Module 7: Chemical Formula and Naming

Module 7 of Suharshi’s Basic Chemistry Course:

📘 Module 7: Chemical Formula and Naming

🧠 Learning Objectives:

• Understand how to write chemical formulas using valency

• Learn basic rules for naming compounds (especially ionic compounds)

• Practice identifying and naming common compounds

🔹 What is a Chemical Formula?

A chemical formula is a symbolic representation that shows:

• The elements present in a compound

• The number of atoms of each element

Example: H₂O means 2 hydrogen atoms + 1 oxygen atom → water

🔹 What is Valency?

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

Some common valencies:

Element	Symbol	Valency
Hydrogen	H	1
Oxygen	O	2
Sodium	Na	1
Chlorine	Cl	1
Magnesium	Mg	2
Carbon	C	4
Calcium	Ca	2
Nitrogen	N	3
Aluminium	Al	3

🧪 Writing a Chemical Formula (Using Criss-Cross Method)

Steps:

1. Write the symbols of the two elements.

2. Write the valencies below the symbols.

3. Criss-cross the valencies (swap and bring them to the opposite element as subscript).

4. Simplify if needed.

Examples:

1. Sodium chloride (NaCl)

   • Na (1), Cl (1) → Na1Cl1 → NaCl

2. Water (H₂O)

   • H (1), O (2) → H2O1 → H₂O

3. Carbon dioxide (CO₂)

   • C (4), O (2) → C2O4 → Simplify → CO₂

4. Magnesium chloride (MgCl₂)

   • Mg (2), Cl (1) → Mg1Cl2 → MgCl₂

5. Aluminium oxide (Al₂O₃)

   • Al (3), O (2) → Al2O3 → Al₂O₃

🧠 Tip: Don’t write subscript “1” (e.g., NaCl not Na1Cl1)

🔹 Rules for Naming Compounds

1. In ionic compounds (metal + non-metal):

   • Write the metal’s name first.

   • Then the non-metal’s name with the ending changed to “-ide”.

   Examples:

   • NaCl → Sodium chloride

   • MgO → Magnesium oxide

   • CaCl₂ → Calcium chloride

2. Compounds with two non-metals (covalent compounds):

   • Use prefixes to indicate number of atoms (mono-, di-, tri-, etc.)

   • Example:

     • CO → Carbon monoxide

     • CO₂ → Carbon dioxide

     • NO₂ → Nitrogen dioxide

Prefixes:

Number	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-

📝 Summary:

• Valency helps you know how atoms combine.

• Criss-cross valencies to write formulas.

• In naming: Metal first, then non-metal with “-ide”.

• For non-metal + non-metal: Use prefixes to indicate quantity.

🎲 Activity Ideas:

• Matching game: Match chemical formulas to names.

• Fill in the blanks: Write formulas from names and vice versa.

• Group practice: Create your own compounds with a valency chart.

📘 Understanding the Suffixes: “-ide”, “-ate”, and “-ite”

🧠 Learning Objectives:

• Understand what “-ide”, “-ate”, and “-ite” indicate in chemical names

• Learn how these suffixes are related to the composition of the compound

• Identify and name basic compounds using these suffixes

🔹 1. The Suffix “-ide”

Usage: Used for compounds containing only two elements (binary compounds)

Structure: Usually Metal + Non-metal (ionic compounds)
Or: Non-metal + Non-metal (in covalent compounds)

How it works:

• The second element (usually a non-metal) gets the “-ide” ending.

• No oxygen atoms involved!

Examples:

Compound	Elements	Name
NaCl	Sodium + Chlorine	Sodium chloride
HCl	Hydrogen + Chlorine	Hydrogen chloride
MgO	Magnesium + Oxygen	Magnesium oxide
NH₃	Nitrogen + Hydrogen	Ammonia (special)
CO₂	Carbon + Oxygen	Carbon dioxide

✅ Quick Tip: “-ide” = two elements only (binary), no polyatomic ions involved

🔹 2. The Suffix “-ate”

Usage: Used for compounds that include a polyatomic ion containing oxygen (oxo-anions)

Structure: Element + Oxygen (Oxyanions)

How it works:

• “-ate” is used when the polyatomic ion has more oxygen atoms.

Examples:

Polyatomic Ion	Formula	Name
NO₃⁻	N + O₃	Nitrate
SO₄²⁻	S + O₄	Sulfate
PO₄³⁻	P + O₄	Phosphate
ClO₃⁻	Cl + O₃	Chlorate

When combined with a metal:

Compound	Formula	Name
NaNO₃	Sodium nitrate
CaSO₄	Calcium sulfate
AlPO₄	Aluminium phosphate

✅ Quick Tip: “-ate” = More oxygen atoms in a polyatomic ion

🔹 3. The Suffix “-ite”

Usage: Also used for polyatomic ions with oxygen, but with fewer oxygen atoms than the “-ate” form.

How it works:

• “-ite” indicates fewer oxygen atoms than “-ate”.

Examples:

Polyatomic Ion	Formula	Name
NO₂⁻	N + O₂	Nitrite
SO₃²⁻	S + O₃	Sulfite
ClO₂⁻	Cl + O₂	Chlorite

When combined with a metal:

Compound	Formula	Name
NaNO₂	Sodium nitrite
CaSO₃	Calcium sulfite
KClO₂	Potassium chlorite

✅ Quick Tip: “-ite” = Fewer oxygen atoms than “-ate”

🔁 Comparing “-ate” and “-ite”

Ion Pair	“-ate” Form	“-ite” Form
Nitrate/Nitrite	NO₃⁻ (more O)	NO₂⁻ (less O)
Sulfate/Sulfite	SO₄²⁻	SO₃²⁻
Chlorate/Chlorite	ClO₃⁻	ClO₂⁻
Phosphate/Phosphite	PO₄³⁻	PO₃³⁻

🔬 Why this Matters in Chemistry:

• These suffixes help you immediately identify what type of atoms are in a compound.

• Especially important when naming or writing chemical formulas in chemistry exams or labs.

📝 Summary:

• “-ide”: Two elements only, no oxygen involved. Example: Sodium chloride (NaCl)

• “-ate”: Polyatomic ion with more oxygen. Example: Sodium nitrate (NaNO₃)

• “-ite”: Polyatomic ion with fewer oxygen atoms than the “-ate” form. Example: Sodium nitrite (NaNO₂)

🎯 Fun Activity:

Give Suharshi the name of a compound and ask her to:

• Write its formula

• Say whether it ends in “-ide”, “-ate”, or “-ite”

• Identify the polyatomic ion, if any