298Therefore, it does not contract when it freezes like most substances, but expands, similar
to how ice is less dense than water and has less mass per unit of volume than liquid water.
With a relatively high thermal conductivity of 149 Wꞏm−1ꞏK−1, silicon conducts heat well
and as a result is not often used to insulate hot objects.
In its crystalline form, pure silicon has a gray color and a metallic luster. Like germanium,
silicon is rather strong, very brittle, and prone to chipping. Silicon, like carbon and
germanium, crystallizes in a diamond cubic crystal structure, with a lattice spacing of
approximately 0.5430710 nm (5.430710 Å).
The outer electron orbital of silicon, like that of carbon, has four valence electrons. The
1s, 2s, 2p and 3s subshells are completely filled while the 3p subshell contains two
electrons out of a possible six.
Silicon is a semiconductor. It has a negative temperature coefficient of resistance, since
the number of free charge carriers increases with temperature. The electrical resistance
of single crystal silicon significantly changes under the application of mechanical stress
due to the piezoresistive effect.
Isotopes of Silicon
Naturally occurring silicon is composed of three stable isotopes, silicon-28, silicon-29, and
silicon-30, with silicon-28 being the most abundant (92% natural abundance). Out of
these, only silicon-29 is of use in NMR and EPR spectroscopy. Twenty radioisotopes have
been characterized, with the most stable being silicon-32 with a half-life of 170 years, and
silicon-31 with a half-life of 157.3 minutes. All of the remaining radioactive isotopes have
half-lives that are less than seven seconds, and the majority of these have half-lives that
are less than one tenth of a second. Silicon does not have any known nuclear isomers.
The isotopes of silicon range in mass number from 22 to 44. The most common decay
mode of six isotopes with mass numbers lower than the most abundant stable isotope,
silicon-28, is Beta Positive β+, primarily forming aluminum isotopes (13 protons) as decay
products. The most common decay mode(s) for 16 isotopes with mass numbers higher
than silicon-28 is beta negative β−, primarily forming phosphorus isotopes (15 protons) as
decay products.
Occurrence
Measured by mass, silicon makes up 27.7% of the Earth's crust and is the second most
abundant element in the crust, with only oxygen having a greater abundance. Silicon is
usually found in the form of complex silicate minerals, and less often as silicon dioxide
(silica, a major component of common sand). Pure silicon crystals are very rarely found
in nature.
The silicate minerals—various minerals containing silicon, oxygen and reactive metals—
account for 90% of the mass of the Earth's crust. This is due to the fact that at the high
temperatures characteristic of the formation of the inner solar system, silicon and oxygen
have a great affinity for each other, forming networks of silicon and oxygen in chemical
compounds of very low volatility. Since oxygen and silicon were the most common non-
gaseous and non-metallic elements in the debris from supernova dust which formed the
protoplanetary disk in the formation and evolution of the Solar System, they formed many
complex silicates which accreted into larger rocky planetesimals that formed the terrestrial
planets.