MetalloidElement 5 · p-block

Boron (B)

Boron (B) is a metalloid with atomic number 5 and a standard atomic weight of 10.81. At room temperature it is a solid that melts at 2,077 °C and boils at 4,000 °C.

5BBoron10.81

Atomic number

protons

Atomic weight

10.81

CIAAW 2024

Melting point

2,077 °C

2,350.15 K

Boiling point

4,000 °C

4,273.15 K

Density

2.34 g/cm³

near room temperature

Electronegativity

2.04

Pauling scale

State at 25 °C

Solid

Discovered

1808

France / England

Boron at a glance

Identity

Symbol	B
Atomic number (Z)	5
Atomic weight	10.81 ± 0.02 (CIAAW 2024)
Category	Metalloid
Group	13 — boron group
Period	2
Block	p-block
CAS number	7440-42-8

Atomic properties

Electron configuration	[He] 2s² 2p¹ Full: 1s² 2s² 2p¹
Electrons per shell	2, 3
Oxidation states	+3 Less common: -5, -1, 0, +1, +2
Electronegativity (Pauling)	2.04
First ionisation energy	8.298 eV (800.6 kJ/mol) 2nd: 25.155 eV · 3rd: 37.931 eV
Electron affinity	0.28 eV
Atomic radius (empirical)	85 pm
Covalent radius	85 pm
Van der Waals radius	192 pm

Physical properties

State at 25 °C	Solid
Melting point	2,077 °C · 3,770.6 °F · 2,350.15 K
Boiling point	4,000 °C · 7,232 °F · 4,273.15 K
Density	2.34 g/cm³
Specific heat capacity	1.026 J/(g·K)
Thermal conductivity	27.4 W/(m·K)
Crystal structure	Tetragonal

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	¹¹B 80.35%, ¹⁰B 19.65%
Stable isotopes	3
Known isotopes	16
Abundance in Earth's crust	10 mg/kg
Abundance in seawater	4.44 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Boron electron configuration

Boron has 5 electrons. Written in short form, its ground-state configuration is [He] 2s² 2p¹ — the electrons of the noble gas helium, plus 2s² 2p¹.Written out in full: 1s² 2s² 2p¹.

As a member of group 13 it has 3 valence electrons in its outer shell (shell 2), which largely decide its chemistry.

2s↑↓

2p↑

Discovery and name

Discovered

1808 — Joseph Louis Gay-Lussac and Louis Jacques Thénard; Humphry Davy · France / England.

Name

From borax, the mineral it was isolated from.

Known for

Borosilicate glass (Pyrex), boron fibres, detergents and neodymium magnets (Nd₂Fe₁₄B) all rely on boron; boron-10 absorbs neutrons in nuclear control rods.

Isotopes of boron

Naturally occurring boron is a mix of 2 isotopes. Each has 5 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
¹¹B	5	6	80.35%	stable
¹⁰B	5	5	19.65%	stable

16 isotopes of boron are known in our data set (half-lives as published in nuclear data tables).

Boron compared with group 13

The other elements in group 13 (boron group), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Boron	5	10.81	2,077 °C	4,000 °C	2.34 g/cm³	2.04
Aluminum	13	26.982	660 °C	2,519 °C	2.7 g/cm³	1.61
Gallium	31	69.723	30 °C	2,229 °C	5.91 g/cm³	1.81
Indium	49	114.82	157 °C	2,027 °C	7.31 g/cm³	1.78
Thallium	81	204.38	304 °C	1,473 °C	11.8 g/cm³	1.62
Nihonium	113	[286]	—	—	16 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of boron?

The standard atomic weight of boron is 10.81 ± 0.02 (CIAAW 2024), so one mole weighs about 10.81 grams.

How many protons, neutrons and electrons does boron have?

A neutral boron atom has 5 protons and 5 electrons. Its most common isotope, boron-11, has 6 neutrons.

What is the electron configuration of boron?

[He] 2s² 2p¹ (in full: 1s² 2s² 2p¹).

What are the melting and boiling points of boron?

Boron melts at 2,077 °C (3,770.6 °F) and boils at 4,000 °C (7,232 °F).

Is boron a metal?

Boron is a metalloid — it has properties between metals and nonmetals. It is in group 13, period 2, and is a solid at room temperature.

Who discovered boron?

1808 — Joseph Louis Gay-Lussac and Louis Jacques Thénard; Humphry Davy (France / England).