CHEM1902 Structure of Phosphorus, Sulfur and Iodine (2022)

Lecture 5c. Structure of the elements, continued P, S and I.

Allotropes of phosphorus

Elemental phosphorusexists in a number of different allotropes.

CHEM1902 Structure of Phosphorus, Sulfur and Iodine (1)

White phosphorus
The most important form of elemental phosphorus from the perspective ofapplications and the chemical literature is white phosphorus.It consists of tetrahedral P4 molecules,in which each atom is bound to the other three atoms by a single bond.This P4 tetrahedron is also present in liquid and gaseous phosphorusup to the temperature of 800 °C when it starts decomposing to P2 molecules.Solid white phosphorus exists in two forms. At low-temperatures, the β form is stable.At high-temperatures the α form is predominant. These forms differ in termsof the relative orientations of the constituent P4 tetrahedra.

The history of the match is linked to the discovery of the allotropes of phosphorus.

Allotropes of Phosphorus



violet - Hittorf

Phosphoreneis an allotrope of phosphorus normally used to designate a single layer of black phosphorus that may be somewhat flattened. Conceptually the structure is similar to the carbon-based graphene, hence the name phosphorene. However phosphorene is a semiconductor, unlike graphene which is a semimetal. Recentlya sample that was about 20 layers thick was shown to demonstrate high-speed data communication on nanoscale optical circuits.

White phosphorus is the most reactive, the least stable, the most volatile,the least dense, and the most toxic of the allotropes. White phosphorusgradually changes to red phosphorus. This transformation is acceleratedby light and heat, and samples of white phosphorus almost always containsome red phosphorus and accordingly appear yellow. For this reason,white phosphorus that is aged or otherwise impure is sometimes called yellow phosphorus. White phosphorus glowsin the dark (when exposed to oxygen) with a very faint tinge of green and blue,is highly flammable and pyrophoric (self-igniting) upon contact with air andis toxic (causing severe liver damage on ingestion). Owing to its pyrophoricity,white phosphorus has been used as an additive in napalm. The odour of combustion ofthis form has a characteristic garlic smell, and samples are commonly coatedwith white "phosphorus pentoxide", which consists of P4O10tetrahedra with oxygen inserted between the phosphorus atoms and attheir vertices. White phosphorus is insoluble in water but soluble incarbon disulfide.

Red phosphorus
In 1847 Anton von Schrotter found that sunlight changed white/yellow intored phosphorus, even when moisture and atmospheric oxygen were rigorously excluded.The red product was separated from the residual yellow phosphorus by treatmentwith carbon disulfide. Red phosphorus was also prepared from the yellow variety byheating it to about 250 °C. in an inert gas. Heating to higher temperaturesreconverted the red modification to the yellow one.
Red phosphorus exists as an amorphous network and does not ignite in airat temperatures below 240 °C.

Violet phosphorus
In 1865, Johann Hittorf heatedred phosphorus in a sealed tube at 530 °C. The upper part of the tube waskept at 444 °C. Brilliant opaque monoclinic, or rhombohedral, crystals sublimed.
This form is sometimes known as "Hittorf's phosphorus" (or violet or α-metallic phosphorus).

Black phosphorus
Black phosphorus is the thermodynamically stable form of phosphorus atroom temperature and pressure. It is obtained by heating white phosphorus underhigh pressures (12,000 atmospheres). In appearance, properties and structureit is similar to graphite,being black and flaky, a conductor of electricity, and having puckered sheetsof linked atoms.
Black phosphorus has an orthorhombic structure and is the least reactive allotrope:a result of its lattice of interlinked six-membered rings.Each atom is bonded to three other atoms.

Allotropes of sulfur

No other element forms more solid allotropes than sulfur. At present,about 30 well characterized sulfur allotropes are known of which the most common form found in nature is the greenish-yellow orthorhombic α-sulfur, containing puckered rings of S8.


When pure it has a greenish-yellow colour (traces of cyclo-S7 in commercially available samples make it appear yellower). It is practically insoluble in water and is a good electrical insulator with poor thermal conductivity. It is quite soluble in carbon disulfide: 35.5 g/100 g solvent at 25 °C. It has a rhombohedral crystal structure. This is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur". It contains S8 puckered rings, alternatively called a crown shape. The S-S bond lengths are all 206 pm and the S-S-S angles are 108° with a dihedral angle of 98°. At 95.3 °C, α-sulfur converts to β-sulfur.


This is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. Like the α- form it contains puckered S8 rings and only differs from it in the way the rings are packed in the crystal. It is unusual because it is only stable above 95.3 °C, below this it converts to α-sulfur. It can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur. It has a melting point of about 120 °C and decomposes at around this temperature.


