Isotopes of silicon

Silicon (₁₄Si) has 25 known isotopes, with mass numbers ranging from 22 to 46. ²⁸Si (the most abundant isotope, at 92.23%), ²⁹Si (4.67%), and ³⁰Si (3.1%) are stable. The longest-lived radioisotope is ³²Si, which is produced by cosmic ray spallation of argon. Its half-life has been determined to be approximately 150 years (with decay energy 0.21 MeV), and it decays by beta emission to ³²P (which has a 14.27-day half-life)^[1] and then to ³²S. After ³²Si, ³¹Si has the second longest half-life at 157.3 minutes. All others have half-lives under 7 seconds.

A chart showing the relative abundances of the naturally occurring isotopes of silicon.

Isotopes of silicon (₁₄Si)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 28Si 92.2% stable 29Si 4.7% stable 30Si 3.1% stable 31Si trace 2.62 h β− 31P 32Si trace 153 y β− 32P
Standard atomic weight Ar°(Si)
	[28.084, 28.086][2] 28.085±0.001 (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
22Si	14	8	22.03611(54)#	28.7(11) ms	β+, p (62%)	21Mg	0+
					β+ (37%)	22Al
					β+, 2p (0.7%)	20Na
23Si	14	9	23.02571(54)#	42.3(4) ms	β+, p (88%)	22Mg	3/2+#
					β+ (8%)	23Al
					β+, 2p (3.6%)	21Na
24Si	14	10	24.011535(21)	143.2 (21) ms	β+ (65.5%)	24Al	0+
					β+, p (34.5%)	23Mg
25Si	14	11	25.004109(11)	220.6(10) ms	β+ (65%)	25Al	5/2+
					β+, p (35%)	24Mg
26Si	14	12	25.99233382(12)	2.2453(7) s	β+	26Al	0+
27Si	14	13	26.98670469(12)	4.117(14) s	β+	27Al	5/2+
28Si	14	14	27.97692653442(55)	Stable			0+	0.92223(19)	0.92205–0.92241
29Si	14	15	28.97649466434(60)	Stable			1/2+	0.04685(8)	0.04678–0.04692
30Si	14	16	29.973770137(23)	Stable			0+	0.03092(11)	0.03082–0.03102
31Si	14	17	30.975363196(46)	157.16(20) min	β−	31P	3/2+
32Si	14	18	31.97415154(32)	157(7) y	β−	32P	0+	trace	cosmogenic
33Si	14	19	32.97797696(75)	6.18(18) s	β−	33P	3/2+
34Si	14	20	33.97853805(86)	2.77(20) s	β−	34P	0+
34mSi	4256.1(4) keV			<210 ns	IT	34Si	(3−)
35Si	14	21	34.984550(38)	780(120) ms	β−	35P	7/2−#
					β−, n?	34P
36Si	14	22	35.986649(77)	503(2) ms	β− (88%)	36P	0+
					β−, n (12%)	35P
37Si	14	23	36.99295(12)	141.0(35) ms	β− (83%)	37P	(5/2−)
					β−, n (17%)	36P
					β−, 2n?	35P
38Si	14	24	37.99552(11)	63(8) ms	β− (75%)	38P	0+
					β−, n (25%)	37P
39Si	14	25	39.00249(15)	41.2(41) ms	β− (67%)	39P	(5/2−)
					β−, n (33%)	38P
					β−, 2n?	37P
40Si	14	26	40.00608(13)	31.2(26) ms	β− (62%)	40P	0+
					β−, n (38%)	39P
					β−, 2n?	38P
41Si	14	27	41.01417(32)#	20.0(25) ms	β−, n (>55%)	40P	7/2−#
					β− (<45%)	41P
					β−, 2n?	39P
42Si	14	28	42.01808(32)#	15.5(4 (stat), 16 (sys)) ms[5]	β− (51%)	42P	0+
					β−, n (48%)	41P
					β−, 2n (1%)	40P
43Si	14	29	43.02612(43)#	13(4 (stat), 2 (sys)) ms[5]	β−, n (52%)	42P	3/2−#
					β− (27%)	43P
					β−, 2n (21%)	41P
44Si	14	30	44.03147(54)#	4# ms [>360 ns]	β−?	44P	0+
					β−, n?	43P
					β−, 2n?	42P
45Si[6]	14	31	45.03982(64)#	4# ms			3/2−#
46Si[6]	14	32
This table header & footer: view

1. ^mSi – 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.

Silicon-28

Silicon-28, the most abundant isotope of silicon, is of particular interest in the construction of quantum computers when highly enriched, as the presence of ²⁹Si in a sample of silicon contributes to quantum decoherence.^[7] Extremely pure (>99.9998%) samples of ²⁸Si can be produced through selective ionization and deposition of ²⁸Si from silane gas.^[8] Due to the extremely high purity that can be obtained in this manner, the Avogadro project sought to develop a new definition of the kilogram by making a 93.75 mm (3.691 in) sphere of the isotope and determining the exact number of atoms in the sample.^[9][10]

Silicon-28 is produced in stars during the alpha process and the oxygen-burning process, and drives the silicon-burning process in massive stars shortly before they go supernova.^[11][12]

Silicon-29

Silicon-29 is of note as the only stable silicon isotope with a nonzero nuclear spin (I = 1/2).^[13] As such, it can be employed in nuclear magnetic resonance and hyperfine transition studies, for example to study the properties of the so-called A-center defect in pure silicon.^[14]

Silicon-34

Silicon-34 is a radioactive isotope with a half-life of 2.8 seconds.^[1] In addition to the usual N = 20 closed shell, the nucleus also shows a strong Z = 14 shell closure, making it behave like a doubly magic spherical nucleus, except that it is also located two protons above an island of inversion.^[15] Silicon-34 has an unusual "bubble" structure where the proton distribution is less dense at the center than near the surface, as the 2s_1/2 proton orbital is almost unoccupied in the ground state, unlike in ³⁶S where it is almost full.^[16][17] Silicon-34 is one of the known cluster decay emission particles; it is produced in the decay of ²⁴²Cm with a branching ratio of approximately 1×10⁻¹⁶.^[18]