question: If Sulfur can only have two covalent bonds, then how is something like SO3 possible?
comment: Hybrid Orbital theory allows for delocalized electrons. Thats a little fancy as an answer but what it means is that basically they can share an electron between them rather than having 2 distinct bonds.
passages: BULLET::::- η-SO, O-bonded. In such cases, SO attaches to a metal via one of its two oxygen centres. Such complexes are prevalent for hard metal cations such as Na and Al. In these compounds the M-O interaction is usually weak.
  
  More exotic bonding modes are known for clusters.
  
  Section::::Preparation.
  
  Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO. The adducts are often weak. In some cases, SO displaces other ligands.
   Lower sulfur oxides
  
  The lower sulfur oxides are a group of inorganic compounds with the formula SO, where m < 2n. These species are often unstable and thus rarely encountered in everyday life. They are significant intermediates in the combustion of elemental sulfur. Some well characterized examples include sulfur monoxide (SO), its dimer SO, and a series of cyclic sulfur oxides, SOx (x = 1, 2), based on cyclic S rings.
   In most cases SO binds in monodentate fashion, attaching to the metal through sulfur. Such complexes are further subdivided according to the planarity or pyramidalization at sulfur. The various bonding modes are:
  
  BULLET::::- η-SO, planar (meaning that the MSO subunit forms a plane). In such complexes, SO is classified as a 2e donor complemented by pi-back bonding into the empty p orbital localized on sulfur.
   BULLET::::- η-SO, pyramidal (meaning that the MSO subunit is pyramidal at sulfur). In such complexes, SO is classified as a pure Lewis acid. The structure is similar to that for conventional Lewis base adducts of SO.
  
  BULLET::::- η-SO. Both S and one O centre are attached to the metal. The MSO subunit is pyramidal at sulfur. This bonding mode is more common for early metals, which are typically strongly pi-donating.
   Industrially SO is made by the contact process. Sulfur dioxide, which in turn is produced by the burning of sulfur or iron pyrite (a sulfide ore of iron). After being purified by electrostatic precipitation, the SO is then oxidised by atmospheric oxygen at between 400 and 600 °C over a catalyst. A typical catalyst consists of vanadium pentoxide (VO) activated with potassium oxide KO on kieselguhr or silica support. Platinum also works very well but is too expensive and is poisoned (rendered ineffective) much more easily by impurities.
   Many clusters have been synthesized in the laboratory with the formula [FeS(SR)], which are known for many R substituents, and with many cations. Variations have been prepared including the incomplete cubanes [FeS(SR)].
   Sulfur dioxide can bind to metal ions as a ligand to form metal sulfur dioxide complexes, typically where the transition metal is in oxidation state 0 or +1. Many different bonding modes (geometries) are recognized, but in most cases, the ligand is monodentate, attached to the metal through sulfur, which can be either planar and pyramidal η.
  
  Section::::Uses.
  
  Section::::Uses.:Precursor to sulfuric acid.
   Section::::Bonding.
  
  The first description of the bonding in modern terms was by Gilbert Lewis in his groundbreaking paper of 1916 where he described the bonding in terms of electron octets around each atom, that is no double bonds and a formal charge of +2 on the sulfur atom.
   The sulfur–oxygen bond has a bond order of 1.5. There is support for this simple approach that does not invoke "d" orbital participation.
  
  In terms of electron-counting formalism, the sulfur atom has an oxidation state of +4 and a formal charge of +1.
  
  Section::::Occurrence.
  
  It is found on Earth and exists in very small concentrations and in the atmosphere at about 1 ppm.
   Section::::Cheletropic reactions involving SO.:Kinetics.
  
