Did you know that phosphorus pentachloride can have a molecular structure in the gas phase but an ionic structure as a solid? In its familiar intact gaseous molecular form, PCl₅ has a trigonal-bipyramidal arrangement, with three equatorial chlorine atoms and two axial chlorine atoms around phosphorus. Yet crystalline solid PCl₅ is commonly described by the ions [PCl₄]⁺ and [PCl₆]⁻. This is a striking reminder that a chemical formula alone does not always determine the bonding arrangement of a substance in every physical state.
For JC students searching for an H2 Chemistry tutor in Hougang, PCl5 gas versus solid structure, phosphorus pentachloride ionic solid, PCl4+ and PCl6− shapes, trigonal bipyramidal bond angles, VSEPR phosphorus chlorine chemistry or A-Level Chemistry 9476 molecular shapes, the key question is what species are actually present under the conditions being described?
The short comparison is:
- Intact PCl₅ molecule in the gas phase: five bonding directions around phosphorus, trigonal bipyramidal.
- Solid PCl₅ under the familiar crystalline description: ionic assembly containing [PCl₄]⁺ and [PCl₆]⁻.
- [PCl₄]⁺ ion: four P–Cl bonding directions, tetrahedral.
- [PCl₆]⁻ ion: six P–Cl bonding directions, octahedral.
- Oxidation state of phosphorus: remains +5 in each of these species.
- Important distinction: forming the ionic solid arrangement does not mean phosphorus has been oxidised or reduced.
I would ask a student to draw an intact PCl₅ molecule, then draw the two ions found in the solid. The three pictures require three different geometries. They are all compatible with phosphorus being bonded to chlorine, but the number of directly coordinated chlorines and the charge on each species differ. That is why copying the gas-phase structure into the crystal is not a complete description of solid phosphorus pentachloride.
The 2027 Singapore H2 Chemistry 9476 syllabus covers electron-pair repulsion, the shapes of species, bond angles, covalent and ionic bonding, and reaction chemistry of Period 3 compounds. The solid-state self-ionisation of PCl₅ and more advanced bonding explanations are useful structure–property enrichment, not an assumption that every examination requires recall of specialised solid-state details unless the syllabus or question provides them.
At eduKate, the small-group model uses at most three learners so that each can draw and explain an unfamiliar species independently. Families searching for Chemistry support from Hougang should verify the actual teaching venue, current JC timetable and travel arrangements directly, rather than infer that the educational page identifies a physical Hougang classroom.
Hougang Chemistry Tutor Library · Official SEAB H2 Chemistry syllabus directory · OpenStax: phosphorus halide structures
Start with the formula PCl₅
Phosphorus pentachloride contains phosphorus and chlorine in a 1:5 stoichiometric ratio.
The name tells us that the simplest formula unit is PCl₅.
Phosphorus lies in Group 15 of the Periodic Table and has five valence electrons in the familiar main-group counting system.
Chlorine lies in Group 17, with seven valence electrons per neutral atom.
A conventional Lewis picture of a single gas-phase PCl₅ molecule uses phosphorus as the central atom bonded to five chlorine atoms.
But the chemical formula does not guarantee that the crystalline solid must consist of isolated copies of that same neutral five-coordinate molecule.
The ratio of elements and structure of the material are related, but they answer different questions.
That distinction is also important for SiO₂, which forms a giant Si–O network in common condensed phases despite the formula looking similar to molecular CO₂.
What does PCl₅ look like in the gas phase?
When an intact PCl₅ molecule is considered in the gas phase, the familiar VSEPR structure is trigonal bipyramidal.
Three P–Cl bonds lie in an approximately triangular equatorial plane around the phosphorus centre.
Two other P–Cl bonds point in opposite directions above and below that plane, forming the axial positions.
The central phosphorus is conventionally described as having five bonding electron domains and no nonbonding electron pair in the simple VSEPR structure of PCl₅.
Those electron domains arrange to minimise repulsions according to the molecular geometry model.
Because the five positions are not all geometrically equivalent, the structure has two kinds of P–Cl bond position: axial and equatorial.
This is important for interpreting angles and bonding. A trigonal bipyramid is not just an octahedron missing a ligand with all remaining angles unchanged.
