sulfur halides (SF6)
Combine sulfur with a halogen, especially fluorine, and you get the sulfur halides, the most famous of which is sulfur hexafluoride, SF6 — a colourless, odourless, astonishingly inert gas. It is the party trick that deepens your voice when inhaled (denser than air, so sound travels slower through it; never actually do this, it can be dangerous), and it is also a serious workhorse of the electrical industry. Sulfur fluorides showcase how the heavier p-block elements can carry many bonds, and how a molecule's shape and stability are not the same thing.
SF6 is a textbook example of an octahedral molecule: a sulfur atom at the centre with six fluorines at the corners of a perfect octahedron, all bond angles 90 degrees, no lone pairs on sulfur, beautifully symmetric. By VSEPR this six-bond geometry is exactly what you expect. Older texts said sulfur "expanded its octet using d orbitals" to reach six bonds, but, as with PCl5, that picture is now regarded as largely incorrect — the bonding is better described by delocalised molecular orbitals over the sulfur and the very electronegative fluorines, no d orbitals required. What makes SF6 special is its extraordinary inertness: the six fluorines wrap the sulfur in a tight, even shell that no reagent, not even hot water or strong acid, can easily get past, so although SF6 is thermodynamically capable of reacting, it is kinetically locked — another reminder that stability and reactivity are separate ideas. Its lower cousin SF4, by contrast, has a lone pair, takes a lopsided see-saw shape, and is a violently reactive fluorinating agent.
These differences matter in the real world. SF6's inertness and high electrical breakdown strength make it the standard insulating gas inside high-voltage switchgear, where it quenches sparks; SF4 is used to put fluorine into molecules in synthesis. The cautionary note is environmental: SF6 is so stable that, once released, it persists in the atmosphere for thousands of years and is among the most potent greenhouse gases known per molecule, so its very inertness, the property that makes it useful, is also what makes its leakage a long-lived climate problem.
SF6 is so unreactive it is sealed inside the giant switches of the power grid: when a circuit opens under load, the gas smothers the arc that would otherwise blaze across the gap.
An almost unreactive molecule put to work precisely because it does nothing.
SF6 is not stable because it cannot react thermodynamically — in principle it can; it is kinetically inert because the snug shell of six fluorines blocks any reagent from reaching the sulfur, a clean illustration that stability and reactivity are not the same thing.