Chemical element · reference page

Boron

Boron is element 5 (B), located in period 2, group 13 and the p block. At room temperature its reference phase is solid. Its electron configuration is [He] 2s2 2p1.

Atomic number5

Standard atomic weight[10.806, 10.821]

Group13

Period2

Blockp

Electron configuration[He] 2s2 2p1

Electronic structure

What controls its chemistry

Atoms in group 13 have three valence electrons in the outer s and p subshells. The balance between covalent bonding and positive oxidation states changes down the group as atomic size and metallic character increase.

Boron is therefore best understood by comparing it with the elements immediately above and below it rather than by treating group 13 as chemically uniform.

Chemical forms

Compounds, ions and materials

Group 13 chemistry includes oxides, halides and covalent or ionic compounds in positive oxidation states. The lighter elements show more covalent character, while the heavier members become progressively more metallic.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 2.34g/cm³. The melting point is 2 365 K (2 092 °C). The boiling point is 4 200 K (3 927 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where boron is found and used

In practice, boron is encountered in borosilicate glass, glass fibre and plant nutrition.

In practice

Examples of real uses

Borosilicate glass

Boron-containing glass resists thermal shock because its composition reduces thermal expansion.

Glass fibre

Boron is used in glass compositions or fibres to adjust thermal, mechanical or chemical properties.

Plant nutrition

Boron is required in plant growth or supplied through compounds that plants can absorb.

Periodic position

What its neighbours tell us

Boron is in period 2 and group 13. In the neighbourhood shown here, carbon is immediately to its right in the same period; aluminium is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationmetalloid

Phase at room temperatureSolid

Electron shells2 · 3

Density2.34g/cm³

Melting point2 365 K (2 092 °C)

Boiling point4 200 K (3 927 °C)

Pauling electronegativity2.04

First ionisation energy800.6 kJ/mol

Electron affinity26.99 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of boron is associated with Joseph Louis Gay-Lussac. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is given as the interval [10.806, 10.821]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Boron has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with boron depend on chemical form, dose and route of exposure. Pure boron, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.