The three important bond angles
For an ideal trigonal-bipyramidal arrangement:
Equatorial–equatorial bond angle ≈ 120°.
Axial–equatorial bond angle ≈ 90°.
Axial–axial bond angle = 180°.
The three equatorial bonds are spread around the plane, creating three roughly 120° separations.
Each axial bond is perpendicular to the equatorial plane, giving a 90° angle to an equatorial bond.
The two axial bonds point in opposite directions, giving 180° between them.
Actual molecular bond angles and bond lengths can differ slightly from idealised VSEPR values depending on the molecule and experimental conditions.
For an H2 question asking the ideal shape and angles, the three values 90°, 120° and 180° are the key result.
Do not report a single universal “bond angle 109.5°” merely because phosphorus is a central main-group atom.
That value belongs to an ideal tetrahedral arrangement, not to intact gas-phase PCl₅.
Why a trigonal bipyramid has axial and equatorial positions
Five bonding directions cannot all point equally far apart in three-dimensional space using the familiar trigonal-bipyramidal arrangement.
The three equatorial positions form a triangle in a plane, while the axial bonds extend above and below.
A simple VSEPR argument compares electron-domain repulsions.
Each axial domain lies at about 90° to all three equatorial domains.
Each equatorial domain lies at about 90° to the two axial domains but 120° to the other equatorial domains.
That is why the sites have different angular environments.
In the simple electron-domain model, lone pairs often prefer equatorial positions in trigonal-bipyramidal arrangements because that position can reduce the number of 90° interactions.
However, intact PCl₅ has no central lone pair under its conventional VSEPR model, so that lone-pair placement rule is not the reason for the geometry here.
The basic task is to recognise five bonding directions and arrange them appropriately.
Is gas-phase PCl₅ always the only gas present?
Not necessarily.
At elevated temperature, phosphorus pentachloride can participate in the dissociation equilibrium:
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g).
Therefore an actual heated gas sample may contain PCl₅, phosphorus trichloride and chlorine, depending on temperature, pressure and equilibrium composition.
The phrase “PCl₅ is trigonal bipyramidal in the gas phase” describes the geometry of an intact PCl₅ molecule, not the claim that every heated sample with bulk phosphorus-to-chlorine composition must consist of 100% PCl₅ molecules.
This distinction is important when interpreting gas-volume, molar-mass or equilibrium measurements.
An examiner may provide conditions under which some dissociation occurs. In that case we must count the actual gaseous species rather than simply apply one PCl₅ particle to every formula unit.
The molecular structure and the reaction equilibrium are different layers of the chemistry.
What about the structure of solid PCl₅?
X-ray structural studies show that ordinary crystalline phosphorus pentachloride is commonly represented as an ionic assembly of:
[PCl₄]⁺ and [PCl₆]⁻.
This is the distinctive fact.
Instead of treating every formula unit in the solid as a separate neutral trigonal-bipyramidal PCl₅ molecule, the familiar crystal is described in terms of positively charged tetrachlorophosphonium ions and negatively charged hexachlorophosphate(V) ions.
These ions are arranged in the solid through electrostatic interactions, with covalent P–Cl bonds within each polyatomic ion.
So the solid contains both ionic attraction between ions and covalent bonding inside the ions.
The fact that a compound can contain more than one bonding category does not contradict chemical bonding theory.
The relevant structure depends on how the atoms reorganise to reach an energetically favourable condensed phase.
OpenStax explicitly identifies the crystalline solid as [PCl₄]⁺[PCl₆]⁻, while showing intact PCl₅ as a molecular species in the gas.
How can [PCl₄]⁺ and [PCl₆]⁻ still represent PCl₅ overall?
At first glance, the ions appear to contain four and six chlorines rather than five.
Add their atom counts:
[PCl₄]⁺ + [PCl₆]⁻ contains:
- Two phosphorus atoms total.
- Ten chlorine atoms total.
The overall formula is therefore:
P₂Cl₁₀.
Divide the atom counts by two to obtain the simplest empirical ratio:
PCl₅.
This means the crystal’s empirical composition can be PCl₅ even though the actual ionic species inside it have different chlorine coordination numbers.
