Atomicity, Radicals & Valency — Key Chemistry Concepts

ATOMICITY, RADICALS & VALENCY

1️⃣ ATOMICITY

Definition

Atomicity is the number of atoms present in one molecule of a substance.

Example:

• O₂ = atomicity 2
• P₄ = atomicity 4
• S₈ = atomicity 8

ATOMICITY OF DIFFERENT CLASSES OF ELEMENTS

a. Noble gases → Monoatomic (Atomicity = 1)

Examples: He, Ne, Ar, Kr, Xe, Rn

These exist as single atoms because they have stable electron configurations.

b. Metals → Mostly Monoatomic

Examples: Na, K, Fe, Cu, Zn, Al, Mg

Metals exist as giant metallic lattices, but each particle is considered a single atom.

c. Non-metals → Mostly Diatomic (Atomicity = 2)

Examples:

• H₂, O₂, N₂
• Cl₂, Br₂, I₂

These exist naturally as pairs of atoms.

d. Triatomic Molecules (Atomicity = 3)

Examples:

• O₃ (ozone)
• CO₂ (carbon dioxide)

e. Tetra-atomic Molecules (Atomicity = 4)

Examples:

• P₄ (phosphorus)
• SO₃ (actually triatomic for the molecule SO₃, but the oxide ion in SO₃²⁻ has 4 atoms)

(Your list had a small mix-up, but P₄ is definitely tetra-atomic.)

f. Polyatomic Molecules (Atomicity > 4)

Examples:

• S₈ (sulphur)
• C₆H₁₂O₆ (glucose)
• C₁₂H₂₂O₁₁ (sucrose)

These contain many atoms.

2️⃣ RADICALS (Ions)

Definition

A radical (also called a polyatomic ion) is a group of atoms that carry a charge and behave like a single ion.

Examples:

Radical	Formula	Charge
Hydroxide	OH⁻	–1
Nitrate	NO₃⁻	–1
Sulphate	SO₄²⁻	–2
Carbonate	CO₃²⁻	–2
Ammonium	NH₄⁺	+1

Radicals do not exist freely in nature; they occur inside compounds.

3️⃣ VALENCY

Definition

Valency is the combining capacity of an element — how many electrons an atom can gain, lose, or share to form a stable compound.

Examples of valency

Element	Valency
H	1
O	2
N	3
C	4
Na	1
Mg	2
Al	3
Cl	1
S	2 or 6
P	3 or 5

Relationship with radicals

Radicals also have valencies which equal their charges.

Radical	Charge	Valency
OH⁻	–1	1
SO₄²⁻	–2	2
CO₃²⁻	–2	2

✔ SUMMARY

• Atomicity → number of atoms in a molecule
• Radicals → charged groups of atoms behaving as a single ion
• Valency → combining power of an atom or radical

 

DETERMINING ATOMICITY

Atomicity means the number of atoms present in one molecule of a substance.

There are two main methods used to determine atomicity:

1️⃣ Counting the Number of Atoms in the Molecule

This is the simplest method.

• Simply look at the chemical formula and count how many atoms are present.

Examples:

• H₂ → atomicity = 2
• O₃ → atomicity = 3
• P₄ → atomicity = 4
• S₈ → atomicity = 8

2️⃣ Using the Ratio of Molecular Mass to Atomic Mass

(✨ This method is used only for molecules made of the same element, e.g., H₂, O₂, P₄, S₈.)

Formula:

Atomicity=Molecular MassAtomic Mass\text{Atomicity} = \frac{\text{Molecular Mass}}{\text{Atomic Mass}}Atomicity=Atomic MassMolecular Mass​

Examples

a. Hydrogen molecule (H₂)

• Atomic mass of H = 1
• Molecular mass of H₂ = 2 × 1 = 2

Atomicity=21=2\text{Atomicity} = \frac{2}{1} = 2Atomicity=12​=2

b. Oxygen molecule (O₂)

• Atomic mass of O = 16
• Molecular mass of O₂ = 2 × 16 = 32

Atomicity=3216=2\text{Atomicity} = \frac{32}{16} = 2Atomicity=1632​=2

This ratio method only works for:

