Chemical Formulas and Naming
In 1827, Swedish chemist Jöns Jacob Berzelius replaced hundreds of alchemical symbols with just 1–2 letters per element, giving the world H₂O and CO₂ — a notation system used in every chemistry lab on Earth since.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · You see H₂O written on a water bottle. What do you think the "2" means? What does the "O" stand for?
Q2 · Without looking it up, guess how many atoms of each element are in CO₂. Write your thinking.
● Know
- What a chemical formula is and what a subscript tells you
- The seven diatomic elements and a memory trick to recall them
- The -ide naming rule for binary compounds
● Understand
- How to read any chemical formula to count atoms of each element
- Why diatomic elements exist as pairs in their natural state
- How to name a two-element compound from its formula
● Can do
- Count atoms in formulas including brackets (e.g. Ca(OH)₂)
- Identify whether a formula is an element, diatomic element or compound
- Name simple binary compounds using the -ide rule (e.g. NaCl = sodium chloride)
A chemical formula uses element and subscript numbers to show what atoms are in a substance. The subscript number tells you how many atoms of element are present. A element naturally exists as a 2-atom molecule. A compound is made of exactly two different elements.
A recipe tells you "2 cups flour, 1 egg". A chemical formula does the same thing for atoms — it's the recipe for a molecule. The element symbol tells you which atom, and the subscript number tells you how many.
Rules for reading a formula:
- Each capital letter starts a new element symbol.
- The subscript number (small, after the symbol) says how many atoms of that element are present.
- No subscript = 1 atom. The 1 is never written.
| Formula | Elements present | Atom count | Total atoms |
|---|---|---|---|
| H₂O | Hydrogen, Oxygen | 2 H + 1 O | 3 |
| CO₂ | Carbon, Oxygen | 1 C + 2 O | 3 |
| NaCl | Sodium, Chlorine | 1 Na + 1 Cl | 2 |
| NaHCO₃ | Sodium, Hydrogen, Carbon, Oxygen | 1 Na + 1 H + 1 C + 3 O | 6 |
| C₆H₁₂O₆ | Carbon, Hydrogen, Oxygen | 6 C + 12 H + 6 O | 24 |
Brackets: When a formula has brackets, the number outside the bracket multiplies everything inside. For example, Ca(OH)₂ means 1 Ca, and (O + H) × 2 = 2 O + 2 H. Total: 1 + 2 + 2 = 5 atoms.
In the formula H₂SO₄, there are hydrogen atoms, sulfur atom, and oxygen atoms, giving a total of atoms in the molecule.
Use these worked examples as your model whenever you meet a new formula:
| Formula | Step-by-step reading | What it is |
|---|---|---|
| H₂O | H₂ = 2 hydrogen atoms; O = 1 oxygen atom → 3 atoms total | Water |
| CO₂ | C = 1 carbon; O₂ = 2 oxygens → 3 atoms total | Carbon dioxide |
| NaHCO₃ | Na=1, H=1, C=1, O₃=3 → 6 atoms total | Sodium bicarbonate (bicarb soda) |
| C₆H₁₂O₆ | C₆=6, H₁₂=12, O₆=6 → 24 atoms total | Glucose (sugar) |
| Ca(OH)₂ | Ca=1; (OH)₂ means O×2=2 and H×2=2 → 1+2+2=5 atoms total | Calcium hydroxide (lime) |
How to approach any formula:
- Find each capital letter — that's a new element.
- Read the subscript after it — that's how many of that atom.
- If there are brackets, multiply everything inside by the number outside.
- Add up all the atoms for the total count.
Australian context: Calcium carbonate (CaCO₃) is the main mineral in limestone, which forms the spectacular Nullarbor karst caves and the White Cliffs of New South Wales. NaCl (sodium chloride) is the salt from Australian saltpans like Lake Eyre (Kati Thanda).
- H₂O
- CO₂
- NaCl
- NaHCO₃
- Ca(OH)₂
- 6 atoms total (Na + H + C + 3O)
- 2 atoms total (Na + Cl)
- 5 atoms total (Ca + 2O + 2H)
- 3 atoms total (2H + O)
- 3 atoms total (C + 2O)
Diatomic elements are seven elements that naturally exist as two-atom molecules. They are never found as single atoms in their natural state — they always come in pairs. The seven are:
| Element | Symbol | Formula |
|---|---|---|
| Hydrogen | H | H₂ |
| Oxygen | O | O₂ |
| Fluorine | F | F₂ |
| Bromine | Br | Br₂ |
| Iodine | I | I₂ |
| Nitrogen | N | N₂ |
| Chlorine | Cl | Cl₂ |
Memory trick: "HOFBrINCl" — pronounced "hof-brinkle". Or think: Have One Friend, But I'm Not Counting Lies.
Naming binary compounds — the -ide rule: A binary compound is made of exactly two different elements. To name it:
- Write the name of the first element unchanged.
