Chemical Formulae
The Big Idea
- A chemical formula is a shorthand way to show which elements are in a substance and how many atoms of each are present (or the simplest ratio, depending on the type of substance).
- Getting formulae right matters because even a small change can describe a different substance with different properties.
Key Definitions
Chemical formula
Symbolic representation showing the elements in a substance and their numbers or ratios.
Molecular formula
Shows the actual number of each type of atom in a single molecule (common for covalent substances).
Empirical formula
Shows the simplest whole-number ratio of atoms in a compound.
Ionic compound
Formed from positive and negative ions (usually metal + non-metal, or metal + polyatomic ion) held together in a giant ionic lattice.
Covalent substance
Forms when non-metal atoms share electrons. It may form small molecules (like $CO_2$) or giant covalent structures (like diamond).
Key Principles For Writing Formulae
- Valency
- Valency is the typical number of bonds an atom forms (or the combining power it has).
- At this level, it helps you predict formulae quickly.
- Ionic charges
- In ionic compounds, the total positive charge must equal the total negative charge, so the overall compound is neutral.
Writing Formulae For Ionic Compounds
Ionic Compounds And Charge Balance
- Ionic compounds contain:
- Cations (positive ions), often from metals (e.g., $Na^+$, $Mg^{2+}$)
- Anions (negative ions), often from non-metals (e.g., $Cl^-$, $O^{2-}$) or polyatomic ions (e.g., $NO_3^-$)
- The formula shows the simplest ratio that makes the total charge add to zero.
Think of ionic compounds like making a “charge-zero team”: you must choose enough positives and negatives so the total score is 0.
Step-By-Step Method For Ionic Formulae
- Step 1: Identify The Ions
- Write the ion symbols and charges.
- Step 2: Balance The Total Charge
- Choose subscripts so total positive charge = total negative charge.
- Step 3: Write The Simplest Whole-Number Ratio
- If both subscripts share a factor, divide down.
- Step 4: Write The Formula
- Write the cation first, then the anion. Do not include charges in the final formula.
- Na⁺ and Cl⁻
- Charges balance 1:1 → NaCl
- Mg²⁺ and O²⁻
- Charges balance 1:1 → MgO
- Ca²⁺ and Cl⁻
- Need 2 chloride ions to balance +2 → CaCl2
The Criss-Cross Method
A quick method is to “criss-cross” the charge numbers to become subscripts.
For example, aluminium oxide:
- Al³⁺ and O²⁻
- Criss-cross → $Al_2O_3$
- Then check if it can be simplified; here it cannot.
Criss-cross is a shortcut, but you must still:
- check the final total charge is zero
- simplify the ratio if needed
Polyatomic Ions
Polyatomic ions
A polyatomic ion is a group of atoms bonded together that carries an overall charge and behaves as one unit in ionic compounds.
Common Polyatomic Ions
| Ion name | Formula | Charge |
|---|---|---|
| Ammonium | $NH_4$ | +1 |
| Hydroxide | $OH$ | -1 |
| Nitrate | $NO_3$ | -1 |
| Sulfate | $SO_4$ | -2 |
| Carbonate | $CO_3$ | -2 |
| Phosphate | $PO_4$ | -3 |
Writing Formulae With Polyatomic Ions
Rule: Use parentheses when needed
If you need more than one polyatomic ion, use parentheses around the ion.
- Ca²⁺ and $NO_3$⁻
- Need two nitrates to balance +2 → $Ca(NO_3)_2$
- Al³⁺ and $SO_4$²⁻
- Charge balance: 2 × (+3) = +6 and 3 × (−2) = −6 → $Al_2(SO_4)_3$
Writing Formulae For Covalent Substances
Covalent Formulae Reflect Bonding
- In covalent substances, atoms share electrons to form bonds.
- The formula shows how many atoms are joined in each molecule (or in the repeating network).
Typical Valencies For Common Non-Metals
| Element | Typical valency |
|---|---|
| H | 1 |
| O | 2 |
| N | 3 |
| C | 4 |
| Halogens (F, Cl, Br, I) | 1 |
Water, $H_2O$
- Oxygen forms 2 bonds
- Each hydrogen forms 1 bond
- So 2 H atoms bond to 1 O atom → $H_2O$
Ammonia, $NH_3$
- Nitrogen forms 3 bonds
- Hydrogen forms 1 bond
- So 3 H atoms bond to 1 N atom → $NH_3$
Methane, $CH_4$
- Carbon forms 4 bonds
- Hydrogen forms 1 bond
- So 4 H atoms bond to 1 C atom → $CH_4$
Carbon Dioxide, $CO_2$
- Carbon forms 4 bonds total
- Oxygen forms 2 bonds each
- So 2 O atoms bond to 1 C atom → $CO_2$
- (Each oxygen forms a double bond to carbon.)
Naming Clues: Prefixes In Covalent Names
For many covalent compounds, prefixes can tell you the number of atoms:
- mono- (1)
- di- (2)
- tri- (3)
- tetra- (4)
- penta- (5)
- Carbon dioxide → $CO_2$
- Dinitrogen tetroxide → $N_2O_4$
Ionic compounds do not use these prefixes in the same way (because charge balance determines the ratio).
Common Compounds To Know
| Compound | Formula |
|---|---|
| Water | $H_2O$ |
| Ammonia | $NH_3$ |
| Methane | $CH_4$ |
| Carbon dioxide | $CO_2$ |
| Hydrogen peroxide | $H_2O_2$ |
| Nitric acid | $HNO_3$ |
| Sodium hydroxide | $NaOH$ |
| Calcium carbonate | $CaCO_3$ |
- Write the formula for the ionic compound formed from $Al^{3+}$ and $Cl^-$.
- Write the formula for the ionic compound formed from $Mg^{2+}$ and $NO_3^-$.
- Explain why $MgO$ is correct but $Mg_2O_2$ should be simplified.
- Use valency to explain why ammonia is $NH_3$ and not $NH_2$.
- Glucose is $C_6H_{12}O_6$. What is its empirical formula?
- Give two reasons why correct chemical formulae are important in chemistry.
