Boron
Boron (B) is a metalloid with atomic number 5 and an atomic mass of 10.81. Its electron configuration is [He] 2s2 2p1, which gives it 3 valence electrons.
Boron key data
| Symbol | B |
|---|---|
| Atomic number | 5 |
| Atomic mass (standard atomic weight) | 10.81 |
| Molar mass | 10.81 g/mol |
| Protons | 5 |
| Neutrons (most abundant isotope, boron-11) | 6 |
| Electrons (neutral atom) | 5 |
| Valence electrons | 3 |
| Category | Metalloid |
| Period | 2 |
| Group | 13 |
| Block | p-block |
| Noble gas notation | [He] 2s2 2p1 |
| Full electron configuration | 1s2 2s2 2p1 |
| Electrons per shell | 2, 3 |
Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Measured properties of boron
From PubChem, the US National Institutes of Health. Public domain.
| Melting point | 2348 K (2074.9 °C), 12th of 103 by melting point |
|---|---|
| Boiling point | 4273 K (3999.9 °C), 14th of 93 by boiling point |
| Density | 2.37 g/cm³, 73rd of 96 by density |
| State at room temperature | Solid |
| Electronegativity | 2.04 (Pauling), 28th of 95 by electronegativity |
| Van der Waals radius | 192 pm, joint 69th of 99 by atomic radius |
| First ionisation energy | 8.298 eV (800.6 kJ/mol), 32nd of 102 by first ionisation energy |
| Electron affinity | 0.277 eV (26.7 kJ/mol), 53rd of 57 by electron affinity |
| Common oxidation states | +3 |
| Discovered | 1808 |
Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.
Boron electron configuration
The full electron configuration of boron is 1s2 2s2 2p1. With the noble-gas core collapsed it is [He] 2s2 2p1, and the outermost subshell is 2p1.
Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.
Mass of one boron atom
A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.
Mass of one atom
- one atom = 10.81 / (6.02214076 × 10²³)
- = 1.7950 × 10⁻²³ g
Atoms in one gram
- atoms per gram = 6.02214076 × 10²³ / 10.81
- = 5.5709 × 10²² atoms
Boron on the periodic table
Boron is in period 2, group 13, in the p-block.
The other elements of group 13 are aluminium, gallium, indium, thallium and nihonium.