This form, first prepared by F.W Muthmann in 1890, is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It contains puckered S8 rings like α-sulfur and β-sulfur and only differs from them in the way that these rings are packed. It is the densest form of the three. It can be prepared by slowly cooling molten sulfur that has been heated above 150 °C or by chilling solutions of sulfur in carbon disulfide, ethyl alcohol or hydrocarbons. It is found in nature as the mineral rosickyite.

Some allotropes of Sulfur

S6 - cyclohexasulfur


S12 - cyclododecasulfur

S6 - cyclo-hexasulfur
This was first prepared by M.R. Engel in 1891 who reacted HCl with thiosulfate, HS2O3-. Cyclo-S6 is orange-red and forms rhombohedral crystals. It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves reacting a polysulfane with sulfur monochloride:

H2S4 + S2Cl2 → cyclo-S6 + 2 HCl (dilute solution in diethyl ether)

The sulfur ring in cyclo-S6 has a "chair" conformation, reminiscent of the chair form of cyclohexane. All of the sulfur atoms are equivalent.

Thermodynamically, S12 is the second most stable sulfur ring after S8. Therefore,S12 is formed in many chemical reactions in which elemental sulfur is aproduct. In addition, S12 is a component of liquid sulfur at all temperatures.The same holds for S18 and S20 which are often formed together with S12.Its structure can be visualised as having sulfur atoms in three parallel planes, 3 in the top, 6 in the middle and three in the bottom.

Liquid sulfur after equilibration contains sulfur homocycles of all sizes and some of these can be isolated by quenching, extraction, fractional precipitation and crystallization depending on their differing solubilities.

Cyclo-S12 can be prepared by heating elemental sulfur to about 200 °C for 5-10 min and then allowing the mixture to cool to 140-160 °C within about15 min. Once the melt has become less viscous, it is poured in as thina stream as possible into liquid nitrogen in order to quench the equilibrium.Recrystallization of the yellow powder from CS2 allows the isolation of an adduct which slowly loses the solvent to give the cyclo-dodecasulfur.

Note that both B and S form stable E12 species but the structures (and coordination numbers) are quite different.


Iodine was discovered by French chemist Bernard Courtois in 1811. His father was a manufacturer of saltpeter (a vital part of gunpowder) andat the time of the French Napoleonic Wars, saltpeter was in great demand. Saltpeter produced from French niter beds required sodium carbonate, which could be isolated from seaweed collected on the coasts of Normandy and Brittany. To isolate the sodium carbonate, the seaweed was burned and the ash washed with water. The remaining waste was destroyed by adding sulfuric acid. Courtois once added excessive sulfuric acid and a cloud of purple vapour rose. He noted that the vapour crystallized on cold surfaces, making dark crystals. Courtois suspected that this was a new element but lacked the funds to pursue it further.

Samples of the material reached Humphry Davy and Joseph Louis Gay-Lussacand in early December 1813 both claimed that they had identified a new element.Arguments erupted between them over who had identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element.

Iodine is found on Earth mainly as the highly water-soluble iodide ion I-, concentrated in oceans and brine pools. Like the other halogens, free iodine occurs mainly as a diatomic molecule I2.In the universe and on Earth, iodine's high atomic number makes it a relatively rare element. However, its presence in ocean water has given it a role in biology. It is the heaviest essential element widely utilized by life in biological functions.

Under standard conditions, iodine is a bluish-black solid that sublimes to form a noxious violet-pink gas. It melts at 113.7 °C (386.85 K) and forms compounds with many elements but is less reactive than the other halogens, and has some metallic light reflectance.

Elemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C; potassium iodide may be added to increase solubility via formation of triiodide ions (I3-). Nonpolar solvents such as hexane and carbon tetrachloride provide a higher solubility.

Iodine normally exists as a diatomic molecule with an I-I bond length of 270 pm, one of the longest single bonds known. The I2 molecules tend to interact via weak London dispersion forces, and this interaction is responsible for the higher melting point compared to more compact halogens, which are also diatomic. Since the atomic size of iodine is larger, its melting point is higher.

The I-I bond is relatively weak, with a bond dissociation energy of 151 kJmol-1, and most bonds to iodine are weaker than for the lighter halides. One consequence of this weak bonding is the relatively high tendency of I2 molecules to dissociate into atomic iodine.

orthorhombic structure of I2
a= 0.72701, b= 0.97934, c= 0.47900 nm

The halogens, Cl2, Br2, and I2 adopt similar orthorhombic structures in which diatomic molecules lie in layers:
Cl a= 0.624 b= 0.826 c= 0.448 nm
Br a= 0.667 b= 0.872 c= 0.448 nm
I a= 0.72701, b= 0.97934, c= 0.47900 nm

Return to the course outline or move on to Lecture 6:Acids, Bases and Solvent Systems.