  The cheletropic reactions of 1,3-dienes with sulfur dioxide have been extensively investigated in terms of kinetics (see above for general reaction).
   But "how" do abundance and isotopic composition of different sulfur reservoirs inform our understanding of biogeochemical processes? The oxidation and reduction of sulfur species often involves the breakage or formation of chemical bonds involving S atoms. Because the thermodynamic stability of certain bonds is often greater when they involve heavier isotopes, an oxidation or reduction reaction can enrich the reactant pool (reservoir) or product pool in compounds containing the heavier isotope, relative to each other. This is known as an isotope effect. The extent to which such a mass-dependent reaction operates in the world's oceans or atmosphere determines how much heavier or lighter various reservoirs of sulfur species will become.
   Section::::Solid cyclo-sulfur allotropes.:"Cyclo"-S (n = 9–15, 18, 20).
  
  These allotropes have been synthesised by various methods for example, treating titanocene pentasulfide and a dichlorosulfane of suitable sulfur chain length, SCl:
  
  or alternatively treating a dichlorosulfane, SCl and a polysulfane, HS:
  
  S, S, and S can also be prepared from S. With the exception of cyclo-S, the rings contain S-S bond lengths and S-S-S bond angle that differ one from another.
   Metal sulfur dioxide complex
  
  Metal sulfur dioxide complexes are complexes that contain sulfur dioxide, SO, bonded to a transition metal. Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion reactions in organometallic chemistry.
  
  Section::::Bonding modes.
  
  Sulfur dioxide forms complexes with many transition metals. For the most stable complex the metal is in oxidation state 0 or +1.
   BULLET::::1. Oxidation of SO into SO by V:
  
  BULLET::::- 2SO + 4V + 2O → 2SO + 4V
  
  BULLET::::2. Oxidation of V back into V by dioxygen (catalyst regeneration):
  
  BULLET::::- 4V + O → 4V + 2O
  
  Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated HSO in the absorption tower to form oleum:
  
  Note that directly dissolving SO in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.
  
  Oleum is reacted with water to form concentrated HSO.
  
  Section::::Purification unit.
   Section::::Summary of known binary interchalcogens.
  
  Section::::Summary of known binary interchalcogens.:Sulfur chalcogenides.
  
  BULLET::::- Lower sulfur oxides, SO where the ratio X:Y is greater than 1:2
  
  BULLET::::- Disulfur monoxide, SO
  
  BULLET::::- Disulfur dioxide, SO
  
  BULLET::::- Sulfur monoxide, SO
  
  BULLET::::- Sulfur dioxide, SO
  
  BULLET::::- Sulfur trioxide, SO
  
  BULLET::::- Higher sulfur oxides, SO where x3
  
  Section::::Summary of known binary interchalcogens.:Selenium chalcogenides.
  
  BULLET::::- Selenium dioxide, SeO
  
  BULLET::::- Selenium trioxide, SeO
  
  BULLET::::- Many "alloys" of selenium and sulfur in different concentrations with semimetallic bonding, SeS
  
  BULLET::::- "Selenium monosulfide", SeS
  
  BULLET::::- "Selenium disulfide", SeS, actually a 2:1 mixture of cyclo-SeS and cyclo-SeS
   As the chalcogens S, Se and Te are less electronegative than oxygen, the bonding in these materials is generally more covalent than in the oxospinels. Some are (magnetic) semiconductors but others display, at times complicated, metallic behavior, often coupled with equally complicated magnetic properties. The assignment of oxidation states is also not always as straightforward as in the more ionic oxocompounds, as is shown in the case of carrollite. 
  
  Group members include:
  
  BULLET::::- Cadmoindite - CdInS
  
  BULLET::::- Carrollite - CuCoS
  
  BULLET::::- Fletcherite - Cu(Ni,Co)S
  
  BULLET::::- Greigite - FeFeS
  
  BULLET::::- Indite - FeInS
  
  BULLET::::- Kalininite - ZnCrS
   The addition of Cl to SCl has been proposed to proceed via a mixed valence intermediate ClS-SCl. SCl undergoes even further chlorination to give SCl, but this species is unstable at near room temperature. It is likely that several SCl exist where x  2.
   On the other hand, at least three pathways exist for the oxidation of thiosulfate (SO) :
  
  BULLET::::- The aforementioned Sox pathway, through which both sulfur atoms in thiosulfate are reduced to sulfate without the formation of any free intermediate.
  