The overall ratio does not identify one neutral molecular building block.
This is the same counting principle used for extended lattices: the empirical formula expresses composition; the structural formula identifies the particles and bonds.
A good student can explain both without assuming one must invalidate the other.
Write the self-ionisation equation
A convenient overall representation of the solid-state rearrangement is:
2PCl₅ ⇌ [PCl₄]⁺ + [PCl₆]⁻.
Check phosphorus:
Left has 2 P atoms. Right has 1 + 1 = 2 P atoms.
Check chlorine:
Left has 2 × 5 = 10 Cl atoms. Right has 4 + 6 = 10 Cl atoms.
Check charge:
Left has total zero. Right has +1 and −1, totalling zero.
Thus the equation conserves atoms and charge.
This is a structural and stoichiometric summary of the ionic-solid composition, not a claim that a real crystal forms through one isolated elementary collision between two PCl₅ gas molecules in precisely that time sequence.
The solid’s actual formation and rearrangement involve the thermodynamics and structure of the condensed material.
The equation is nevertheless useful for understanding why two ionic species can correspond to the single empirical formula PCl₅.
Find the shape of [PCl₄]⁺
The positive ion [PCl₄]⁺ contains one phosphorus centre with four chlorine donor/bonding directions in the familiar Lewis/VSEPR model.
There is no central lone pair in the conventional four-coordinate representation.
The ideal shape is therefore tetrahedral.
The ideal Cl–P–Cl angle is:
109.5°.
Notice that this is different from the gas-phase PCl₅ molecule.
PCl₅ has five bonding domains and trigonal-bipyramidal geometry.
[PCl₄]⁺ has four bonding domains and tetrahedral geometry.
The ion’s positive charge does not automatically mean that it has a lone pair or that it becomes planar. We must count the electron domains around the central phosphorus in the actual ion.
A student should not draw [PCl₄]⁺ as square planar simply because four atoms surround phosphorus.
In the familiar main-group VSEPR model, the ion is tetrahedral.
Find the shape of [PCl₆]⁻
The negative ion [PCl₆]⁻ contains one phosphorus centre with six P–Cl bonding directions in the familiar structure.
There are no central lone pairs in the simple six-coordinate VSEPR representation.
Six bonding domains arrange in an octahedral geometry.
The ideal adjacent Cl–P–Cl bond angles are:
90°.
Between opposite chlorine positions, the angle is:
180°.
The ion is therefore not a trigonal bipyramid with one extra chlorine simply attached arbitrarily.
Its six-coordinate geometry has a different symmetrical arrangement.
This contrast gives a useful three-way VSEPR exercise:
PCl₅ molecule: trigonal bipyramidal.
[PCl₄]⁺ ion: tetrahedral.
[PCl₆]⁻ ion: octahedral.
All use phosphorus as the central atom, but the number of bonding domains differs.
Three structures in one table
| Species | Central bonding domains | Simple VSEPR shape | Ideal key angles | Formal charge |
|---|---|---|---|---|
| PCl₅ molecule | 5 | Trigonal bipyramidal | 90°, 120°, 180° | 0 |
| [PCl₄]⁺ | 4 | Tetrahedral | 109.5° | +1 |
| [PCl₆]⁻ | 6 | Octahedral | 90°, 180° | −1 |
The table is a concise structural summary. It does not mean PCl₅ has three different shapes simultaneously as one intact molecule.
The species differ in composition and charge, and the physical state determines which representation is relevant.
Why the solid can be ionic even though the gas molecule is covalent
In a gas of intact PCl₅ molecules, the individual molecules can be separated from one another under the appropriate conditions.
The bonding within each molecule is described using covalent P–Cl interactions.
In a solid, the chemical system can achieve a favourable structure by arranging [PCl₄]⁺ and [PCl₆]⁻ ions into a crystal, with electrostatic attraction between the charged species.
The formation of the ionic assembly depends on the balance of energetic contributions: bond rearrangement, ion formation and stabilization within the crystal.
It would be misleading to say that the solid becomes ionic simply because lower temperature always creates ions in every covalent molecule.
Many covalent molecular compounds remain molecular solids.
