | |-id=Francium-212m3 | style="text-indent:1em" | 212m3Fr | | colspan="3" style="text-indent:2em" | 5854.7(6) keV | style="text-align:center" | 1986 | 312(21) ns | IT | 212Fr | 27− | |-id=Francium-212m4 | style="text-indent:1em" | 212m4Fr | | colspan="3" style="text-indent:2em" | 8533.4(11) keV | style="text-align:center" | 1990 | 23.6(21) μs | IT | 212Fr | 34+# | |-id=Francium-213 | rowspan=2|213Fr | rowspan=2| | rowspan=2 style="text-align:right" | 87 | rowspan=2 style="text-align:right" | 126 | rowspan=2|212.996184(5) | rowspan=2 style="text-align:center" | 1964 | rowspan=2|34.14(6) s | α (99.44%) | 209At | rowspan=2|9/2− | rowspan=2| |- | β+ (0.56%) | 213Rn |-id=Francium-213m1 | style="text-indent:1em" | 213m1Fr | | colspan="3" style="text-indent:2em" | 1590.41(18) keV | style="text-align:center" | 1976 | 505(14) ns | IT | 213Fr | 21/2− | |-id=Francium-213m2 | style="text-indent:1em" | 213m2Fr | | colspan="3" style="text-indent:2em" | 2537.62(23) keV | style="text-align:center" | 1976 | 238(6) ns | IT | 213Fr | 29/2+ | |-id=Francium-213m3 | style="text-indent:1em" | 213m3Fr | | colspan="3" style="text-indent:2em" | 8094.8(7) keV | style="text-align:center" | 1989 | 3.1(2) μs | IT | 213Fr | (65/2−) | |-id=Francium-214 | 214Fr | | style="text-align:right" | 87 | style="text-align:right" | 127 | 213.998971(9) | style="text-align:center" | 1966 | 5.51(13) ms | α | 210At | (1−) | |-id=Francium-214m1 | style="text-indent:1em" | 214m1Fr | | colspan="3" style="text-indent:2em" | 121(5) keV | style="text-align:center" | 1968 | 3.35(5) ms | α | 210At | (8−) | |-id=Francium-214m2 | style="text-indent:1em" | 214m2Fr | | colspan="3" style="text-indent:2em" | 638(5) keV | style="text-align:center" | 1994 | 103(4) ns | IT | 214Fr | (11+) | |-id=Francium-214m3 | style="text-indent:1em" | 214m3Fr | | colspan="3" style="text-indent:2em" | 6577(100)# keV | style="text-align:center" | 1994 | 108(7) ns | IT | 214Fr | (33+) | |-id=Francium-215 | 215Fr | | style="text-align:right" | 87 | style="text-align:right" | 128 | 215.000342(8) | style="text-align:center" | 1970 | 90(4) ns | α | 211At | 9/2− | |-id=Francium-216 |216Fr | | style="text-align:right" | 87 | style="text-align:right" | 129 | 216.003190(4) | style="text-align:center" | 1970 | 700(20) ns | α | 212At | (1−) | |-id=Francium-216m