With Groups 1, 2, and 13 covered, it’s time to visit Group 14. This is known as the carbon group,
since carbon sits right at the top. The other members of this group are silicon, germanium,
tin, lead, and flerovium. These elements have electron configurations that end in ns2np2,
and they are quite interesting in that both 4+ and 4- ions are possible, depending on what they are
interacting with. In terms of oxidation states, +4 is all that carbon and silicon will exhibit, while
germanium, tin, and lead can exhibit both +4 and +2, with +4 being more common, except for in lead,
where +2 becomes more common, due to the inert pair effect. Again, this is where the s electrons
in the outermost shell tend to remain unshared in post-transition metals. In this group we also
encounter our first nonmetal apart from hydrogen, and that is the ubiquitous carbon, which we know
so much about from organic chemistry. Since that entire series was essentially all about what
carbon can do, we won’t bother summarizing any of that here. Silicon and germanium,
on the other hand, are both metalloids that often form covalent bonds, while tin and lead are
borderline metals that have significant covalent character. Flerovium, like nihonium from the
previous tutorial, is a synthetic transactinide element that was first formed in 1998.
It is extremely radioactive, with the most stable isotope having a half-life of just under
one second, so there is essentially nothing to discuss in terms of reactivity and applications.
So where can we find these elements? As we mentioned, carbon is just about everywhere,
especially in living organisms, which are carbon-based. Silicon is even more abundant in
the form of silicon dioxide. This actually makes up 59% of Earth’s crust, and is easily found in
sand at any beach. Germanium is found in ores like sphalerite, which has germanium sulfide impurity,
as well as silver, lead, and copper sulfide ores. Russia and China are primary producers.
Tin is available in the form of cassiterite, found primarily in China and Indonesia.
And lead is found in ores like galena, cerussite, and anglesite.
To get into bonding behavior, the elements in this group all have four valence electrons,
with allows for a range of hybridizations and therefore diverse structures. For example,
pi bonds are possible between adjacent p orbitals. As we go down the group,
these outermost p orbitals become larger, but they also get further away from one another,
as the covalent bond length increases. This weakens the interaction, thus while pi bonds
are very common between carbon atoms, they are already quite rare between silicon atoms.
On the other hand, pi bonding between one p orbital and one d orbital is also possible,
and this actually strengthens going down the group as these orbitals get bigger, due to the angle
in which the lobes of the d orbitals project, and the resulting enhanced overlap with the p
orbital. So while carbon readily forms double and triple bonds, silicon prefers single bonds,
especially with oxygen, and metal-oxygen bond strength increases going down the group.
Continuing with elemental forms, carbon has several common allotropes. First there is
graphite, which consists of sheets of fused benzene rings around 3.35 Angstroms apart.
These layers make electrostatic interactions but slide very easily over one another,
giving graphite application as a lubricant. This extended pi system makes graphite a semimetal
with some overlap between its valence band and conduction band so it is also able to conduct
electricity. Visit my general chemistry tutorial on conductors, insulators, and semiconductors
if you wish to review information regarding molecular orbitals and electrical conductivity.
Diamond is an extended lattice of sp3 hybridized carbon atoms,
making this one of the hardest and most durable substances known. Then a bit more exotic are the
fullerenes, such as buckminsterfullerene, commonly referred to as bucky balls,
as well as things like carbon nanotubes which have astonishing tensile strength. These have
tremendous application in nanotechnology and are the subject of ongoing research and development.
Silicon also takes on a diamond-like structure, and is a semiconductor, due to the small band gap
between its valence band and conduction band. Germanium also has a diamond-like structure,
while tin has both metallic and diamond allotropes which are stable. The metallic is
malleable and shiny while the diamond is brittle and gray. Cold weather encourages transition from
the metal allotrope to the diamond, which is why objects made of tin tend to disintegrate
in cold weather, and in catalytic fashion, a phenomenon known as tin disease or tin pest.
Lead, unlike tin, is exclusively metallic, and less reactive than the other metals in this group.
Now in terms of reactivity, again we don’t need to say much about carbon since that’s
what organic chemistry is for. But we want to be aware that beyond its covalent bonding behavior,
carbon atoms can interact with transition metals to form interstitial carbides,
where carbon atoms fit into octahedral holes in close-packed metal lattices. This produces alloys
that are very hard and with high melting points, such as when combined with iron to make steel.
Covalently speaking, carbon-oxygen bonds are extremely strong,
and we will see the CO ligand in many transition metal complexes. Carbon-nitrogen
bonds are also strong, so we can say the same for CN ligands. More on these later.
Silicon is similar to carbon, and therefore tends to be tetravalent, but with an extra
shell it has occupied d orbitals, which makes for easy hybridization and changing of geometry.
More atoms can fit around the larger silicon atom than the smaller carbon atom. This means
silicon can be pentavalent while carbon cannot, save for fleeting moments under certain extreme
conditions we will not discuss here. Silicon will bond with carbon and hydrogen to form silanes,
such as tetramethylsilane, or TMS, which should be familiar from our study of NMR spectroscopy. Like
carbon, silicon makes very strong bonds to oxygen, so these are quite common. Silicones are compounds
with a silicon-oxygen backbone, and R groups attached to the silicon atoms. And silicates
are salts in which the anion contains silicon and oxygen. These are abundant in Earth’s crust,
and are studied in detail over in the geology series. It is possible to produce pure silicon by
the high-temperature reduction of silicon dioxide with a strong reducing agent, like carbon or
magnesium solid, which yields silicon and either carbon monoxide gas or magnesium oxide solid.
In terms of applications, again for carbon these are limitless, given carbon’s omnipresence in
natural products, drugs, and the exotic allotropes we mentioned. For silicon, the applications should
also be quite familiar, as it is crucial for microchip electronics, and therefore virtually
every bit of advanced technology we use every day. Silicon is also used in aluminum-silicon
alloys for auto parts. Silicones are used for waterproofing, casting materials, high temperature
greases and waxes, caulking, and medical implants. Silicon dioxide is used for concrete, cement,
glass, enamels, and pottery. Some organisms, like diatoms, even have silicon dioxide exoskeletons.
Germanium first found application in transistors and diodes. Germanium oxide has a high index of
refraction and low dispersion so it is essential in keeping light within fiber optics. For this
reason it has replaced titanium oxide in glass for fiber optics. Germanium is also transparent
to infrared electromagnetic radiation, so it is used for IR optics, thermal imaging cameras,
and night vision equipment. Silicon-germanium alloys are also finding use in microchips.
Tin is an interesting metal, as it is non-toxic, does not rust, and is very easy to melt and cast.
Tin is present in some of the most classic alloys, such as bronze, an alloy of tin and copper;
solder, an alloy of tin and lead; and pewter, an alloy of predominately tin
and mixtures of other metals like silver, copper, zinc, and manganese. Tin-niobium
alloys are used in superconducting magnets. Tin plating of iron, lead, zinc, or steel helps
prevent corrosion. And tin is used in organ pipes as it is the most resonant of all the metals.
Lead has tremendous application in lead-acid batteries.
These are capable of delivering high currents, and were among the first rechargeable batteries.
Lead is used in bullets due to its high density while being soft enough to not jam the barrel.
Lead is also used in pewters, in radiation shields, and was long used for pipes as these are
easy to weld and repair and they do not corrode very easily, though some corrosion does occur,
which is why lead pipes are no longer advisable for carrying drinking water, due to the potential
danger of lead poisoning. And as we mentioned, flerovium has no useful applications due to its
extremely short half-life. And with that, we wrap up another group from the periodic table.