Phosphorus pentachloride is a specific example where the condensed solid’s energetic and structural preferences favour an ionic form.
The useful scientific principle is that phase and surrounding environment can change which chemical species are most stable.
Why this is not an oxidation–reduction reaction
A change from neutral PCl₅ molecules to the ionic species [PCl₄]⁺ and [PCl₆]⁻ might look like a redox process.
But check the formal oxidation state of phosphorus in each species.
For PCl₅, assign chlorine −1:
x + 5(−1) = 0, so x = +5.
For [PCl₄]⁺:
x + 4(−1) = +1, so x = +5.
For [PCl₆]⁻:
x + 6(−1) = −1, so x = +5.
Phosphorus remains in oxidation state +5 throughout the familiar structural comparison.
There is no change in phosphorus’s formal oxidation number, and no corresponding electron-transfer half-equations are required for the net ionic association model.
Therefore 2PCl₅ ⇌ [PCl₄]⁺ + [PCl₆]⁻ is not an ordinary oxidation–reduction reaction.
It is a rearrangement of bonding and coordination into oppositely charged ionic species.
Oxidation number is not equal to the charge on the entire complex
For [PCl₄]⁺, the net ion charge is +1, but phosphorus’s oxidation number is +5.
For [PCl₆]⁻, the net ion charge is −1, yet phosphorus’s oxidation number is again +5.
This is because the formal oxidation-number calculation assigns a contribution of −1 to each chlorine atom in the simple convention.
Thus:
Complex charge ≠ central atom’s oxidation number when ligands or bonded atoms carry formal electron-accounting contributions.
The distinction is the same one students meet in metal complexes such as [Cu(NH₃)₄(H₂O)₂]²⁺ or hydrolysed iron aqua ions.
A student who says phosphorus must be −1 in [PCl₆]⁻ has confused the total ion charge with the oxidation state of one atom inside that ion.
Charge-balancing equations are the best way to avoid that mistake.
How can phosphorus have five or six bonding directions?
A familiar elementary octet rule says that main-group atoms often achieve eight valence electrons in stable bonding arrangements.
But the octet rule is a pattern, not a universal law for every main-group species.
Gas-phase PCl₅ and [PCl₆]⁻ are examples of species described in school Chemistry as having expanded valence coordination beyond the simplest four-electron-pair octet model.
In the basic Lewis electron-counting picture, phosphorus is shown with five bonding directions in PCl₅ and six in [PCl₆]⁻.
At a more advanced level, bonding in hypervalent compounds is described using molecular orbitals, polar bonds and multicentre bonding, rather than requiring a literal picture of many phosphorus 3d orbitals forming ordinary localised bonds.
This is an important scientific qualification. Older explanations sometimes claim that phosphorus simply “promotes electrons into empty 3d orbitals” as the sole cause of fivefold coordination. That is not a reliable complete modern bonding explanation.
For H2 VSEPR, count the bonding domains and predict the shape; do not invent a detailed orbital-hybridisation mechanism that the question does not require.
The role of Lewis acid–base interactions
Phosphorus pentachloride can behave as a Lewis acid in suitable chemical contexts because it can accept electron density from appropriate donors such as chloride.
An illustrative equation is:
PCl₅ + Cl⁻ ⇌ [PCl₆]⁻.
The chloride ion donates electron density, and the resulting anion contains six chlorine neighbours around phosphorus in the familiar model.
We can also describe a complementary formal ion-forming step:
PCl₅ ⇌ [PCl₄]⁺ + Cl⁻,
as an electron-pair and chloride-transfer bookkeeping representation.
Adding these formal equations gives:
2PCl₅ ⇌ [PCl₄]⁺ + [PCl₆]⁻.
This provides a useful Lewis acid–base interpretation of how the ionic species relate to the neutral formula.
But do not assume these two equations establish a complete experimentally measured elementary pathway by which a crystal forms.
They express a net chemical rearrangement and coordination picture.
What is the significance of [PCl₄]⁺ having positive charge?
The [PCl₄]⁺ ion is positively charged overall, even though the phosphorus has four covalent P–Cl bonds in the familiar Lewis model.
A positively charged polyatomic ion can still contain covalent bonds inside it.
