Isotopes of aluminium

Aluminium (Al) has 22 known isotopes from ²¹Al to ⁴²Al and 4 known isomers. Only ²⁷Al (stable isotope) and ²⁶Al (radioactive isotope, t_1/2 = 7.2 × 10⁵ y) occur naturally, however ²⁷Al has a natural abundance of 99.9+ %. Other than ²⁶Al, all radioisotopes have half-lives under 7 minutes, most under a second. Relative atomic mass is 26.9815386(8). ²⁶Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of ²⁶Al to ¹⁰Be has been used to study the role of sediment transport, deposition, and storage, as well as burial times, and erosion, on 10⁵ to 10⁶ year time scales.

Cosmogenic Aluminium-26 was first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial ²⁶Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further ²⁶Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that ²⁶Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists believe that the energy released by the decay of ²⁶Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
	excitation energy
19Al	13	6	19.0218#	<35 ns	p	18Mg
20Al	13	7	20.0194#	<35 ns	p	19Mg
21Al	13	8	21.02804(32)#	<35 ns	p	20Mg	1/2+#
22Al	13	9	22.01952(10)#	59(3) ms	β+ (96.7%)	22Mg	(3)+
					β+, 2p (2.5%)	20Ne
					β+, p (0.8%)	21Na
23Al	13	10	23.007267(20)	470(30) ms	β+ (92%)	23Mg	5/2+#
					β+, p (8%)	22Na
23mAl				~0.35 s			#79
24Al	13	11	23.9999389(30)	2.053(4) s	β+ (99.95%)	24Mg	4+
					β+, α (.0349%)	20Ne
					β+, p (.0159%)	23Na
24mAl	425.8(1) keV			131.3(25) ms	IT (82%)	24Al	1+
					β+ (18%)	24Mg
					β+, α	20Ne
25Al	13	12	24.9904281(5)	7.183(12) s	β+	25Mg	5/2+
26Al	13	13	25.98689169(6)	7.17(24)×105 years	β+	26Mg	5+	Trace
26mAl	228.305(13) keV			6.3452(19) s	β+	26Mg	0+
27Al	13	14	26.98153863(12)	Stable			5/2+	1.0000
28Al	13	15	27.98191031(14)	2.2414(12) min	β−	28Si	3+
29Al	13	16	28.9804450(13)	6.56(6) min	β−	29Si	5/2+
30Al	13	17	29.982960(15)	3.60(6) s	β−	30Si	3+
31Al	13	18	30.983947(22)	644(25) ms	β− (98.4%)	31Si	(3/2,5/2)+
					β−, n (1.6%)	30Si
32Al	13	19	31.98812(9)	31.7(8) ms	β− (99.3%)	32Si	1+
					β−, n (.7%)	31Si
32mAl	955.7(4) keV			200(20) ns			(4+)
33Al	13	20	32.99084(8)	41.7(2) ms	β− (91.5%)	33Si	(5/2+)#
					β−, n (8.5%)	32Si
34Al	13	21	33.99685(12)	56.3(5) ms	β− (87.5%)	34Si	4−#
					β−, n (12.5%)	33Si
35Al	13	22	34.99986(19)	38.6(4) ms	β− (74%)	35Si	5/2+#
					β−, n (26%)	34Si
36Al	13	23	36.00621(23)	90(40) ms	β− (69%)	36Si
					β−, n (31%)	35Si
37Al	13	24	37.01068(36)	10.7(13) ms	β−	37Si	3/2+
38Al	13	25	38.01723(78)	7.6(6) ms	β−	38Si
39Al	13	26	39.02297(158)	7.6(16) ms	β−	39Si	3/2+#
40Al	13	27	40.03145(75)#	10# ms [>260 ns]
41Al	13	28	41.03833(86)#	2# ms [>260 ns]			3/2+#
42Al	13	29	42.04689(97)#	1 ms

Notes

• 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.

References

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:

External links

• Aluminum isotopes data from The Berkeley Laboratory Isotopes Project's

Isotopes of magnesium	Isotopes of aluminium	Isotopes of silicon
Table of nuclides