Boron
Boron is a metalloid chemical element with symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in both the Solar system and the Earth's crust. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals.
At a glance
- Atomic number
- 5
- Atomic mass
- 10.81 u
- Phase
- Solid
- Density
- 2.08 g/cm³
- Melting point
- 2,349 K
- Boiling point
- 4,200 K
- First ionization energy
- 800.6 kJ/mol
- Electronegativity
- 2.04
- Electron affinity
- 26.989 kJ/mol
- Molar heat capacity
- 11.087 J/(mol·K)
- Period
- 2
- Appearance
- black-brown
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Boron’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
Preparing the orbital view…
5 protons6 neutrons
Boron-11: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, boron is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
BNeutral atom · no charge
A neutral boron atom. Removing its first electron takes 801 kJ/mol.
Boron’s outer shell holds 3 electrons. Even inside it there’s a jump: electron 2 takes 3× as much as electron 1, because it sits in a lower-energy subshell. Electron 4 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 6.8× as much energy as the one before.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 3, then 2 bars. Read backwards, that’s 2, 3: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 3 electrons before the first big jump are boron’s valence electrons, the ones it uses in chemical reactions. That’s why boron sits in group 13: every element in the group has the same number of outer electrons, so they react in similar ways.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
- The jump inside the outer shell is evidence that shells are split into subshells (s, p, d and f), each with its own energy. You can explore their shapes in the Orbitals view above.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 2 | 801 | – | Measured |
| 2 | 2 | 2,427 | 3 | Measured |
| 3 | 2 | 3,660 | 1.5 | Measured |
| 4 | 1 | 25,026 | 6.8 | Calculated |
| 5 | 1 | 32,827 | 1.3 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
How does it compare?
Density
2.08 g/cm³24th lowest of 99
Melting point
2,349 K11th highest of 96
Boiling point
4,200 K16th highest of 97
Electronegativity
2.0428th highest of 96
First ionization energy
801 kJ/mol32nd highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- BoronRoyal Society of Chemistry
- BoronWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database