Likewise, [PCl₆]⁻ is negatively charged as a whole while the phosphorus and chlorine atoms are connected through the relevant bonding framework.
In the solid, electrostatic attraction between the cations and anions helps stabilise the lattice.
This is why the phrase ionic compound does not mean there are no covalent bonds anywhere inside the material.
Ammonium chloride is a simple parallel example: NH₄⁺ contains covalent N–H bonds, while NH₄⁺ and Cl⁻ are held together through ionic interactions in the crystal.
The classification depends on the interactions between structural particles as well as the bonds inside them.
Comparison with phosphorus trichloride
Phosphorus trichloride, PCl₃, has one central phosphorus atom bonded to three chlorines in the familiar simple molecular structure.
The central phosphorus has one lone pair as well as three bonding domains.
Thus PCl₃ is trigonal pyramidal under the standard VSEPR model.
The approximate bonding geometry differs from intact PCl₅, which has five bonding domains and no central lone pair in the conventional model.
PCl₃ is also relevant to the gaseous dissociation equilibrium:
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g).
This equilibrium is separate from the solid-state description involving [PCl₄]⁺ and [PCl₆]⁻.
A student must not mix the two equations and claim PCl₃ is one of the two ions that build the solid crystal.
PCl₃ is a neutral molecular phosphorus trichloride species, while [PCl₄]⁺ is a cation with different composition and shape.
Why gaseous PCl₅ dissociation responds to pressure
For the equilibrium:
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g),
the left-hand side contains one mole of gas particles per mole of reaction, while the right contains two.
At constant temperature in a suitable ideal-gas model, compression that increases the relevant reacting gas partial pressures favours the side with fewer gas molecules, PCl₅.
Expansion at constant temperature favours the side with more gas molecules, PCl₃ and Cl₂.
The corresponding concentration-form equilibrium expression is:
Kc = [PCl₃][Cl₂]/[PCl₅].
The square brackets refer to gaseous species present at equilibrium, not to the crystalline ions in solid PCl₅.
The value of Kc depends on temperature for this fixed reaction equation.
Students should not use the self-ionisation equation for the solid to calculate the gaseous Kc expression. They are different chemical processes.
A worked gas-equilibrium example
Suppose a hypothetical sealed 1.00 dm³ container initially holds 0.100 mol gaseous PCl₅, with no PCl₃ or Cl₂ present.
At equilibrium, 0.0200 mol PCl₅ has dissociated according to:
PCl₅ ⇌ PCl₃ + Cl₂.
The amounts are:
| Gas species | Initial moles | Change | Equilibrium moles |
|---|---|---|---|
| PCl₅ | 0.100 | −0.0200 | 0.0800 |
| PCl₃ | 0 | +0.0200 | 0.0200 |
| Cl₂ | 0 | +0.0200 | 0.0200 |
Because the container volume is 1.00 dm³, the numerical concentrations in mol dm⁻³ equal the listed mole amounts divided by 1.00.
Therefore:
Kc = (0.0200 × 0.0200)/0.0800.
Kc = 0.00500 mol dm⁻³ under the direct concentration-form convention.
This is an invented dataset, not the measured equilibrium constant of real PCl₅ at an unstated temperature.
The calculation is useful because it connects PCl₅ molecular composition to equilibrium stoichiometry without confusing the gas reaction with the ionic solid structure.
Why total gas moles increase on dissociation
Initially the hypothetical system contains 0.100 mol gas molecules.
At equilibrium it contains:
0.0800 + 0.0200 + 0.0200 = 0.120 mol gas molecules.
The gas molecule amount has increased.
But phosphorus atoms and chlorine atoms remain conserved.
Each PCl₅ molecule can dissociate into one PCl₃ molecule and one Cl₂ molecule.
Before dissociation, that portion contains one phosphorus atom and five chlorine atoms.
After dissociation, PCl₃ contains one phosphorus and three chlorines, while Cl₂ contains the remaining two chlorines.
So no element appears or disappears; only the number and identities of gas molecules change.
This is why pressure effects can depend on gas particle count without violating conservation of matter.
