Element: Thallium

General data

Element name	Thallium
Chemical symbol	Tl
Atomic number Z	81
Year discovered	1861
Standard state (25°C)	Solid
Category	Post-transition metal
Block	p
Atomic mass	204.38
CPK color	#A6544D

Physical properties

Melting point	577 K (304 °C)
Boiling point	1746 K (1473 °C)
Density	11.8 g/cm³

Energy properties

First ionization energy	6.108 eV
Electron affinity	0.2 eV

Atomic structure

Protons	81 (Atomic number)
Neutrons (Tl-205, most abundant isotope)	124
Electrons	81
Electron configuration	[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹
Oxidation states	+3, +1
Electronegativity (Pauling)	1.62
Atomic radius (van der Waals)	196 pm

Ionization energies - NIST ASD (81 states)

State	Charge	Ioniz. energy (eV)	Type
Tl I	0+	6.1082873 ± 0.0000012	measured
Tl II	1+	20.4283 ± 0.0006	measured
Tl III	2+	29.852 ± 0.0006	measured
Tl IV	3+	51.14 ± 0.04	measured
Tl V	4+	62.6 ± 0.4	measured
Tl VI	5+	80 ± 2	calculated
Tl VII	6+	97.9 ± 2	calculated
Tl VIII	7+	116 ± 2.1	calculated
Tl IX	8+	135 ± 2.2	calculated
Tl X	9+	158 ± 2.2	calculated
Tl XI	10+	177 ± 2.4	calculated
Tl XII	11+	198 ± 2.5	calculated
Tl XIII	12+	218.3 ± 2	semi-empirical
Tl XIV	13+	306.9 ± 1.4	semi-empirical
Tl XV	14+	340 ± 17	calculated
Tl XVI	15+	366 ± 19	calculated
Tl XVII	16+	392 ± 20	calculated
Tl XVIII	17+	439 ± 22	calculated
Tl XIX	18+	467 ± 24	calculated
Tl XX	19+	520 ± 30	calculated
Tl XXI	20+	570 ± 30	calculated
Tl XXII	21+	600 ± 30	calculated
Tl XXIII	22+	640 ± 30	calculated
Tl XXIV	23+	700 ± 30	calculated
Tl XXV	24+	760 ± 40	calculated
Tl XXVI	25+	820 ± 40	calculated
Tl XXVII	26+	880 ± 40	calculated
Tl XXVIII	27+	930 ± 50	calculated
Tl XXIX	28+	990 ± 50	calculated
Tl XXX	29+	1060 ± 50	calculated
Tl XXXI	30+	1110 ± 60	calculated
Tl XXXII	31+	1170 ± 60	calculated
Tl XXXIII	32+	1230 ± 60	calculated
Tl XXXIV	33+	1290 ± 60	calculated
Tl XXXV	34+	1350 ± 60	calculated
Tl XXXVI	35+	1625 ± 5	calculated
Tl XXXVII	36+	1681 ± 5	calculated
Tl XXXVIII	37+	1740 ± 4	calculated
Tl XXXIX	38+	1802 ± 4	calculated
Tl XL	39+	1862 ± 4	calculated
Tl XLI	40+	1920 ± 4	calculated
Tl XLII	41+	2014 ± 4	calculated
Tl XLIII	42+	2075 ± 4	calculated
Tl XLIV	43+	2140 ± 4	calculated
Tl XLV	44+	2202 ± 4	calculated
Tl XLVI	45+	2447 ± 4	calculated
Tl XLVII	46+	2508 ± 4	calculated
Tl XLVIII	47+	2574 ± 4	calculated
Tl XLIX	48+	2635 ± 4	calculated
Tl L	49+	2854 ± 4	calculated
Tl LI	50+	2925 ± 4	calculated
Tl LII	51+	3094 ± 4	calculated
Tl LIII	52+	3164.7 ± 1.9	semi-empirical
Tl LIV	53+	5234 ± 5	calculated
Tl LV	54+	5371 ± 5	calculated
Tl LVI	55+	5518 ± 5	calculated
Tl LVII	56+	5674 ± 5	calculated
Tl LVIII	57+	5824 ± 6	calculated
Tl LIX	58+	5969 ± 6	calculated
Tl LX	59+	6241 ± 7	calculated
Tl LXI	60+	6392 ± 10	calculated
Tl LXII	61+	6560 ± 12	calculated
Tl LXIII	62+	6714 ± 15	calculated
Tl LXIV	63+	7146 ± 19	calculated
Tl LXV	64+	7281 ± 24	calculated
Tl LXVI	65+	7430 ± 30	calculated
Tl LXVII	66+	7570 ± 40	calculated
Tl LXVIII	67+	8260 ± 40	calculated
Tl LXIX	68+	8420 ± 50	calculated
Tl LXX	69+	8730 ± 50	calculated
Tl LXXI	70+	8880 ± 60	calculated
Tl LXXII	71+	19070 ± 60	calculated
Tl LXXIII	72+	19440 ± 70	calculated
Tl LXXIV	73+	19840 ± 90	calculated
Tl LXXV	74+	20210 ± 90	calculated
Tl LXXVI	75+	22780 ± 100	calculated
Tl LXXVII	76+	23250 ± 110	calculated
Tl LXXVIII	77+	23850 ± 120	calculated
Tl LXXIX	78+	24234.1 ± 0.4	calculated
Tl LXXX	79+	96783.2 ± 0.3	calculated
Tl LXXXI	80+	98592.12 ± 0.25	calculated

