Valency is the combining power or capacity of an atom. It shows how many electrons an atom will lose, gain, or share to form chemical bonds.

How to Determine Valency from the Periodic Table

• Group I elements (Li, Na, K, etc.) have 1 electron in their outer shell → valency = 1
• Group II elements (Be, Mg, Ca, etc.) have 2 electrons in their outer shell → valency = 2
• Group III elements (B, Al, etc.) have 3 electrons in their outer shell → valency = 3

For elements with more than 4 electrons in the outer shell, calculate valency by subtracting the number of outer electrons from 8:

$\text{Valency} = 8 - \text{number of outer electrons}$

Example: Oxygen has 6 outer electrons → valency = 8 − 6 = 2

Some elements have variable valency (more than one valency). For example, iron has valencies of 2 and 3, while copper has valencies of 1 and 2.

Oxidation state (or oxidation number) is the total number of electrons an atom gains or loses to form a bond. It indicates the charge an atom would have if all bonds were ionic.

Rules for Assigning Oxidation States

1. Free elements have oxidation state = 0 (e.g., O₂, Na, Cl₂)
2. The sum of oxidation states in a neutral compound = 0
3. In simple ions, oxidation state equals the charge (Na⁺ = +1, Cl⁻ = −1)
4. Group I metals = +1, Group II = +2, Group III = +3
5. Hydrogen = +1 (except in hydrides where it is −1)
6. Oxygen = −2 (except in peroxides where it is −1)
7. Fluorine = −1 (always, when combined)

Worked Example

Find the oxidation state of chlorine in KClO₃.

• K = +1
• O = −2 (three O atoms = −2 × 3 = −6)
• Let Cl = x

$+1 + x + (-6) = 0$ $x - 5 = 0$ $x = +5$

The oxidation state of chlorine in KClO₃ is +5.

A radical (or polyatomic ion) is a group of atoms that behaves as a single unit and carries a charge. Examples include:

The valency of a radical equals the numerical value of its charge.

Steps to Write a Formula

1. Write the symbols of the two elements or radicals
2. Write their charges (valencies) as superscripts
3. Interchange the charges and write as subscripts
4. Simplify if the numbers are equal

Worked Example

Write the formula of calcium chloride.

1. Symbols: Ca Cl
2. Charges: Ca²⁺ Cl⁻
3. Interchange: Ca₁Cl₂
4. Formula: CaCl₂

Formula with Radicals

When a radical appears more than once, use brackets:

Example: Aluminium sulfate

1. Al³⁺ and SO₄²⁻
2. Interchange: Al₂(SO₄)₃
3. Formula: Al₂(SO₄)₃

Binary Ionic Compounds (Metal + Non-metal)

1. Name the metal first (using its element name)
2. Name the non-metal second, changing the ending to -ide

Examples:

• NaCl → Sodium chloride
• MgO → Magnesium oxide
• CaS → Calcium sulfide

For Variable Valency Metals

Use Roman numerals in brackets to show the oxidation state:

• FeCl₂ → Iron(II) chloride
• FeCl₃ → Iron(III) chloride
• Cu₂O → Copper(I) oxide
• CuO → Copper(II) oxide

Binary Covalent Compounds (Non-metal + Non-metal)

Use prefixes to show the number of atoms:

Examples:

• CO₂ → Carbon dioxide
• CO → Carbon monoxide
• PCl₃ → Phosphorus trichloride
• N₂O₄ → Dinitrogen tetraoxide
• SF₆ → Sulfur hexafluoride

• Valency shows the combining power of an atom
• Oxidation state is the charge an atom would have in an ionic compound
• Radicals are polyatomic ions with positive or negative charges
• Chemical formulas show the ratio of elements in a compound
• IUPAC naming uses systematic rules: metals named first, non-metals end with "-ide", prefixes indicate numbers of atoms in covalent compounds

In Tanzania, chemical formulae and nomenclature are used in everyday shopping and farming. When a farmer buys fertilizer from a local agro dealer, the packaging often lists chemicals like urea (NH₄NO₃) or DAP (diammonium phosphate, (NH₄)₂HPO₄). Understanding oxidation states helps farmers recognize that DAP provides nitrogen in the ammonium form (oxidation state −3) which is readily available to plants. Similarly, when buying water treatment chemicals like chlorine (Cl₂) or aluminium sulfate (Al₂(SO₄)₃) for cleaning community water tanks, knowing the chemical names ensures the correct product is purchased for purifying drinking water in villages.