The same general reasoning applies to N₂O₄ ⇌ 2NO₂, another useful gaseous dimer/dissociation equilibrium.
What if the examiner asks about solid PCl₅ conductivity?
An ionic solid often contains charged particles fixed in a crystal structure.
In the simple school model, those ions are not free to migrate over long distances in the solid, so an ionic crystal is not generally an effective ionic conductor in its solid state.
But when an ionic substance forms an appropriate mobile-ion phase, ionic conduction may become possible.
For PCl₅, detailed experimental behaviour can also involve phase changes, dissociation and chemical reactivity, so one should not automatically assume it behaves exactly like molten sodium chloride at an arbitrary temperature.
The correct prediction depends on the actual phase and species present.
For a structural question, the main conclusion is:
Solid PCl₅ has ions in an ionic crystal but those ions are not freely mobile in the ordinary solid-state model.
That is different from a statement about whether any heated, molten or decomposing PCl₅ sample conducts well.
A structural comparison with SiCl₄
Silicon tetrachloride, SiCl₄, is a familiar molecular chloride with a tetrahedral arrangement around silicon.
The formula describes one discrete molecule under ordinary molecular conditions.
PCl₅ is more complicated because intact gas-phase molecules have a trigonal-bipyramidal geometry, while the crystalline solid is ionic.
Therefore not every Period 3 chloride has exactly the same structural behaviour.
A student should not classify all Period 3 chlorides as either purely covalent molecular in every phase or purely ionic in every phase.
The bonding and phase behaviour depend on the substance and conditions.
This is related to the broader Period 3 chloride chemistry where NaCl is a familiar ionic lattice, SiCl₄ is molecular and AlCl₃ displays significant covalent character and association.
PCl₅ offers another clear reminder to read the exact chemical state.
Why PCl₅ can react with water
Phosphorus pentachloride is moisture-sensitive and can hydrolyse.
Under an excess-of-water textbook model, its overall transformation can be represented as:
PCl₅ + 4H₂O → H₃PO₄ + 5HCl.
Check atoms:
- One phosphorus on both sides.
- Five chlorines on both sides.
- Eight hydrogens on the left and three plus five on the right.
- Four oxygens on both sides.
A simpler stepwise bookkeeping route may include phosphorus oxychloride, POCl₃:
PCl₅ + H₂O → POCl₃ + 2HCl.
Then:
POCl₃ + 3H₂O → H₃PO₄ + 3HCl.
Adding those two equations gives the overall excess-water equation.
These equations illustrate covalent chloride hydrolysis and the importance of reaction conditions. They are not instructions for handling PCl₅, which is corrosive and can produce hazardous hydrogen chloride.
The hydrolysis chemistry is a different question from whether solid PCl₅ is ionic or gas-phase PCl₅ is trigonal bipyramidal.
Why the formula PCl₅ does not mean five independent chloride ions
In an intact gaseous PCl₅ molecule, the chlorine atoms are covalently associated with the central phosphorus in a neutral molecular species.
It would be wrong to represent one gaseous PCl₅ molecule as:
P⁵⁺ + 5Cl⁻,
as if it were simply a group of six isolated gas-phase ions.
In the crystalline solid, the observed ions are [PCl₄]⁺ and [PCl₆]⁻, each containing internal P–Cl bonding.
Thus even the ionic solid is not a lattice of bare P⁵⁺ ions surrounded only by separate free Cl⁻ ions.
This is why oxidation-number formalism must not be confused with the actual identity of particles in a sample.
The formal phosphorus oxidation state is +5 in PCl₅, [PCl₄]⁺ and [PCl₆]⁻, but that does not require a bare P⁵⁺ ion to be present.
This distinction is valuable across covalent oxoanions, transition-metal complexes and polyatomic salts.
Why a bond-counting model is useful but incomplete
The familiar VSEPR model predicts molecular shape from the number of electron domains around a central atom.
It works well for identifying the ideal geometries of:
PCl₅: five bonding domains.
[PCl₄]⁺: four bonding domains.
[PCl₆]⁻: six bonding domains.
But VSEPR does not, by itself, predict that crystalline PCl₅ must be composed of [PCl₄]⁺ and [PCl₆]⁻ rather than neutral PCl₅ molecules.
