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Phosphorus and Its Compounds: Phosphides, Phosphine and Fertilisers

Phosphorus is a non-metal in group 15, just below nitrogen. White phosphorus is P₄ (a strained 3D pyramid) and catches fire in air; red phosphorus is a stable chain. It burns to P₄O₆ or P₄O₁₀, and P₄O₁₀ with water gives phosphoric acid. Metal phosphides such as Ca₃P₂ give poisonous phosphine PH₃ with water. Phosphates from phosphoric acid are the key fertilisers.

🎬 Step-by-step story

  1. White phosphorus is four P atoms in a tiny pyramid: count 4 atoms and 6 bonds. The bonds are bent to 60°, so the molecule is strained and very eager to react.
  2. Heat white phosphorus without air and the pyramids break open and join in long chains. This is red phosphorus. No strain, so it is calm and safe to keep in the open.
  3. Burn it with little air. One oxygen atom (red) slips between every two P atoms, on all 6 edges. This is P₄O₆.
  4. With plenty of air, 4 more oxygen atoms attach, one on each P by a double bond. This is P₄O₁₀, the white smoke.
  5. Metal phosphides like Ca₃P₂ meet water and release phosphine, PH₃: a P with 3 H and a lone pair. It is a poisonous gas that smells of rotten fish.
  6. P₄O₁₀ plus water gives phosphoric acid, H₃PO₄. Its salts, phosphates, feed plants as fertiliser. Pick any molecule and count its atoms.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why is P₄ so ready to catch fire?

The P–P–P angle is only 60°, far from what P likes. The strain stores energy and makes the bonds easy to break.

Why is red phosphorus safe?

The pyramids opened into chains. With no strained angles the bonds are stronger, so it needs about 260 °C to ignite.

Why do we get P₄O₆ sometimes and P₄O₁₀ at other times?

It depends on the air. With little oxygen only the 6 edges get an O. With plenty, each P also takes an extra O.

What is the P=O in P₄O₁₀?

A double bond from P to an end O atom. The 4 end O atoms plus the 6 bridge O atoms make 10 O.

Why is phosphine such a weak base, and why does it smell?

P is big, so its lone pair is spread out and does not grab H⁺ well. The smell of rotten fish comes from traces of related impure compounds, and the gas itself is dangerous even at low levels.

How is phosphoric acid linked to fertilisers?

Its salts are phosphates. Mixing H₃PO₄ with ammonia or with phosphate rock gives soluble fertilisers that supply P to plants.

Phosphorus and its allotropes

Phosphorus (symbol P, atomic number 15) is a non-metal in group 15, the same family as nitrogen. It does not occur free in nature; it is found as phosphates, for example phosphate rock, Ca₃(PO₄)₂.

Allotropes are different forms of the same element:

Making it: heat phosphate rock with sand and coke in an electric furnace: 2Ca₃(PO₄)₂ + 6SiO₂ + 10C → 6CaSiO₃ + 10CO + P₄. The P₄ vapour is cooled under water.

Why is phosphorus so much more reactive than nitrogen (N₂)? N≡N has a very strong triple bond; P–P single bonds in a strained P₄ are weak.

Burning phosphorus: oxides and phosphoric acid

Phosphorus burns in oxygen. The product depends on how much air there is.

P₄O₁₀ loves water, so it is used as a drying agent. With water it gives phosphoric acid:

P₄O₁₀ + 6 H₂O → 4 H₃PO₄

H₃PO₄ has three –OH groups that can give H⁺, so it is tribasic. It forms three kinds of salt with NaOH: NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ (phosphates). It is used in soft drinks, rust removers and fertilisers.

Phosphides and phosphine

A phosphide is a compound of phosphorus with a metal, such as calcium phosphide Ca₃P₂ or aluminium phosphide AlP. (P here is P³⁻.)

Phosphides react with water or acid and give phosphine, PH₃:

You can also make it by boiling white phosphorus with NaOH solution: P₄ + 3 NaOH + 3 H₂O → PH₃ + 3 NaH₂PO₂.

Phosphine, PH₃: a colourless, very poisonous gas with a smell like rotten fish or garlic. The molecule is a pyramid (P on top, three H below, one lone pair), bond angle about 93.5°, smaller than ammonia (107°). It is only a very weak base: PH₃ + HI → PH₄I (phosphonium iodide). It burns: 2 PH₃ + 4 O₂ → P₂O₅ + 3 H₂O. Impure phosphine can catch fire by itself, which may explain strange "ghost lights" over marshes.

