Element: Cesium

General data

Element name	Cesium
Chemical symbol	Cs
Atomic number Z	55
Year discovered	1860
Standard state (25°C)	Solid
Category	Alkali metal
Block	s
Atomic mass	132.91
CPK color	#57178F

Physical properties

Melting point	301.59 K (28.44 °C)
Boiling point	944 K (671 °C)
Density	1.93 g/cm³

Energy properties

First ionization energy	3.894 eV
Electron affinity	0.472 eV

Atomic structure

Protons	55 (Atomic number)
Neutrons (Cs-133, most abundant isotope)	78
Electrons	55
Electron configuration	[Xe] 6s¹
Oxidation states	+1
Electronegativity (Pauling)	0.79
Atomic radius (van der Waals)	343 pm

Ionization energies - NIST ASD (55 states)

State	Charge	Ioniz. energy (eV)	Type
Cs I	0+	3.89390572743 ± 0.00000000017	measured
Cs II	1+	23.15745 ± 0.00006	measured
Cs III	2+	33.195 ± 0.004	semi-empirical
Cs IV	3+	43 ± 1.7	calculated
Cs V	4+	56 ± 1.9	calculated
Cs VI	5+	69.1 ± 2	calculated
Cs VII	6+	82.9 ± 2	calculated
Cs VIII	7+	110.1 ± 0.6	semi-empirical
Cs IX	8+	125.61 ± 0.09	semi-empirical
Cs X	9+	213.3 ± 2.5	measured
Cs XI	10+	233 ± 2.4	calculated
Cs XII	11+	261 ± 2.5	calculated
Cs XIII	12+	289 ± 3	calculated
Cs XIV	13+	316 ± 3	calculated
Cs XV	14+	352 ± 3	calculated
Cs XVI	15+	382 ± 3	calculated
Cs XVII	16+	413 ± 3	calculated
Cs XVIII	17+	445 ± 3	calculated
Cs XIX	18+	476 ± 3	calculated
Cs XX	19+	597 ± 4	calculated
Cs XXI	20+	629 ± 4	calculated
Cs XXII	21+	666 ± 4	calculated
Cs XXIII	22+	700 ± 4	calculated
Cs XXIV	23+	753 ± 4	calculated
Cs XXV	24+	791 ± 4	calculated
Cs XXVI	25+	875 ± 4	calculated
Cs XXVII	26+	916.1 ± 0.5	semi-empirical
Cs XXVIII	27+	1592 ± 4	calculated
Cs XXIX	28+	1672 ± 4	calculated
Cs XXX	29+	1757 ± 4	calculated
Cs XXXI	30+	1848 ± 4	calculated
Cs XXXII	31+	1936 ± 4	calculated
Cs XXXIII	32+	2029 ± 4	calculated
Cs XXXIV	33+	2137 ± 5	calculated
Cs XXXV	34+	2230 ± 5	calculated
Cs XXXVI	35+	2329 ± 5	calculated
Cs XXXVII	36+	2422 ± 5	calculated
Cs XXXVIII	37+	2683 ± 4	calculated
Cs XXXIX	38+	2767 ± 4	calculated
Cs XL	39+	2859 ± 4	calculated
Cs XLI	40+	2945 ± 4	calculated
Cs XLII	41+	3118 ± 4	calculated
Cs XLIII	42+	3214 ± 4	calculated
Cs XLIV	43+	3392 ± 4	calculated
Cs XLV	44+	3485 ± 4	calculated
Cs XLVI	45+	7989 ± 4	calculated
Cs XLVII	46+	8224 ± 4	calculated
Cs XLVIII	47+	8484 ± 4	calculated
Cs XLIX	48+	8726 ± 4	calculated
Cs L	49+	9350 ± 4	calculated
Cs LI	50+	9629 ± 4	calculated
Cs LII	51+	9974 ± 4	calculated
Cs LIII	52+	10208.78 ± 0.05	calculated
Cs LIV	53+	41861.08 ± 0.04	calculated
Cs LV	54+	42913.144 ± 0.02	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

no data

Isotopes (41)

A	N	Half-life	Decay
Cs-112	57	490 μs	p
Cs-113	58	16.9 μs	p
Cs-114	59	0.57 s	EC/β⁺
Cs-115	60	1.03 s	EC/β⁺
Cs-116	61	0.70 s	EC/β⁺
Cs-117	62	8.4 s	EC/β⁺
Cs-118	63	14 s	EC/β⁺
Cs-119	64	29.1 s	EC/β⁺
Cs-120	65	60.4 s	EC/β⁺
Cs-121	66	155 s	EC/β⁺
Cs-122	67	21.18 s	EC/β⁺
Cs-123	68	5.91 min	EC/β⁺
Cs-124	69	30.9 s	EC/β⁺
Cs-125	70	44.4 min	EC/β⁺
Cs-126	71	1.643 min	EC/β⁺
Cs-127	72	6.25 h	EC/β⁺
Cs-128	73	3.64 min	EC/β⁺
Cs-129	74	32.13 h	EC/β⁺
Cs-130	75	29.20 min	EC/β⁺
Cs-131	76	9.688 d	EC
Cs-132	77	6.480 d	EC/β⁺
Cs-133	78	stable	-
Cs-134	79	2.0656 a	β⁻
Cs-135	80	1.33×10⁶ a	β⁻
Cs-136	81	13.01 d	β⁻
Cs-137	82	30.007 a	β⁻
Cs-138	83	32.48 min	β⁻
Cs-139	84	9.25 min	β⁻
Cs-140	85	64.0 s	β⁻
Cs-141	86	24.86 s	β⁻
Cs-142	87	1.687 s	β⁻
Cs-143	88	1.795 s	β⁻
Cs-144	89	0.993 s	β⁻
Cs-145	90	588 ms	β⁻
Cs-146	91	322 ms	β⁻
Cs-147	92	232 ms	β⁻
Cs-148	93	151 ms	β⁻
Cs-149	94	112 ms	β⁻
Cs-150	95	81 ms	β⁻
Cs-151	96	59 ms	β⁻
Cs-152	97	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

More about the data sources