• Diatomic molecules (H₂, O₂, Cl₂, N₂)
• Polyatomic elements (P₄, S₈, O₃)
• Not for compounds (e.g., CO₂, H₂O)

✔ Quick Examples Using the Ratio Method

Sulphur (S₈)

• Atomic mass of S = 32
• Molecular mass = 8 × 32 = 256

25632=8⇒Atomicity = 8\frac{256}{32} = 8 \Rightarrow \text{Atomicity = 8}32256​=8⇒Atomicity = 8

Phosphorus (P₄)

• Atomic mass of P = 31
• Molecular mass = 4 × 31 = 124

12431=4⇒Atomicity = 4\frac{124}{31} = 4 \Rightarrow \text{Atomicity = 4}31124​=4⇒Atomicity = 4

✔ SUMMARY (Easy to Remember)

• Atomicity = number of atoms in a molecule.
• Can be found by:
  1. Counting atoms in the formula.
  2. Dividing molecular mass by atomic mass (only for elemental molecules).

 

✅ Atomicity Table (Corrected and Completed)

Atomicity = total number of atoms in one molecule of the substance.

1. Calcium carbonate

• Use: Limestone / Bones
• Formula: CaCO₃
• Atomicity: Ca(1) + C(1) + O₃(3) = 5

2. Aspirin (Acetylsalicylic acid)

Your formula had errors. Correct formula = C₉H₈O₄

• Use: Analgesic (pain relief)
• Formula: C₉H₈O₄
• Atomicity: 9 + 8 + 4 = 21

3. Iron(III) hydroxide

• Use: Cosmetics / Water treatment
• Formula: Fe(OH)₃
• Atomicity: Fe(1) + O₃(3) + H₃(3)
  = 1 + 3 + 3 → 7

(You had 27 and 21, both incorrect.)

4. Paradichlorobenzene (PDCB)

Correct formula = C₆H₄Cl₂

• Use: Moth balls
• Formula: C₆H₄Cl₂
• Atomicity: 6 + 4 + 2 = 12

5. Magnesium hydroxide

• Use: Milk of magnesia
• Formula: Mg(OH)₂
• Atomicity: Mg(1) + O₂(2) + H₂(2)
  = 1 + 2 + 2 → 5

6. TNT (Trinitrotoluene)

Formula: C₇H₅(NO₂)₃

• Use: Explosive
• Formula: C₇H₅(NO₂)₃
  Atomicity:
• C: 7
• H: 5
• (NO₂)₃ → N₃O₆ → total = 3 + 6 = 9
  Total atomicity = 7 + 5 + 9 = 21

7. Pyrite

Formula: FeS₂

• Use: Fool’s gold
• Formula: FeS₂
• Atomicity: 1 + 2 = 3

8. Calcium dihydrogen phosphate

Formula: Ca(H₂PO₄)₂

• Use: Fertilizer
  Atomicity:
• Ca = 1
• (H₂PO₄)₂ →
  • H = 4
  • P = 2
  • O = 8
    Total = 1 + 4 + 2 + 8 = 15

9. Tetraoxosulphate(VI) acid (Sulphuric acid)

Correct formula = H₂SO₄

• Use: Fertilizer / Paper industry
• Formula: H₂SO₄
• Atomicity: 2 + 1 + 4 = 7

✅ FINAL CORRECTED TABLE

Substance	Use	Formula	Atomicity
Calcium carbonate	Limestone/Bones	CaCO₃	5
Aspirin	Analgesic	C₉H₈O₄	21
Iron(III) hydroxide	Cosmetics/Water treatment	Fe(OH)₃	7
Paradichlorobenzene	Moth balls	C₆H₄Cl₂	12
Magnesium hydroxide	Milk of magnesia	Mg(OH)₂	5
TNT	Explosive	C₇H₅(NO₂)₃	21
Pyrite	Fool’s gold	FeS₂	3
Calcium dihydrogen phosphate	Fertilizer	Ca(H₂PO₄)₂	15
Tetraoxosulphate(VI) acid	Fertilizer/Paper industry	H₂SO₄	7