State the formulae of the elements and compounds named in the subject content

Stoichiometry – Formulae 📐

1️⃣ Introduction

In chemistry, a formula tells us the types and numbers of atoms in a substance. Think of it like a recipe: the ingredients are atoms, and the numbers are the amounts needed to make a molecule. Understanding formulae is the first step to mastering stoichiometry, which is all about measuring how much of each substance is involved in a reaction.

2️⃣ Elements – Simple Symbols

Elements are written with one or two letters. The first letter is always uppercase, the second (if present) is lowercase. Examples:

• Hydrogen – H
• Carbon – C
• Oxygen – O
• Iron – Fe
• Silver – Ag

3️⃣ Binary Compounds – Two Elements

Binary compounds consist of two different elements. The formula lists the elements in the order of the catalytic (more reactive) element first. Subscripts show the number of atoms of each element in one molecule. If the number is 1, the subscript is omitted.

4️⃣ Polyatomic Compounds – More Than Two Elements

Polyatomic compounds contain three or more different elements. They are often named with prefixes (mono-, di-, tri-, etc.) to indicate the number of atoms of each element. Common examples include:

• Ammonium – NH₄⁺
• Nitrate – NO₃⁻
• Sulfate – SO₄²⁻
• Phosphate – PO₄³⁻

5️⃣ Naming Conventions – A Quick Guide

1. Binary ionic compounds (metal + non‑metal): Metal + non‑metal (e.g., NaCl – Sodium chloride).
2. Binary covalent compounds (two non‑metals): Use prefixes to show atom counts (e.g., CO₂ – Carbon dioxide).
3. Polyatomic ions are named after the ion itself (e.g., SO₄²⁻ – sulfate).
4. When a metal can have more than one oxidation state, use Roman numerals in parentheses (e.g., Fe₂O₃ – Iron(III) oxide).

6️⃣ Analogy: Building with LEGO Blocks

Imagine each atom is a LEGO block. A formula is the instruction sheet that tells you how many of each block you need to build a specific structure. If you have the wrong number of blocks, the structure won’t hold together – just like a chemical reaction that doesn’t balance.

7️⃣ Quick Practice – Write the Formulae

1. Water – \$H_2O\$
2. Carbon monoxide – \$CO\$
3. Magnesium chloride – \$MgCl_2\$
4. Calcium carbonate – \$CaCO_3\$
5. Potassium nitrate – \$KNO_3\$

8️⃣ Summary ??

- Elements use single symbols.

- Binary compounds list elements in order of reactivity.

- Polyatomic compounds use prefixes or ion names.

- Roman numerals indicate metal oxidation states.

- Remember: a formula is like a recipe – the right ingredients in the right amounts make the reaction work!