Phosphates and fertilisers

Plants need N, P and K (nitrogen, phosphorus, potassium). Phosphorus is in DNA, in ATP (the cell's energy packet) and in bones. Plants take it as soluble phosphate ions.

Problem: phosphate rock Ca₃(PO₄)₂ is insoluble in water, so roots cannot use it. Fix: turn it into a soluble salt.

Care: too much phosphate washes into ponds and lakes and feeds algae. The algae cover the water and later rot, using up oxygen so fish die. This is eutrophication.

Try it: predict, then check

In the 3D board pick P₄O₆ and count the red O atoms before reading the caption (answer 6). Then pick P₄O₁₀ and count again (10). Safe home task: look at the strip on the side of a matchbox. It is red phosphorus. Strike a match and watch the head burn: that is phosphorus compounds reacting with oxygen. Never handle white phosphorus. Look at a fertiliser bag label and find the three numbers N-P-K.

Key formulas and definitions

Worked examples

1. Balance: P₄ + O₂ → P₄O₁₀.

Right side has 10 O atoms, so we need 5 O₂ on the left. P₄ + 5 O₂ → P₄O₁₀. P: 4 = 4, O: 10 = 10.

2. What mass of P₄O₁₀ forms when 6.2 g of phosphorus burns in plenty of air? (P = 31, O = 16)

Moles of P atoms = 6.2 ÷ 31 = 0.2. Each P₄O₁₀ holds 4 P atoms, so 0.2 ÷ 4 = 0.05 mol P₄O₁₀. Molar mass P₄O₁₀ = 4×31 + 10×16 = 284 g/mol. Mass = 0.05 × 284 = 14.2 g.

3. 18.2 g of Ca₃P₂ reacts with excess water. Find the mass of PH₃ formed. (Ca = 40, P = 31, H = 1)

Molar mass Ca₃P₂ = 3×40 + 2×31 = 182 g/mol. Moles = 18.2 ÷ 182 = 0.1. The equation gives 2 PH₃ per Ca₃P₂, so 0.2 mol PH₃. Molar mass PH₃ = 34. Mass = 0.2 × 34 = 6.8 g. (At STP the volume is 0.2 × 22.4 = 4.48 L.)

4. Why is red phosphorus safe to keep in the open but white phosphorus is not?

White phosphorus is P₄ with 60° bond angles. The strain makes the bonds weak, so it reacts with oxygen at about 30 °C and catches fire. Red phosphorus is a chain with no such strain and ignites only near 260 °C.

5. How many moles of NaOH fully neutralise 0.1 mol of H₃PO₄?

H₃PO₄ is tribasic: H₃PO₄ + 3 NaOH → Na₃PO₄ + 3 H₂O. So 0.1 × 3 = 0.3 mol NaOH.

6. Find the percentage of phosphorus by mass in H₃PO₄.

Molar mass = 3×1 + 31 + 4×16 = 98 g/mol. Percentage of P = 31 ÷ 98 × 100 ≈ 31.6 %.

7. What mass of H₂SO₄ is needed to turn 31 g of Ca₃(PO₄)₂ into single superphosphate? (Ca = 40, P = 31, O = 16, S = 32)

Molar mass Ca₃(PO₄)₂ = 3×40 + 2×(31 + 64) = 310 g/mol. Moles = 31 ÷ 310 = 0.1. Equation: 1 Ca₃(PO₄)₂ needs 2 H₂SO₄, so 0.2 mol. Molar mass H₂SO₄ = 98. Mass = 0.2 × 98 = 19.6 g.

8. Why is the H–P–H angle in PH₃ (about 93.5°) smaller than the H–N–H angle in NH₃ (107°)?

Phosphorus is a bigger atom and its bonding orbitals are longer and mix less. The lone pair takes more room, so the three P–H bonds are squeezed closer to 90°.

Common mistakes

Practice quiz

1. The formula of white phosphorus is:
2. White phosphorus is stored:
3. Ca₃P₂ + water gives:
4. H₃PO₄ is:
5. Superphosphate is made so that the phosphate becomes:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

Why is phosphorus kept under water?

White phosphorus catches fire in air. Water keeps oxygen away and phosphorus does not dissolve in it.

What is the difference between phosphorus, phosphide and phosphate?

Phosphorus is the element P. A phosphide contains P³⁻ joined to a metal (like Ca₃P₂). A phosphate contains PO₄³⁻ (like Ca₃(PO₄)₂).

Where is phosphorus taught?

In many countries it appears in the non-metals or group 15 part of senior school chemistry, next to nitrogen.

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