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Chemistry: Atoms First 2e · Representative Metals, Metalloids, and Nonmetals

Occurrence, Preparation, and Properties of Phosphorus

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Reference-values note: atomic masses, furnace temperatures (~1400–1500 °C), and white phosphorus autoignition (~30–40 °C) are commonly taught reference values; use the periodic table provided in your course for graded work. Safety note: white phosphorus ignites in air and is toxic — general principles (store under water, avoid skin contact and inhalation, trained supervision) only, no lab procedures.
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Phosphorus (P, atomic number 15) is nitrogen's larger, more restless cousin in Group 15: it is never found free in nature because it is too reactive and too easily oxidized. Every phosphorus atom on Earth is locked inside compounds: in rocks such as , in soils, and in every living cell, where phosphate groups build DNA, RNA, and the energy currency ATP. The element comes in striking allotropes: white phosphorus, a waxy, toxic solid that bursts into flame in air and glows in the dark; red phosphorus, a safer polymer used on match striking surfaces; and black phosphorus, a stable layered form. This topic covers where phosphorus is found, how it is extracted from phosphate rock in an electric furnace, and the compounds that make it indispensable to agriculture, biology, and industry.

Why this matters

Phosphorus is a limiting nutrient: plants cannot grow without it, and phosphate fertilizer is mined, not made from air — a finite strategic resource. Understanding phosphate chemistry explains why fertilizer contains phosphorus, why phosphate runoff causes algal blooms, why bones are built from calcium phosphate, and why ATP is the cell's "energy currency". White phosphorus is also a safety story: it ignites spontaneously in air, so it is stored under water. On exams, phosphorus tests your grasp of allotropy (same element, different structures and reactivities), oxidation states (+3 vs +5), and furnace-reaction stoichiometry.

The college version

Core Concepts

Occurrence: phosphate rock and living matter

Phosphorus is about the 11th most abundant element in Earth's crust by mass, but it always appears combined as phosphates. The main minerals are apatite, Ca5(PO4)3(F,OH,Cl) — most commonly fluorapatite — and phosphorite (phosphate rock), essentially calcium phosphate, Ca3(PO4)2, with impurities. In biology, phosphorus appears in bones and teeth (hydroxyapatite, Ca5(PO4)3OH), in the phosphate backbone of DNA and RNA, in phospholipid membranes, and in ATP.

Preparation: the electric furnace

Because phosphorus is too reactive to occur free, it is made by reducing phosphate rock with carbon at very high temperature, using sand (SiO2) to remove calcium as slag:

2 Ca3(PO4)2(s) + 6 SiO2(s) + 10 C(s) Δ⟶ P4(g) + 6 CaSiO3(l) + 10 CO(g)

The reaction runs at about 1400–1500 °C in an electric furnace. The P4 vapor (white phosphorus) distills off, condenses under water, and is stored there because it ignites in air.

Allotropes of phosphorus

  • White phosphorus (P4): four phosphorus atoms at the corners of a tetrahedron with strained ~60° bond angles, which explains its high reactivity. It is a waxy, poisonous solid, insoluble in water, soluble in carbon disulfide, and ignites spontaneously in air at about 30–40 °C. It glows faintly in the dark (chemiluminescence from slow oxidation).
  • Red phosphorus: heating white phosphorus without air gives a polymeric form that is much less reactive and used on match striking surfaces.
  • Black phosphorus: the most stable , graphite-like and layered; produced under high pressure.

Properties and oxidation states

Phosphorus has five valence electrons. With active metals it gains three electrons to form ionic phosphides containing P3-, such as Ca3P2, which reacts with water to give phosphine, PH3. With nonmetals, phosphorus is covalent, showing +3 and +5 oxidation states: PCl3 and PCl5, and the oxoacids phosphorous acid H3PO3 (+3) and H3PO4 (+5). The +5 state dominates.

Key compounds

Phosphoric acid, H3PO4, is made industrially by treating phosphate rock with sulfuric acid:

Ca3(PO4)2(s) + 3 H2SO4(aq) → 3 CaSO4(s) + 2 H3PO4(aq)

It is used in fertilizers, soft drinks, detergents, and rust removers. Phosphate fertilizers: acid treatment converts insoluble rock phosphate into soluble, plant-available phosphates (superphosphate). Phosphine (PH3) is a toxic, foul-smelling gas analogous to ammonia but far less basic — the P–H bond is weak and the lone pair is less available.

Common Confusions

Do Not ConfuseWithDifference
Phosphide P3-Phosphate PO43-Phosphide is a bare −3 ion; phosphate is the −3 oxoanion of P(+5)
Phosphite PO33-Phosphate PO43-One fewer oxygen: P(+3) vs P(+5) — don't drop oxygens when naming
AllotropesIsotopesAllotropes differ in structure (same atoms); isotopes differ in neutron number (same structure)
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Phosphorus is like a very hungry puppy that never stays put — you never find it alone in nature because it grabs onto other atoms right away. One form, white phosphorus, is so eager to react that it catches fire just from touching air, so chemists keep it underwater. Its favorite job is being part of phosphate, the "handle" every living cell uses to store energy and build DNA — no phosphorus, no life.