- Write the name of the second element with "-ide" on the end.
| Formula | Elements | Name |
|---|---|---|
| NaCl | Sodium + Chlorine | Sodium chloride |
| MgO | Magnesium + Oxygen | Magnesium oxide |
| HCl | Hydrogen + Chlorine | Hydrogen chloride |
| CaS | Calcium + Sulfur | Calcium sulfide |
| AlCl₃ | Aluminium + Chlorine | Aluminium chloride |
Note: In this lesson we are only reading and naming formulas — we are not yet writing formulas from scratch. That skill (which requires valency) comes in Year 9.
A student reads the formula Al₂O₃ (aluminium oxide). They say: "There are 2 aluminium atoms and 3 oxygen atoms, so 5 atoms total. This is a binary compound." Are they right? How would you name this compound using the -ide rule?
How close was your prediction?
Great — you applied both the atom-counting and naming rules correctly.
Remember: subscript goes with the element just before it. O₃ = 3 oxygens, not 3 of everything.
For each formula below, state: (a) how many elements are present, (b) how many atoms of each element, and (c) the total atom count.
| Formula | Elements (how many types) | Atoms of each | Total atoms |
|---|---|---|---|
| O₂ | |||
| NH₃ | |||
| H₂SO₄ | |||
| Fe₂O₃ | |||
| Mg(OH)₂ |
Name each of the following binary compounds using the -ide rule. Then classify each as either an element, diatomic element, or binary compound.
| Formula | Name (using -ide rule) | Type |
|---|---|---|
| LiF | ||
| KBr | ||
| CaO | ||
| N₂ | ||
| ZnS | ||
| FeS |
Earlier you were asked: H₂O is written on a water bottle. What do you think the "2" means?
Now that you've worked through the lesson, write a fuller answer. Use the words subscript, hydrogen, and binary compound in your response.
Q1. For the formula C₆H₁₂O₆ (glucose), state: (a) how many types of element it contains, (b) the total number of atoms, and (c) whether it is an element, compound or mixture. (3 marks)
Q2. Name the following compounds using the -ide naming rule: (a) MgO (b) CaS (c) AlCl₃. (3 marks)
Q3. Explain the difference between O (an oxygen atom), O₂ (oxygen gas) and H₂O (water) using both particle descriptions and written descriptions. Why is O₂ a diatomic element but H₂O is not? (4 marks)
Answers
▾MCQ 1
C — In H₂SO₄ the subscript 4 is after O, so there are 4 oxygen atoms. The full count is: H₂ = 2 hydrogens, S = 1 sulfur, O₄ = 4 oxygens → 7 atoms total. The question asks only about oxygen = 4.
MCQ 2
B — CO₂: C = 1 carbon (no subscript = 1), O₂ = 2 oxygens. The subscript 2 belongs to the oxygen (O), not the carbon (C). Carbon dioxide = 1 C + 2 O.
MCQ 3
C — O₂ is a diatomic element — it's one element (oxygen) that naturally exists as a 2-atom molecule. NaCl and H₂O are compounds (2 different elements). CO₂ is a compound (carbon + oxygen).
MCQ 4
C — FeS has two different elements (iron and sulfur) chemically bonded → binary compound. It's not a mixture (can't be physically separated), not an element (has 2 types of atom), and not diatomic (diatomic means one element as a pair).
MCQ 5
B — In Ca(OH)₂ the ₂ outside the bracket multiplies everything inside. Inside the bracket: 1 O + 1 H. Multiplied by 2: 2 O + 2 H. Plus 1 Ca = 5 atoms total. Oxygen count = 2.
Short Answer 1
Model answer: (a) C₆H₁₂O₆ contains 3 types of element: carbon (C), hydrogen (H) and oxygen (O). (b) Total atoms: C₆ = 6, H₁₂ = 12, O₆ = 6 → 24 atoms total. (c) It is a compound — it is made of different elements chemically bonded, and it is a pure substance (not a mixture).
Short Answer 2
Model answer: (a) MgO = magnesium oxide (magnesium unchanged; oxygen → oxide). (b) CaS = calcium sulfide (calcium unchanged; sulfur → sulfide). (c) AlCl₃ = aluminium chloride (aluminium unchanged; chlorine → chloride).
Short Answer 3
Model answer: O is a single oxygen atom on its own — this would be a single particle. O₂ is oxygen gas: two oxygen atoms bonded together into one molecule. This is a diatomic element because it is one element (only oxygen) existing as a 2-atom molecule. H₂O is water: 2 hydrogen atoms bonded to 1 oxygen atom. This is not diatomic because it contains two different elements (hydrogen and oxygen) — it is a compound, not a single element in a paired form. Diatomic specifically means one element forming 2-atom molecules.
- Reading formulas — subscript after a symbol says how many of that atom. No subscript = 1. Brackets multiply everything inside.
- Diatomic elements — 7 elements (HOFBrINCl) that naturally exist as 2-atom molecules: H₂, O₂, F₂, Br₂, I₂, N₂, Cl₂.
- Naming binary compounds — first element unchanged + second element gets "-ide". E.g. NaCl = sodium chloride; MgO = magnesium oxide.