  BULLET::::- The oxidation of thiosulfate (SO) via the formation of tetrathionate (SO) intermediate, that is present in several obligate chemolithotrophic "Gamma" and "Betaproteobacteria" as well as in facultative chemolithotrophic "Alphaproteobacteria".
   A thiosulfoxide is a chemical compound containing a sulfur to sulfur double bond of the type RR'S=S with R and R' both alkyl or aryl residues. The thiosulfoxide has a molecular shape known as trigonal pyramidal. Its coordination is also trigonal pyramidal. The point group of the thiosulfoxide is C. A 1982 review concluded that there was as yet no definitive evidence for the existence of stable thiosulfoxides which can be attributed to the double bond rule which states that elements of period 3 and beyond do not form multiple bonds. The related sulfoxides of the type RR'S=O are very common. Many compounds containing a sulfur-sulfur double bond have been reported in the past although only a few verified classes of actually stable compounds exist, closely related to thiosulfoxides. 
   The structure of the sulfite anion can be described with three equivalent resonance structures. In each resonance structure, the sulfur atom is double-bonded to one oxygen atom with a formal charge of zero (neutral), and sulfur is singly bonded to the other two oxygen atoms, which each carry a formal charge of −1, together accounting for the −2 charge on the anion. There is also a non-bonded lone pair on the sulfur, so the structure predicted by VSEPR theory is trigonal pyramidal, as in ammonia (NH). In the hybrid resonance structure, the S-O bonds are equivalently of bond order one and one-third.
   Seemingly ambiguous oxidation states are obtained on a set of resonance formulas of equal weights for a molecule of heteronuclear bonds where the atom connectivity does not correspond to the number of two-electron bonds dictated by the 8 − "N" rule. An example is SN where four resonance formulas featuring one S=N double bond have oxidation states +2 and +4 on the two sulfur atoms, to be averaged to +3 because the two sulfur atoms are equivalent in this square-shaped molecule.
  
  Section::::Appearances.:Ambiguous oxidation states.:A physical measurement is needed to decide the oxidation state.
   In the structure of rapidcreekite, there are two distinct calcium sites coordinated by six oxygen anions and two HO groups arranged as a square antiprism. The sulfur site is tetrahedrally coordinated by oxygen anions and the carbon site is coordinated by a triangle of oxygen anions. The structural unit of rapidcreekite is a sheet that consists of edge-sharing CaΦ polyhedra (Φ, unspecified species: O or OH) cross-linked by carbonate and sulfate groups. Sheets are held together by weakening hydrogen bonds, accounting for the perfect cleavage along {100}.
   A large number of labile O-bonded SO complexes arise from the oxidation of a suspension of the metals in liquid SO, an excellent solvent.
  
  Section::::Reactions.
  
  The main reaction of sulfur dioxide promoted by transition metals is its reduction by hydrogen sulfide. Known as the Claus process, this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries.
  
  Section::::Reactions.:Insertion of SO into metal-ligand bonds.
   SO3
  
  SO3 may refer to
  
  BULLET::::- Sulfur trioxide, a chemical compound of sulfur and trioxide
  
  BULLET::::- Sulfite, a chemical ion composed of sulfur and oxygen with a 2− charge
  
  BULLET::::- SO(3), the special orthogonal group in 3 dimensions; the rotations that can be given an object in 3-space
  
  BULLET::::- "", the third main game in the "Star Ocean" series
  
  BULLET::::- A staff officer of the third class, usually a junior officer
  
  BULLET::::- Special Operations 3 - Planning, of SOE (Special Operations Executive, British, World War II)
   Higher sulfur oxides
  
  Higher sulfur oxides are a group of chemical compounds with the formula SO where x lies between 0 and 1. They contain peroxo (O−O) groups and the oxidation state of sulfur is +6 as in SO.
  
  Monomeric SO can be isolated at low temperatures (below 78 K) following the reaction of SO and atomic oxygen or photolysis of SO–ozone mixtures. The favoured structure is:
  