Worked example

Example 1: Percent phosphorus in phosphate rock

What is the mass percent of phosphorus in calcium phosphate, Ca3(PO4)2? (Atomic masses: Ca = 40.08, P = 30.97, O = 16.00 g/mol.)

Step 1 — Formula for molar mass:

M(Ca3(PO4)2) = 3(40.08) + 2[30.97 + 4(16.00)] = 120.24 + 2(94.97) = 310.18 g/mol

Step 2 — Divide the phosphorus mass by the formula mass:

%P = 2(30.97) g P310.18 g Ca3(PO4)2 × 100% = 20.0%

Check: two P atoms (62 g) in a 310 g formula unit is one-fifth — 20%.

Example 2: Furnace stoichiometry — making white phosphorus

How many grams of P4 can be produced from 620.4 g of Ca3(PO4)2? (Molar masses: Ca₃(PO₄)₂ = 310.18 g/mol, P₄ = 123.88 g/mol.)

Step 1 — Grams to moles:

n = 620.4 g × 1 mol Ca3(PO4)2310.18 g = 2.00 mol Ca3(PO4)2

Step 2 — Mole ratio (1 mol P₄ per 2 mol Ca₃(PO₄)₂):

2.00 mol Ca3(PO4)2 × 1 mol P42 mol Ca3(PO4)2 = 1.00 mol P4

Step 3 — Moles to grams:

1.00 mol P4 × 123.88 g P41 mol P4 = 123.9 g P4

Unit check: g rock → mol rock → mol P₄ → g P₄. Answer: 123.9 g P₄.

Example 3: Making phosphoric acid

How many grams of H3PO4 form when 310.2 g of Ca3(PO4)2 reacts with excess sulfuric acid? (Molar masses: Ca₃(PO₄)₂ = 310.18 g/mol, H₃PO₄ = 98.00 g/mol.)

Step 1 — Moles of rock phosphate:

n = 310.2 g × 1 mol310.18 g = 1.00 mol Ca3(PO4)2

Step 2 — Mole ratio (2 mol H₃PO₄ per 1 mol Ca₃(PO₄)₂):

1.00 mol Ca3(PO4)2 × 2 mol H3PO41 mol Ca3(PO4)2 = 2.00 mol H3PO4

Step 3 — Moles to grams:

2.00 mol H3PO4 × 98.00 g1 mol = 196.0 g H3PO4

Every phosphate group ends up in product acid — atoms are conserved.

Key takeaways

  • Phosphorus is never found free; it occurs as phosphates (apatite, phosphate rock, Ca3(PO4)2).
  • Furnace preparation: 2 Ca3(PO4)2 + 6 SiO2 + 10 C → P4 + 6 CaSiO3 + 10 CO, ~1400–1500 °C; product is P4 (white P).
  • Allotropes in order of stability: white (most reactive, toxic, ignites in air, stored under water) → red (polymeric, matchboxes) → black (most stable, layered).
  • White phosphorus is P4, a tetrahedron with strained ~60° angles — the strain drives its reactivity.
  • Oxidation states: −3 (phosphides), +3 (PCl₃, H₃PO₃), +5 (PCl₅, H₃PO₄ — dominant).
  • H3PO4 is made from rock phosphate + H2SO4; a triprotic acid and major industrial chemical.
  • Phosphate is essential to DNA, RNA, ATP, and bone (hydroxyapatite).
  • Safety principle: white phosphorus ignites in air and is toxic — trained handling and containment required.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Why is phosphorus never found as a free element in nature?

    Show answer

    Phosphorus is too reactive toward oxygen (and other elements) to survive free; it is always found oxidized as phosphate minerals.

  2. Name the three allotropes of phosphorus in increasing order of stability, and one use of each.

    Show answer

    White (most reactive; stored under water) → red (polymeric; match striking surfaces) → black (most stable; layered semiconductor).

  3. In the furnace reaction, what roles do carbon and silicon dioxide play?

    Show answer

    Carbon is the reducing agent (oxidized to CO, reducing P(+5) to P(0) in P₄); SiO₂ combines with CaO to form calcium silicate slag, removing calcium from the reaction.

  4. What is the oxidation state of phosphorus in H3PO4, and in Ca3P2?

    Show answer

    In H3PO4: 3(+1) + x + 4(−2) = 0 → x = +5. In Ca3P2: 3(+2) + 2x = 0 → x = −3.

  5. How many grams of P4 come from 1.00 mol of Ca3(PO4)2 (molar mass P₄ = 123.88 g/mol)?

    Show answer

    1.00 mol Ca3(PO4)2 × 1 mol P42 mol Ca3(PO4)2 × 123.88 g1 mol = 61.9 g P4.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Phosphate
PO43- ion, or any compound containing it
Allotrope
Different structural form of the same element
Apatite
Ca5(PO4)3(F,OH,Cl), the main phosphate mineral
Phosphoric acid
H3PO4, a triprotic acid with P in +5

Sources & references

  1. openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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