Much of the information in these course notes has been sourced from Wikipedia underthe Creative Commons License.
'Inorganic Chemistry' - C. Housecroft and A.G. Sharpe, PrenticeHall, 4th Ed., 2012, ISBN13: 978-0273742753, pps 24-27, 43-50,172-176, 552-558, 299-301, 207-212
'Basic Inorganic Chemistry' - F.A. Cotton, G. Wilkinson and P.L.Gaus, John Wiley and Sons, Inc. 3rd Ed., 1994.
'Introduction to Modern Inorganic Chemistry' - K.M. Mackay, R.A.Mackay and W. Henderson, International Textbook Company, 5th Ed.,1996.
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CHEM1902 Structure of Phosphorus, Sulfur and Iodine (3)
This work is licensed under a Creative CommonsAttribution-ShareAlike 3.0 Unported License.

Created and maintained by Prof. Robert J.Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created November 2014. Links checked and/or lastmodified 2nd April 2015.


What type of structure is phosphorus? ›

It has a tetrahedral shape and has the formula P4. The two main forms of phosphorus are white phosphorus and red phosphorus.
Discovery date1669
AllotropesWhite P, Red P, Black P, P2
2 more rows

What kind of bond is phosphorus and sulfur? ›

Each phosphorus atom is connected to 3 or 4 atoms, at most one of which is phosphorus. Each sulfur atom is connected to 1 or 2 atoms, which must all be phosphorus. The pictures seem pretty consistent about showing a 'double bond' when a sulfur atom is connected to just 1 phosphorus.

What happens when iodine and phosphorus reacts? ›

Reaction of phosphorus with the halogens

White phosphorus, P4, reacts with iodine, I2, in carbon disulphide (CS2) to form phosphorus(II) iodide, P2I4.

What is phosphorus Sulphur? ›

Phosphorus sulfides comprise a family of inorganic compounds containing only phosphorus and sulfur. These compounds have the formula P4Sx with x ≤ 10.

Does sulfur have a giant structure? ›

Phosphorus, sulphur, chlorine and argon are simple molecular substances with only van der Waals attractions between the molecules. Their melting or boiling points will be lower than those of the first four members of the period which have giant structures.

Does iodine have a simple molecular structure? ›

Simple molecules held together by weak forces of attraction. Examples include iodine, methane and carbon dioxide.

Is Phosphorus sulfide ionic or covalent? ›

Phosphine sulfides are formed from the reaction of organic phosphines with sulfur, in which the sulfur atom is linked to the phosphorus by a bond that has both covalent and ionic properties.

Is PCl ionic or covalent? ›

A bond with two identical atoms is always pure covalent, while a covalent bond with two different atoms is likely to be polar covalent.
Part A: Covalent or Ionic Compound?
Covalent CompoundsIonic Compounds
2 more rows

What is the typical bond pattern of phosphorus? ›

Phosphorus typically forms five bonds without a formal charge. Usually P bonds to four oxygen atoms. Halogens are the elements found in column 7 of the periodic table. These atoms are important tools in organic chemistry labs.

What does phosphorus and iodine make? ›

The reduction of iodine by phosphorus(I) (hypophosphorous acid) yields iodide and phosphorus(III) (phosphorous acid). The reaction is autocatalytic in hydrogen ion, autoinhibitory in iodide ion, and zero order in iodine over a wide concentration range.

What is the name of PI3? ›

Phosphorus triiodide | PI3 - PubChem.

What is the function of red phosphorus with i2? ›

The role of the red phosphorus in the reduction reaction is elucidated: it intervenes in a catalytic cycle by an oxido-reductive disproportionation with the liberated iodine, affording either hypophosphorous acid in aqueous media, or P214 in anhydrous media with concomitant regeneration of HI.

What is the difference between sulphur and phosphorus? ›

Answer and Explanation: One of the main differences between the phosphorus and sulfur cycles is that the sulfur cycle includes an atmospheric component, while the phosphorus...

What is the formula of sulphur? ›

The most common form of the element sulfur is composed of molecules that consist of eight atoms of sulfur and its molecular formula is S8.

What do phosphorus and sulfur have in common? ›

They have the same number of protons. Transcribed Image Text:What do the clements sulfur (S), nitrogen (N), phosphorus (P), and bromine (Br) have in common? A. They are noble (inert) gases.

Is Phosphorus a giant covalent lattice? ›

Silicon Giant covalent lattice Break strong covalent bonds. Phosphorus (P4) Simple molecular (lattice/covalent) Break weak London forces between the molecules. Sulfur (S8) Simple molecular (lattice/covalent) Break weak London forces between the molecules.

