Silicon
Silicon is a chemical element with symbol Si and atomic number 14. It is a tetravalent metalloid, more reactive than germanium, the metalloid directly below it in the table. Controversy about silicon's character dates to its discovery.
At a glance
- Atomic number
- 14
- Atomic mass
- 28.085 u
- Phase
- Solid
- Density
- 2.329 g/cm³
- Melting point
- 1,687 K
- Boiling point
- 3,538 K
- First ionization energy
- 786.5 kJ/mol
- Electronegativity
- 1.9
- Electron affinity
- 134.0684 kJ/mol
- Molar heat capacity
- 19.789 J/(mol·K)
- Period
- 3
- Appearance
- crystalline, reflective with bluish-tinged faces
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Silicon’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.
10 is the [Ne] core total across two shells.
Preparing the orbital view…
14 protons14 neutrons
Silicon-28: 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, silicon 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
SiNeutral atom · no charge
A neutral silicon atom. Removing its first electron takes 787 kJ/mol.
Silicon’s outer shell holds 4 electrons. Electron 5 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 3.7× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 4, then 8, then 2 bars. Read backwards, that’s 2, 8, 4: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 4 electrons before the first big jump are silicon’s valence electrons, the ones it uses in chemical reactions. That’s why silicon sits in group 14: 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.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 3 | 787 | – | Measured |
| 2 | 3 | 1,577 | 2 | Measured |
| 3 | 3 | 3,232 | 2 | Measured |
| 4 | 3 | 4,356 | 1.3 | Measured |
| 5 | 2 | 16,091 | 3.7 | Measured |
| 6 | 2 | 19,806 | 1.2 | Estimated |
| 7 | 2 | 23,790 | 1.2 | Estimated |
| 8 | 2 | 29,292 | 1.2 | Estimated |
| 9 | 2 | 33,893 | 1.2 | Estimated |
| 10 | 2 | 38,727 | 1.1 | Estimated |
| 11 | 2 | 45,953 | 1.2 | Estimated |
| 12 | 2 | 50,502 | 1.1 | Calculated |
| 13 | 1 | 235,198 | 4.7 | Calculated |
| 14 | 1 | 257,922 | 1.1 | 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.
Doping: make it conduct
Pure silicon barely conducts: every outer electron is locked in a bond. Swap in a few atoms of another element, called doping, and it starts to conduct. Every computer chip is built this way.
- −Free electron
- +Hole: a bond missing an electron
- P⁺ B⁻ Dopant atoms that gave or took one electron
Pick phosphorus or boron, then click an atom to swap it in (or use “Add one”). Click it again to swap it back.
Pure silicon
- Free electrons
- 0
- Holes
- 0
- Current
- None: switch the battery on
Every outer electron is locked in a bond, so there’s almost nothing free to carry a current.
Where doping matters
- Every chip
A computer chip holds billions of tiny switches called transistors. Each is a pattern of n-type and p-type silicon, made by doping different areas.
- Solar cells
A solar panel is a sheet of n-type silicon on p-type silicon. Light frees electrons near the join, and the join pushes them one way: a current.
- Germanium came first
The first transistor, built in 1947, was made of germanium. Silicon took over because its bonds are stronger (a band gap of 1.12 eV against 0.66), so it keeps working when it gets hot.
The crystal is drawn flat: really each atom bonds to four neighbours in 3D, as in diamond. Doping is hugely exaggerated: chips use about one dopant atom per million atoms or fewer. Carrier speeds use room temperature mobilities (silicon: electrons 1,400, holes 450 cm²/V·s; Sze, Physics of Semiconductor Devices). The meter shows current in relative units.
How does it compare?
Density
2.33 g/cm³25th lowest of 99
Melting point
1,687 K30th highest of 96
Boiling point
3,538 K24th highest of 97
Electronegativity
1.936th highest of 96
First ionization energy
787 kJ/mol33rd 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.
- SiliconRoyal Society of Chemistry
- SiliconWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database