Atomicity, Radicals (Polyatomic Ions) & Valency
Topic Content
ATOMICITY OF ELEMENTS
Definition:
Atomicity = number of atoms in one molecule of an element.
MATCHING ELEMENTS WITH THEIR ATOMICITY
|
Element |
Atomicity |
Type |
|
Ozone (O₃) |
3 |
Triatomic |
|
Phosphorus (P₄) |
4 |
Tetra-atomic |
|
Sodium (Na) |
1 |
Monoatomic |
|
Sulphur (S₈) |
8 |
Octa-atomic / Polyatomic |
|
Hydrogen (H₂) |
2 |
Diatomic |
Atomicity of Some Common Elements
|
Name |
Atomicity |
Formula |
|
Argon |
1 |
Ar (Monoatomic) |
|
Helium |
1 |
He (Monoatomic) |
|
Oxygen |
2 |
O₂ (Diatomic) |
|
Hydrogen |
2 |
H₂ (Diatomic) |
|
Nitrogen |
2 |
N₂ (Diatomic) |
|
Chlorine |
2 |
Cl₂ (Diatomic) |
|
Phosphorus |
4 |
P₄ (Tetraatomic) |
|
Sulphur |
8 |
S₈ (Polyatomic) |
MOLECULAR OR POLYATOMIC IONS (RADICALS)
Definition:
A molecular or polyatomic ion is a group of atoms (often of different elements) that carry a net charge and act as a single unit in chemical reactions.
Examples of Common Radicals:
|
Radical |
Formula |
Charge |
|
Ammonium |
NH₄⁺ |
+1 |
|
Hydroxide |
OH⁻ |
–1 |
|
Nitrite |
NO₂⁻ |
–1 |
|
Nitrate |
NO₃⁻ |
–1 |
|
Carbonate |
CO₃²⁻ |
–2 |
|
Hydrogen carbonate |
HCO₃⁻ |
–1 |
|
Sulphate |
SO₄²⁻ |
–2 |
|
Phosphate |
PO₄³⁻ |
–3 |
|
Arsenate |
AsO₄³⁻ |
–3 |
Radicals carry charges (oxidation numbers) that are used when writing chemical formulas.
VALENCY AND OXIDATION NUMBERS
1️⃣ Valency
- Definition: Number of electrons in the outermost shell that an atom can lose, gain, or share to form a stable compound.
- Determines how elements bond and combine.
2️⃣ Oxidation Number (Oxidation State)
- Definition: Number of electrons an atom loses, gains, or shares in a compound.
- Loss of electrons → positive charge
- Gain of electrons → negative charge
- Represented as a number with + or – sign.
Example:
- Oxygen: Valency = 2 (can form 2 bonds)
- Oxidation number = –2 (gains 2 electrons in compounds)
Using Valency to Write Chemical Formulas
Rule:
- Charge on cation → subscript on anion
- Charge on anion → subscript on cation
- Simplify to lowest whole-number ratio.
Examples:
|
Metal / Radical |
Formula |
|
Na⁺ + Cl⁻ |
NaCl |
|
Na⁺ + O²⁻ |
Na₂O |
|
Ca²⁺ + Cl⁻ |
CaCl₂ |
|
Ca²⁺ + O²⁻ |
CaO |
|
Al³⁺ + Cl⁻ |
AlCl₃ |
|
Fe³⁺ + OH⁻ |
Fe(OH)₃ |
|
Cu²⁺ + OH⁻ |
Cu(OH)₂ |
|
(NH₄)⁺ + SO₄²⁻ |
(NH₄)₂SO₄ |
|
(NH₄)⁺ + PO₄³⁻ |
(NH₄)₃PO₄ |
|
Mg²⁺ + N³⁻ |
Mg₃N₂ |
Example: Mg²⁺ + O²⁻ → MgO (not Mg₂O₂; simplified ratio).
SUMMARY
- Atomicity: Number of atoms in a molecule.
- Radicals / Polyatomic ions: Group of atoms with a net charge, behaves as a unit.
- Valency: Number of electrons an atom uses to bond.
- Oxidation number: Charge acquired by an atom when forming a compound.
- Formula writing: Use the charges of ions to balance cations and anions.
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Objective Questions