What type of structure is sulfur? ›

Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula S8. Elemental sulfur is a bright yellow, crystalline solid at room temperature.

Is Phosphorus a covalent solid? ›

Due to strong covalent bonding within the layers, graphite has a very high melting point, as expected for a covalent solid (it actually sublimes at about 3915°C).
Covalent Network Solids.
SubstanceΔHsub (kJ/mol)Average Bond Energy (kJ/mol)
phosphorus (s)58.98201
sulfur (s)64.22226
iodine (s)62.42149
Aug 14, 2020

What is the structure of iodine? ›

Gaseous iodine is composed of I2 molecules with an I–I bond length of 266.6 pm. The I–I bond is one of the longest single bonds known. It is even longer (271.5 pm) in solid orthorhombic crystalline iodine, which has the same crystal structure as chlorine and bromine.

What crystal structure is iodine? ›

Iodine is a covalent crystal.

Does iodine have a giant structure? ›

- The structure of iodine crystal is described as a face-centered-cubic structure, since the molecular formula of iodine is ${{I}_{2}}$, so they form a covalent bond between two iodine atoms only and they do not form giant molecules.

Is Iodine an ion? ›

Iodide salts are mild reducing agents and many react with oxygen to give iodine. Iodide is a halide anion and a monoatomic iodine. It has a role as a human metabolite. It is a conjugate base of a hydrogen iodide.
4.3Related Element.
Element NameIodine
Atomic Number53
1 more row

What is the difference between sulfur and sulfide? ›

Sulphur is a chemical compound with the symbol S and has the atomic number 16. Sulphide is an inorganic anion of sulphur. It has the chemical formula S2-. Sulphur is sometimes found in the pure form but mostly occurs as sulphide or minerals of sulfate.

What is difference between sulfide and sulphide? ›

Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2 or a compound containing one or more S2 ions. Solutions of sulfide salts are corrosive.

Why is PCl3 covalent? ›

PCl3 is a covalent compound as electrons are shared between the P atom and the Cl atoms. P atom is three electrons short of achieving an octet, while the Cl atoms are one electron away. The sharing of electrons results in an inert electron configuration for all the atoms.

What type of bond is PCl5? ›

Phosphorus(V) chloride, PCl

In the case of phosphorus, 5 covalent bonds are possible - as in PCl5.

Why is PCl3 not an ionic compound? ›

Is PCl3 (Phosphorus trichloride) Ionic or Covalent/Molecular? - YouTube

How many bonds does sulfur form? ›

Sulfur is a nonmetal in group 6A , and therefore has 6 valence electrons. In order to obey the octet rule, it needs to gain 2 electrons . It can do this by forming 2 single covalent bonds.

How many bonds can iodine form? ›

As the valency of iodine is 1 we can say that iodine can form one single bond with another atom. Thus, iodine can form only one single bond. Note:Atoms having more than 4 valence electrons always tend to gain electrons to complete their octet.

Is sulfur ionic or covalent? ›

SO2 is a covalent molecule. Because the bond is formed between one atom of sulfur (S) and two atoms of oxygen (O) by the sharing of electrons.
Is Sulfur dioxide (SO2) ionic or covalent? Types of bond in SO2.
Name of MoleculeSulfur dioxide (SO2)
Ionic or Covalent?Covalent
Molar mass64.066 g mol1

What type of bond is phosphorus and iodine? ›

Answer and Explanation: Phosphorus triiodide is formed from the covalent bonding of phosphorus to three iodine atoms, hence, the name triiodide.

What is the Lewis structure for pi3? ›

How to Draw the Lewis Structure for PI3 (Phosphorus Triiodide) - YouTube

What is the formula for Diphosphorus Tetraiodide? ›

What bond type is PI3? ›

PI3 or Triiodophosphine is an inorganic compound. The bond formation in the molecule of Triiodophosphine can be understood by covalent bonding concept.

Is iodine a metal? ›

Iodine is a nonmetallic, nearly black solid at room temperature and has a glittering crystalline appearance.

What is the structure of red phosphorus? ›

Red phosphorus exists in a polymeric chain of tetrahedrally structured P4 molecules in which one of the P-P bonds are broken to enable the linking of these tetrahedrons.

What is the role of red phosphorus? ›

It is used in production of pyrotechnics, fertilizers, and pesticides. It is used in electroluminescent coatings. It is used in many organic synthesis reactions. Due to its stable but reactive nature, it is used in smoke bombs.

What is the structure of black phosphorus? ›

Black phosphorus has an orthorhombic pleated honeycomb structure and is the least reactive allotrope, a result of its lattice of interlinked six-membered rings where each atom is bonded to three other atoms. In this structure, each phosphorous atom has 5 outer shell electrons.

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