Element: Rubidium

General data

Element name	Rubidium
Chemical symbol	Rb
Atomic number Z	37
Year discovered	1861
Standard state (25°C)	Solid
Category	Alkali metal
Block	s
Atomic mass	85.468
CPK color	#702EB0

Physical properties

Melting point	312.46 K (39.31 °C)
Boiling point	961 K (688 °C)
Density	1.53 g/cm³

Energy properties

First ionization energy	4.177 eV
Electron affinity	0.468 eV

Atomic structure

Protons	37 (Atomic number)
Neutrons (Rb-85, most abundant isotope)	48
Electrons	37
Electron configuration	[Kr] 5s¹
Oxidation states	+1
Electronegativity (Pauling)	0.82
Atomic radius (van der Waals)	303 pm

Ionization energies - NIST ASD (37 states)

State	Charge	Ioniz. energy (eV)	Type
Rb I	0+	4.1771281 ± 0.0000012	measured
Rb II	1+	27.28954 ± 0.00006	measured
Rb III	2+	39.247 ± 0.003	semi-empirical
Rb IV	3+	52.2 ± 0.25	semi-empirical
Rb V	4+	68.44 ± 0.15	calculated
Rb VI	5+	82.9 ± 2	calculated
Rb VII	6+	98.67 ± 0.04	semi-empirical
Rb VIII	7+	132.79 ± 0.25	semi-empirical
Rb IX	8+	150.628 ± 0.025	measured
Rb X	9+	277.12 ± 0.12	measured
Rb XI	10+	313.1 ± 2.4	calculated
Rb XII	11+	356 ± 2.5	calculated
Rb XIII	12+	400 ± 3	calculated
Rb XIV	13+	443 ± 3	calculated
Rb XV	14+	502 ± 3	calculated
Rb XVI	15+	550 ± 3	calculated
Rb XVII	16+	601 ± 3	calculated
Rb XVIII	17+	654 ± 3	calculated
Rb XIX	18+	706 ± 2.2	calculated
Rb XX	19+	857 ± 3	semi-empirical
Rb XXI	20+	905.3 ± 1	semi-empirical
Rb XXII	21+	958.9 ± 1.4	semi-empirical
Rb XXIII	22+	1024 ± 4	semi-empirical
Rb XXIV	23+	1080 ± 3	semi-empirical
Rb XXV	24+	1125 ± 5	semi-empirical
Rb XXVI	25+	1242.5 ± 1.7	semi-empirical
Rb XXVII	26+	1294.57 ± 0.04	semi-empirical
Rb XXVIII	27+	3133.3 ± 1.8	semi-empirical
Rb XXIX	28+	3281 ± 6	semi-empirical
Rb XXX	29+	3443 ± 8	semi-empirical
Rb XXXI	30+	3600 ± 30	semi-empirical
Rb XXXII	31+	3815 ± 4	semi-empirical
Rb XXXIII	32+	3988 ± 3	semi-empirical
Rb XXXIV	33+	4214 ± 4	semi-empirical
Rb XXXV	34+	4356.865 ± 0.01	calculated
Rb XXXVI	35+	18305.886 ± 0.004	calculated
Rb XXXVII	36+	18965.5484 ± 0.002	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: Rubidium (Rb) — © images-of-elements.com (CC BY 3.0)

Isotopes (35)

A	N	Half-life	Decay
Rb-72	35	103 ns	p
Rb-73	36	< 81 ns	p
Rb-74	37	64.78 ms	EC/β⁺
Rb-75	38	19.0 s	EC/β⁺
Rb-76	39	36.5 s	EC/β⁺
Rb-77	40	3.78 min	EC/β⁺
Rb-78	41	17.66 min	EC/β⁺
Rb-79	42	22.8 min	EC/β⁺
Rb-80	43	33.4 s	EC/β⁺
Rb-81	44	4.571 h	EC/β⁺
Rb-82	45	1.2577 min	EC/β⁺
Rb-83	46	86.2 d	EC
Rb-84	47	32.82 d	EC/β⁺
Rb-85	48	stable	-
Rb-86	49	18.671 d	β⁻
Rb-87	50	4.97×10¹⁰ a	β⁻
Rb-88	51	17.775 min	β⁻
Rb-89	52	15.39 min	β⁻
Rb-90	53	158 s	β⁻
Rb-91	54	58.0 s	β⁻
Rb-92	55	4.48 s	β⁻
Rb-93	56	5.85 s	β⁻
Rb-94	57	2.704 s	β⁻
Rb-95	58	379.9 ms	β⁻
Rb-96	59	201.6 ms	β⁻
Rb-97	60	169.1 ms	β⁻
Rb-98	61	115 ms	β⁻
Rb-99	62	54 ms	β⁻
Rb-100	63	52 ms	β⁻
Rb-101	64	32 ms	β⁻
Rb-102	65	37 ms	β⁻
Rb-103	66	23 ms	β⁻
Rb-104	67	unknown	β⁻
Rb-105	68	unknown	β⁻
Rb-106	69	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