That solid-state structural claim is supported by experimental evidence and more detailed energetic reasoning.
In other words, a correct local bonding model does not automatically determine the most stable bulk crystal phase.
Students should distinguish:
VSEPR: given a species, what geometry is expected?
Solid-state chemistry: which species and packing arrangement are actually present in the crystal?
Both questions matter, but they require different evidence.
How to answer a three-part examination question
Suppose a problem asks:
(a) State the shape of gaseous PCl₅.
Answer: Trigonal bipyramidal, with ideal bond angles 90°, 120° and 180°.
(b) Given that solid PCl₅ consists of [PCl₄]⁺ and [PCl₆]⁻, predict the shapes of these ions.
Answer: [PCl₄]⁺ tetrahedral, [PCl₆]⁻ octahedral.
(c) Explain why the empirical formula of the solid remains PCl₅.
Answer: One [PCl₄]⁺ ion and one [PCl₆]⁻ ion have total composition P₂Cl₁₀ and overall neutral charge. Dividing the atom ratio by two gives PCl₅.
This is a clear integrated answer.
It avoids overcomplicating a straightforward shape question with a speculative orbital description or treating the different physical phases as a contradiction.
A table of misconceptions
| Student statement | First error | Repair question |
|---|---|---|
| PCl₅ has the same trigonal-bipyramidal structure in every phase | Ignored ionic crystalline structure | What ions are identified in solid PCl₅? |
| Solid PCl₅ consists of isolated P⁵⁺ and five Cl⁻ ions | Confused oxidation states with particle identities | What are the actual polyatomic ions? |
| [PCl₄]⁺ must be square planar because it has four chlorines | Misapplied VSEPR | Are there four bonds and zero central lone pairs? |
| [PCl₆]⁻ must be trigonal bipyramidal | Ignored its six bonding domains | What is the geometry for six bond directions? |
| Phosphorus becomes +4 in [PCl₄]⁺ | Ignored chlorine’s oxidation-number assignments | What is x in x − 4 = +1? |
| The [PCl₄]⁺ and [PCl₆]⁻ pair changes the empirical formula to P₂Cl₁₀ only | Forgot simplest formula ratio | Can P₂Cl₁₀ be simplified? |
| Formation of charged ions means oxidation or reduction occurred | Confused net charge redistribution and redox | Does phosphorus’s oxidation state change? |
| Gas-phase PCl₅ can never dissociate | Ignored PCl₅ ⇌ PCl₃ + Cl₂ equilibrium | What happens under changing gas conditions? |
| VSEPR alone proves the solid crystal’s ion identities | Confused molecular shape prediction with structure determination | What evidence establishes the crystal species? |
Eight practice questions
- Give the ideal molecular shape of an intact gaseous PCl₅ molecule.
- State the three ideal bond angles found in a trigonal bipyramid.
- Name the two ions in the familiar ionic structure of solid phosphorus pentachloride.
- Give the shapes and ideal angles of [PCl₄]⁺ and [PCl₆]⁻.
- Show how the combination of one [PCl₄]⁺ and one [PCl₆]⁻ yields the empirical formula PCl₅.
- Calculate the phosphorus oxidation state in each of the three species.
- Write the gaseous dissociation equilibrium involving PCl₅, PCl₃ and Cl₂.
- Explain why the structural change between gaseous PCl₅ and solid PCl₅ does not imply an ordinary redox reaction.
Answers
1. Trigonal bipyramidal. 2. Approximately 90°, 120° and 180°. 3. [PCl₄]⁺ and [PCl₆]⁻. 4. [PCl₄]⁺ tetrahedral, approximately 109.5°; [PCl₆]⁻ octahedral, 90° and 180°. 5. Together the ions give P₂Cl₁₀, simplifying to PCl₅. 6. Phosphorus is +5 in all three. 7. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). 8. The coordination and ionic species change, but the formal oxidation state of phosphorus remains +5.
A student who can answer the first two but not Questions 3–5 needs help distinguishing a molecule’s shape from the actual ions and empirical composition of a crystal.
A seven-session JC1–JC2 revision path
- Session 1: revise the meaning of molecular formula, empirical formula and formula unit.