Value type as given by NIST ASD: measured - from evaluated experimental data; calculated - from ab initio calculations (mostly highly ionised atoms that have not been measured; for the simplest ones, e.g. H I, the calculation is more accurate than measurement); semi-empirical - from interpolation, extrapolation or another semi-empirical method.

Photos

Photo of the element: Thallium (Tl) — © images-of-elements.com (CC BY 3.0)

Isotopes (42)

A	N	Half-life	Decay
Tl-176	95	5 ms	p
Tl-177	96	18 ms	α
Tl-178	97	254 ms	α
Tl-179	98	426 ms	α
Tl-180	99	1.09 s	EC/β⁺
Tl-181	100	2.9 s	EC/β⁺
Tl-182	101	1.9 s	EC/β⁺
Tl-183	102	6.9 s	EC/β⁺
Tl-184	103	9.5 s	EC/β⁺
Tl-185	104	19.5 s	EC/β⁺
Tl-186	105	3.4 s	α
Tl-187	106	≈ 51 s	EC/β⁺
Tl-188	107	71 s	EC/β⁺
Tl-189	108	2.3 min	EC/β⁺
Tl-190	109	2.8 min	EC/β⁺
Tl-191	110	unknown	EC/β⁺
Tl-192	111	9.6 min	EC/β⁺
Tl-193	112	21.6 min	EC/β⁺
Tl-194	113	33.3 min	EC/β⁺
Tl-195	114	1.15 h	EC/β⁺
Tl-196	115	1.84 h	EC/β⁺
Tl-197	116	2.83 h	EC/β⁺
Tl-198	117	5.3 h	EC/β⁺
Tl-199	118	7.42 h	EC/β⁺
Tl-200	119	26.1 h	EC/β⁺
Tl-201	120	3.0421 d	EC
Tl-202	121	12.471 d	EC
Tl-203	122	stable	-
Tl-204	123	3.783 a	β⁻
Tl-205	124	stable	-
Tl-206	125	4.202 min	β⁻
Tl-207	126	4.77 min	β⁻
Tl-208	127	3.053 min	β⁻
Tl-209	128	2.162 min	β⁻
Tl-210	129	1.30 min	β⁻
Tl-211	130	80 s	β⁻
Tl-212	131	31 s	β⁻
Tl-213	132	24 s	β⁻
Tl-214	133	11.0 s	β⁻
Tl-215	134	10 s	β⁻
Tl-216	135	6 s	β⁻
Tl-217	136	unknown	β⁻

Data sources

PubChem Periodic Table API - Retrieved on 29 June 2026 - licence: CC0
IUPAC CIAAW - Abridged Standard Atomic Weights 2024 - Table version: 2024 - licence: Public domain
NNDC Wallet Cards 2023 - Data as of 15 October 2024 - licence: Public domain
Images of Elements - Retrieved on 29 June 2026 - licence: CC BY 3.0

More about the data sources