- Session 2: count five bonding electron domains and draw intact PCl₅ with axial and equatorial positions.
- Session 3: identify the 90°, 120° and 180° ideal gas-phase bond angles.
- Session 4: draw tetrahedral [PCl₄]⁺ and octahedral [PCl₆]⁻ and check their charges.
- Session 5: derive PCl₅ as the simplest ratio from the ionic pair and verify phosphorus oxidation state in each species.
- Session 6: distinguish solid-state self-ionisation from gaseous PCl₅ ⇌ PCl₃ + Cl₂ dissociation.
- Session 7: compare PCl₅ with PCl₃, SiCl₄ and SiO₂, identifying which are molecular or network or ionic in the stated phase.
The final task should use changed species, so the learner has to choose the appropriate structural model before predicting bond angles or properties.
What parents seeking an H2 Chemistry tutor near Hougang can ask
Bring a marked bonding or molecular-shape question. Ask whether the learner confused the compound formula with its particles in a solid, the number of bonding domains with total valence electrons, ionic charge with phosphorus oxidation number or one physical state with another.
A useful three-student or individual tutorial should finish with a changed VSEPR example where students independently draw the species, calculate the formal charge and justify the stated shape.
Confirm current H2 Chemistry syllabus experience, actual lesson venue, timetable, available places and travel requirements. A Hougang-focused educational guide does not itself prove that a physical tuition classroom operates in Hougang.
Why the concept matters beyond the examination
Solid-state and molecular chemistry both describe the same elements, but the stable arrangements can differ with phase and environment.
This matters in materials science, chemical manufacturing, crystal chemistry and the interpretation of structural evidence.
A molecular drawing describes the bonding within a chosen species. A crystal structure describes what species and interactions actually build the solid.
The transferable scientific habit is to ask what the formula describes, which phase is present, what particles are observed and what model the question actually calls for.
That is useful preparation for advanced Chemistry and related university pathways without promising a particular grade or admission outcome.
Frequently asked questions
What is the shape of gaseous PCl₅?
Trigonal bipyramidal, with ideal angles around 90°, 120° and 180°.
Why is solid PCl₅ described as ionic?
The familiar crystalline structure contains [PCl₄]⁺ and [PCl₆]⁻ ions, rather than only isolated neutral PCl₅ molecules.
What is the shape of [PCl₄]⁺?
Tetrahedral, with an ideal bond angle of about 109.5°.
What is the shape of [PCl₆]⁻?
Octahedral, with ideal adjacent angles of 90° and opposite directions of 180°.
Why is the solid’s formula PCl₅ if the ions contain four and six chlorines?
One cation and one anion together contain P₂Cl₁₀, which simplifies to the empirical ratio PCl₅.
Is phosphorus oxidised or reduced in the solid-state self-ionisation?
No. It has formal oxidation state +5 in PCl₅, [PCl₄]⁺ and [PCl₆]⁻.
Can gaseous PCl₅ dissociate?
Yes. It can form an equilibrium with PCl₃(g) and Cl₂(g), depending on conditions.
Is the solid-state ionic structure compulsory for H2 9476?
The core VSEPR and bonding concepts are syllabus-relevant. Treat the particular ionic crystal composition and detailed hypervalent bonding as structure–property enrichment unless the examination-year outcomes or question specifically require it.
Continue learning and references
Why SiO₂ is a giant covalent network but CO₂ is molecular · Why aluminium chloride and sodium chloride behave differently in water · Why silicon tetrachloride hydrolyses but carbon tetrachloride resists · VSEPR shapes, lone pairs and bond angles · OpenStax: phosphorus halides, gas-phase shapes and ionic solid PCl₅ · Hougang Chemistry Tutor Library · eduKate Singapore Chemistry enquiries.
Safety and model note: PCl₅, PCl₃ and chlorine-containing reaction systems are hazardous, moisture-sensitive or corrosive. These structural equations are educational; they are not directions for preparation, heating or sublimation outside professional laboratory controls. The gas equilibrium calculation uses hypothetical data. The solid-state ionic representation is based on reported crystal structures and is distinct from the molecular geometry of intact PCl₅ gas.